Break Line
Depletion of snow and ice by melting and evaporation.

The X-axis on a graph.

Absolute Pressure
A pressure scale that starts with a value of zero for an absolute vacuum.

Absolute Temperature
A temperature scale that starts with zero at absolute zero temperature.

Absolute Zero Temperature
The temperature at which molecular kinetic energy is negligible.

Absorbed radiation
Any radiation that is absorbed by some object on the surface or in the atmosphere, thus reducing or removing the effects of that radiation on the surface or on the atmosphere.

The transfer of molecules from the bulk of the gas to a liquid surface, followed by diffusion of these molecules to the bulk of the liquid.

Compounds that release H+ ions in solution.

Acid fog
The combination of fog droplets with gaseous pollutants, such as oxides of sulfur or nitrogen oxides, creating fog droplets that have a pH of less than 5.0.

Acid rain
The wet deposition of acid from the troposphere to the earth.

Activation Energy (Ea)
The height of the energy threshold that moleculesmust reach in order to react.

Adiabatic charts
Also known as thermodynamic charts, these are meteorological charts that contain sounding data of pressure, temperature, dewpoints, and mixing ratios.

Adiabatic compression
The adiabatic process that results in the warming of the air parcel or layer.

The horizontal movement of an air mass that causes changes in the physical properties of the air such as temperature and moisture.

1) Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.  2) Official information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken. Advisories are also issued to describe: (a) tropical cyclones prior to issuance of watches and warnings and (b) subtropical cyclones.

A current air quality model developed by the US Environmental Protection Agency (EPA).

Aerodynamic Diameter
The diameter of a spherical particle having a density of 1 gm/cm3 that has the same inertial properties (i.e. settling velocity) in the gas as the particle of interest.

The suspension of solid or liquid particles in the atmosphere.

AIRMET (AIRman's METeorological Information)
This NWS aviation product advises of weather that maybe hazardous, other than convective activity, to single engine, other light aircraft, and Visual Flight Rule (VFR) pilots.

AIRMETs are routinely issued for 6 hour periods beginning at 0145 UTC during Central Daylight Time and at 0245 UTC during Central Standard Time. AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancelation of a SIGMET .

Air Mass
A large body of air that has nearly uniform conditions of temperature and humidity.

Air Pollution
Air pollution is any gas or particulate that, at high enough concentrations, may be harmful to life and/or property.

Air Quality
A general term that describes the amount of pollutants in the air.

Air Quality Statement
This National Weather Service product is issued to relay air pollution information and issue Ozone Action Days.

Air Stagnation
A meteorological situation in which there is a major buildup of air pollution in the atmosphere. This usually occurs when the same air mass is parked over the same area for several days. During this time, the light winds cannot "cleanse" the buildup of smoke, dust, gases, and other industrial air pollution.

Air Stagnation Advisory
This National Weather Service product is issued when major buildups of air pollution, smoke, dust, or industrial gases are expected near the ground for a period of time.  This usually results from a stagnant high pressure system with weak winds being unable to bring in fresh air.

Air Toxics (or Hazardous Air Pollutants)
Pollutants that are known to cause or suspected of causing cancer or other serious health effects, such as developmental effects or birth defects.

The portion of incoming radiation which is reflected by a surface.

Alberta Clipper
A fast moving low pressure system that moves southeast out of Canadian Province of Alberta (southwest Canada) through the Plains, Midwest, and Great Lakes region usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".

As pertains to computational science, algorithm refers to the mathematics of the model, including both the mathematical representation of the model and the numerical methods that might be used to solve those equations.

Hydrocarbons that contain only single bonds (general formula CnH2n-2).

The family of chemicals that contain only carbons and hydrogens with carbon-carbon double bonds.

The family of chemicals that contain only carbons and hydrogens with carbon-carbon triple bonds.

Aneroid Barometer
An instrument designed to measure atmospheric pressure. It contains no liquid.

An atom or molecule with a net negative charge due to excess in electrons.

Anthropogenic Pollution
Pollution which comes from man-made sources (ex. automobile emissions, mining, deforestation)

An area of high pressure around which the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern. Also called a high.

A part of a neutron (one of the members of the subatomic zoo)

The flat, spreading top of a Cumulonimbus Cloud (Cb). Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind.

Apparent Temperature
The apparent temperature is a measure of human discomfort due to combined heat and humidity.  It was developed by Dr. R. G. Steadman in 1979 and is based on studies of human physiology and textile (clothing) science.  The apparent temperature is designed so that apparent temperature exceeds the actual air temperature when the humidity is relatively high.  The apparent temperature then measures the increased physiological heat stress and discomfort associated with higher than comfortable humidities.  The apparent temperature is less than the actual air temperature when the humidity is relatively low and that the apparent temperature indicates the reduced stress and increased comfort associated with the higher rate of evaporative cooling of the skin.

Apparent Temperature = 1.03T + T (exp((DP-59)/17)-1)/19-3where T = Observed Temperature (degree F) DP = Dew Point (degree F)

As pertains to computational science, application refers to the science being studied using computing technologies, techniques, and tools. For example, quantum chemistry is a well-known application in computational science, and is known as computational chemistry.

Approaching Severe Levels
A thunderstorm which contains winds of 35 to 49 knots (40 to 57 mph), or hail 1/2 inch or larger but less than 3/4 inch in diameter.

Aqueous Liquid
A liquid whose main component is water but which can also contain significant concentrations of other dissolved solids and liquids (usually < 15% by weight) and/or low-to-moderate levels of suspended solids (usually < 2% by weight).

Area source pollution
pollution which comes from a number of different sources and is emitted in small quantities. An array of pollutant sources, so widely dispersed and uniform in strength that they can be treated in a dispersion model as an aggregate pollutant release from a defined area at a uniform rate. Compare line source and point source.

Aromatic compounds
Organic molecules that contain one or more benzene rings.

Atmospheric Pressure
The pressure exerted by the atmosphere at a given point.

Atmospheric Pressure (Air Pressure)
The weight of air pushing down on a unit area of a planet's surface.

Atomic Mass
The average of the masses of the naturally occurring isotopes of an element weighted according to their abundances.

Atomic number(Z)
Number of protons in an element.

Atomic Weight
See Atomic Mass.

Attainment Area
An area that meets the air quality standard for a criteria pollutant (under NAAQS).

Avagadro's Number
A number (6.022 x 1023 to four significant figures) equal to the number of atoms in exactly 12 g of carbon-12; the number of atoms or molecules in one mole of an element or compound.

AViatioN model
One of the operational forecast models run at NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800 GMT.  As of fall 1996, forecast output was available operationally out to 72 hours only from the 0000 and 1200 runs. At 0600 and 1800, the model is run only out to 54 hours.  It is one of the oldest operational models used by forecasters. The AVN model was developed primarily to aid in forecasting for aviation. The AVN gives short range forecasts like the NGM and ETA models do, but it also forecasts well into the medium range with forecasts up to 72 hours into the future. The resolution of the AVN model is about 100 km, which is not as good as the NGM or ETA models, but it still provides valuable insight into the future state of the atmosphere. The AVN also has its own set of statistical equations that use the AVN model output. The output from the AVN statistical equations is known as AVN MOS or FAN Guidance.

Break Line

Bext Value
The abbreviation for the extinction coefficient, a measure of the visibility of the atmosphere

Back Door Cold Front
A cold front moving south or southwest along the Atlantic seaboard and Great Lakes.

An instrument used for measuring air pressure. The two most common types are the mercury barometer and the aneroid barometer.

Barometric Pressure
The actual pressure value indicated by a pressure sensor.

Barotropic System
A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.  As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.

A substance that produces OH- ions when dissolved in water.

As pertains to computational science, a standard that is used to measure how effectively and efficiently a computer program runs on a given computer platform. Benchmarks are frequently speeds, or timings, but may also be standards for accuracy of the calculation.

Best Available Control Technology (BACT)
An emission limitation based on using the most up-to-date methods, systems, techniques, and production processes available to achieve the greatest feasible reductions with the consideration of energy, environmental, and economic impacts. These limitations are determined on a case-by-case basis as part of the PSD program.

Bernoulli Effect
The reduction of pressure due to such high speed winds through street canyons.

Biogenic pollution
Pollution which comes from biological sources, such as plant and animals

Blackbody radiation
Continuum radiation emitted by an object due to it's heat.

Bond order
The number of electron pairs being shared by any two bonded atoms.

Boundry Layer
In general, a layer of air adjacent to a bounding surface.  Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant.  For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere.  It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface.  The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.  There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer).  This layer is only a part of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it).  The surface boundary layer is roughly 10 meters thick, but again the exact depth is indeterminate.  Like friction, the effects of insolation and radiational cooling are strongest within this layer.

Bowen Ratio
The comparison between the amount of sensible heat and the amount of latent heat, often important when determining energy balance. The equation for the Bowen ratio is sensible heat divided by latent heat.

15 to 25 mph winds

British Thermal Unit (BTU)
The quantity of heat that must be transferred to one pound mass of water to raise the temperature by 1°F.

Broken (BKN)
An official sky cover classification for aviation weather observations, descriptive of a sky cover of 5/8 to 7/8.  This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog.

Brownian Diffusion
The slight deflection of very small particles in a gas stream that occurs when rapidly moving gas molecules strike them.

Bulk Richardson Number (BRN)
It is the ratio of the buoyancy (CAPE) of a lifted parcel to the vertical wind shear of the environment in which the parcel is lifted. It correlates well with observed storm type (single, multicell, supercell), especially for CAPEs between 1500 and 3000 J/kg. BRN's less than 45 tend to support supercell structures, but multicellular convection is favored over 45. While the BRN has shown some value as a predictor of storm type, it is a poor predictor of storm rotation because BRN Shear is a "bulk" measure. For example, it does not take in account the specific effects of directional and speed shear components.  High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear.

A description of the movement of a parcel of air or layer of air in the atmosphere, used as a predictor of the stability of the atmosphere.

Break Line

A condition when no air motion is detected.

Cap or Cap Strength
It measures the ability of stable air aloft (a layer of relatively warm air) to inhibit low-level parcel ascent. Empirical studies show that a cap greater than 2oC often precludes thunderstorms in the absence of a strong dynamical or forced lift. This occurs even when the instability is excessive.  A strong cap prevents widespread convection from occurring; thus, it allows low level heat and moisture to increase over a period of time.  This in turn increases the amount of potential instability.  Also, the air above it can cool, which also increases potential instability.  This delay in the onset of convection increases the severe potential for a limited number of cells that manage to punch through the cap or reach the boundary separating capped from uncapped region. The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above.  Meanwhile, when there is no cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.  As a result, convection tends to be widespread, but less intense. This is a result of developing thunderstorms competing for a limited amount of available moisture.  Therefore, severe storms often form along these lid boundaries, where the release of potential instability is favored. As a result, thunderstorms showing rapid growth within or very near a strongly capped region become severe.  This is also called a lid.

Carbon Bed Adsorber
An air pollution control system that is used to collect and concentrate organic compounds on an activated carbon adsorbent.

Carbon Bond-IV Mechanism
A reaction mechanism model which describes reactants on the basis of their bonding patterns

Carbon Monoxide (CO)
An air pollutant which comes from incomplete fossil fuel combustion, and is both a contributer to the formation of tropospheric ozone as well as a serious health hazard

Cartesian Coordinates
The familiar "x-y" coordinate system, in which the axes are at right angles to each other. Raw radar data, often in polar coordinates, can always be converted to Cartesian coordinates.

A substance, usually present in small amounts compared to the reactants, that speeds up the chemical reaction rate without being consumed in the process.

Catalytic Converter
Catalytic converters convert the three main mobile source pollutants (NOx, HC’s, and CO) to non-harmful species using platinum or palladium as a catalyst for oxidations.

Catalytic Oxidizer
An air pollution control device that uses a catalyst to accelerate the oxidation reaction at lower temperatures than possible in gas phase thermal oxidation.

Ion with a positive charge.

The height of the lowest layer of broken or overcast clouds.

Central Tendencies
A group of statistical methods (mean, median and mode) used to find measures of center for a data set

Chloroflourocarbons (CFCs)
Pollutants involved in the destruction of stratospheric ozone.

Cubic Feet per Second (CFS)
TThe flow rate or discharge equal to one cubic foot (of water, usually) per second. This rate is equivalent to approximately 7.48 gallons per second. This is also referred to as a second-foot.

A set of dimensional units based on three fundamental units: centimeters, grams, and seconds.

The tendency of the wind to follow the axis of a channel or be steered by sloping land, resulting in a change in direction.

Chemical compounds
Substances that consists of two or more different elements in a definite ratio. For example, water is a compound made up of two hydrogen atoms each bonded to a single oxygen atom.

Chemical equation
A statement in formulas that expresses the identities and quantitties of substances involved in a chemical or physical change. The left side of the equation represents the substances before the reaction occurs, known as the reactant. The right side shows the substances that were changed during the reaction, or the products. The arrow indicates that a chemical change occurs.
Chemical Formula
A short-hand way of identifying the components and amounts in a molecule.

Chinook Wind
A warm, dry wind that descends the eastern slope of the Rocky Mountains. The warmth and dryness of this wind can quickly melt and evaporate snowcover. Another name for this type of wind is "foehn".

Class I Areas
Class I areas are protected by the PSD program and include national parks, national wilderness areas, national monuments, national seashores, and other areas of special national or regional natural, recreational, scenic, or historic value.

Class II Areas
Attainment areas that are neither industrialized nor meet the specific requirements for classification as Class I areas. They are protected by the PSD program.

Class III Areas
Industrialized attainment areas. They are protected by the PSD program.

Clean Air Act Amendments (CAAA)
The Clean Air Act Amendments are a series of regulations, passed in 1990, relating to the emission of pollutants, primarily NOx’s and VOC’s. The amendments set in place emission standards and deadlines for companies to come into compliance on the regulations.

Climate Prediction Center (CPC)
One of nine national centers that comprises the National Centers for Environmental Prediction (NCEP). Their mission is to maintain a continuous watch on short-term climate fluctuations and to diagnose and predict them.

Climatological Data (CD)
National Climatic Data Center (NCDC) publication. Monthly editions contain station daily maximum and minimum temperatures and precipitation. 

Closed form
As pertains to computational science, closed form (also known as analytic form) are those equations that can be solved exactly, without use of approximations and/or application of one or more numerical methods.

Cloud Condensation Nuclei
Those aerosols that readily participate in the formation of clouds.

Cold Air Advection
Transport of cold air into a region by horizontal winds.

Cold Front
The leading edge of a relatively colder airmass which separates two air masses in which the gradients of temperature and moisture are maximized.  In the northern hemisphere winds ahead of the front will be typically southwest and shift into the northwest with frontal passage.

Collection Efficiency
A ratio of pollutants entering a control device versus pollutants leaving the device expressed as a percent.

Combined Gas Law
An equation derived from the ideal gas law that allows calculation of pressure, temperature, and volume when a given amount of gas undergoes a change of conditions.

Combustion Particle Burnout
Particulate matter formed and released by the combustion of fossil fuels

Computational expense
A general term that signifies the costs (in terms of computing time, dollars, number of cyles, etc.) of a computer program running. Codes that are computationally expensive are those that take significant amounts of time, money, and/or computing resources to complete.

Computational science
The combination of application, algorithm and architecture to solve complex scientific problems. Sometimes referred to as computer modeling and simulation or scientific computing

Condensable Particulate Matter
Particulate matter formed and released by the combustion of fossil fuels

The process by which a gas or vapor changes into a liquid.

A simple, relatively inexpensive device that normally uses water or air to cool and condense a vapor stream.

A measure of the ability of a material to conduct an electric charge.

Conservation of Matter
A general principle of non-nuclear reactions and processes where matter is neither created nor destroyed.

Constant Flux layer
Another name for the surface layer in the planetary boundary layer, roughly 10% of the height of the PBL.

Generally, transport of heat and moisture by the movement of a fluid. In meteorology, the term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cumulonibus (Cb), towering cumulus clouds, and Altocumulus Castellanus (ACCAS) clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

Convective Available Potential Energy (CAPE or B+)
It defines the vertically integrated positive buoyancy of an adiabatically rising air parcel on a sounding. This is proportional to the amount kinetic energy that the air parcel gains while it is warmer that its surrounding environment. As a result, CAPE provides the best measure of the potential instability available in the atmosphere. Increasing values of CAPE generally lead to progressively vigorous convection. However, severe thunderstorms can form in environments showing weak to moderate CAPE, especially if the Storm Relative Helicity values are high.

Convective Boundary Layer
Another term for the lower layers in the planetary boundary layer, in which buoyancy production of turbulence will be dominant over shear production.

Convective Condensation level (CCL)
It is the height to which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated (condensation starts). It approximates the base height of cumuliform clouds which are, or would be, produced by surface heating.

Convective Stability
The stability of a layer after it has been lifted.

The rise of air when two or more air masses meet head-on.

Convergence Zone
The area where two or more air masses meet and rise. The zone where convergence occurs is the convergence zone.

Coordinated Universal Time (UTC)
The time in the zero degree meridian time zone. In order to convert to Eastern Standard Time, subtract 5 hours (Eastern Daylight Time subtract 4 hours). For example, 0900 UTC is 4:00 AM EST or 5:00 AM EDT.

Coriolis Force
An apparent force observed on any free-moving object in a rotating system. On the Earth, this deflective force results from the Earth's rotation and causes moving particles (including the wind) to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

A measure of the similarity between variables or functions.

Covalent compounds
Compounds that consist of atoms bonded together by shared electron pairs.

Criteria Pollutants
The group of common air pollutants defined by EPA whose concentrations in the ambient air must be maintained at low levels to protect public health. EPA has promulgated a National Ambient Air Quality Standard (NAAQS) for each of the criteria pollutants.

A cloud in the form of individual, detached domes or towers that are usually dense and well-defined. It has a flat base with a bulging upper part that often resembles cauliflower.

Alkanes that form a ring. The prefix cyclo- added to the rest of the name when naming specific cycloalkanes.

An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Break Line
Daily Climatological Report
As the name indicates, this climatological product is issued daily by each National Weather Service office.  Most of the climatological data in this report are presented in a tabular form; however, some narrative statements may also be used in the product.  The report is organized so that similar items are grouped together (i.e., temperature, precipitation, wind, sunrise and sunset times, etc.).

Dalton's Law of Partial Pressures
The law stating that the total pressure of a gas is the sum of the pressures exerted by each component gas.

Data assimilation
Any modification to obtain a suitable data set for starting a numerical simulation. Most models must be initialized with a starting set of data. Most datasets, collected from observed or experimental efforts, must be assimilated, or modified, prior to their input into numerical simulations.

Delta T

1) A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development. 2) A way of determining whether lake effect snow showers are likely to develop over the Great Lakes.  Typically, this is done by taking the absolute value of the difference in temperature between the 850 mb temperature and the lake water temperature.  Typically, 13 degree C or more is needed for lake effect snow showers to develop over The Great Lakes.  However, an 8 degree C difference can help enhance system snow.

The quantity of mass contained in a specific volume.

Dependent variable
The variable, or value, that results from some change or action, usually depicted on the y-axis in an x-y graph. As an example: ozone concentration depends on the amount of emissions put into the atmosphere, so ozone concentration is a dependent variable, and emissions amount is the independent variable

Developing Storm
Used in the National Weather Service High Seas Forecast.  It refers to an extratropical low or an area in which storm force winds of 48 knots (55 mph) or greater are "expected" by a certain time period.  On surface analysis charts a developing storm indicates storm force winds forecast within the next 36 hours. When the term developing storm is used on the 48 hour surface and 96 hour surface charts, storm force winds are expected to develop by 72 hours and 120 hours, respectively.

Water droplets that form upon surfaces on or near the ground when air is cooled toward its dew point.

Dew Point (Dew-Point Temperature)
A measure of atmospheric moisture. The temperature to which air must be cooled, at constant pressure and moisture content, in order for saturation to occur. The higher the dew point, the greater amount of water vapor in the air mass.

Any process in which an exchange of energy, usually int he form of heat, occurs between a parcel of air and the surrounding environment.

Diatomic gases
Nonmetal gases that form covalent bonds when they occur naturally

Diffused Radiation
Radiation that diffuses through particles in the atmosphere before reaching the surface of the earth.

A process by which the molecules of two or more substances gradually mix as a result of random thermal motion.

Measure of the extent to which very small particles are influenced by molecular collisions which cause the particles to move in a random manner across the direction of gas flow.

Dimensional Calculations
A form of mathematical calculation in which the units applying to each value are stated explicitly and are handled algebraically.

Units of measure used to express the magnitude of mass, distance, force, and time.

A molecule consisting of two identical simpler molecules. For example, nitrogen dioxide (NO2) forms a dimer (N2O4).

Direct Radiation
Any radiation that strikes the surface of the earth without any interfering process (scattering, diffusion, etc.)

The process by which substances are moved or diluted. Dispersion is a term typically used in the description of the removal of air pollutants from one location to another.

Displacement Reaction
A reaction in which an atom or ion in one substance displaces an atom or ion in another. Classified as single-displacement and double-displacement (metathesis) reactions.

Dissociation Constant
A number that represents the essentially steady equilibrium constant for a weak solution. Also known as ionization constant.

An atmospheric condition that exists when the winds cause a horizontal net outflow of air from a specific region.

A RADAR or LIDAR that determines the velocity of falling precipitation either toward or away from the sensing device by taking into account the change in frequency of waves (Doppler effect)

A chemcial reaction where different atoms in two different compounds displace each other.

Downslope Wind
Also called a katabatic wind, any wind that blows down the side of a mountain or hill.

Dry Adiabat
A line of constant potential temperature on a thermodynamic chart.

Dry Adiabatic
The rate of change of temperature in a rising or descending unsaturated parcel of air.

Dry Adiabatic Lapse Rate
The rate of change of temperature in rising or descending unsaturated air parcel. The rate of adiabatic cooling or warming is 5.5oF per 1,000 feet (10oC per 1,000 meters).

Dry adiabatic process
The process by which temperature changes in a rising or descending unsaturated parcel of air, without exchange of energy occuring between the parcel and the surrounding environment.

Dry Basis Concentration
The conversion of concentration data or gas flow rate to a moisture-free basis.

Dry Deposition
Dry deposition is the process by which gaseous pollutants are adsorbed by plants and soil through a mass-transfer process. Also defined as the deposition of acids or acid precursors from the atmosphere onto plant foliage and other solid surfaces by adsorption and direct uptake in the absence of liquid water. The rate at which this occurs depends on the 'deposition velocity'. The size of this coefficient varies according to the surface. Typical values for SO2 deposited on foliage are 0.5 - 1.0 cm s-1.

Dry Scrubber
An air pollution control device used to remove an acid gas pollutant from a gas stream. The pollutant is collected on or in a solid or liquid material, which is injected into the gas stream. A dry scrubber produces a dry product that must be collected downstream from this control device.

Dynamic Model
In hydrology, a hydrological computer model that is mathematically and physically correct because it accounts for every molecule of moisture entering, stored in and leaving a basin or an area.

Break Line

A small volume of air (or any fluid) that behaves differently from the larger flow in which it exists.

Eddy Viscosity
The internal friction produced by turbulent flow.

Effective Precipitation (Rainfall)
1) That part of the precipitation that produces runoff.  2) A weighted average of current and antecedent precipitation that is "effective" in correlating with runoff.   3) That part of the precipitation falling on an irrigated area that is effective in meeting the consumptive use requirements.

A substance that produces either (1) a conducting ionic melt when fused, or (2) a conducting ionic solution when dissolved in certain solvents (most often water). All acids, bases and salts are electrolytes.

Electromagnetic spectrum
Light at all different wavelengths, including radio waves, microwaves, infrared light, visible light, ultraviolet light, gamma rays, and x-rays.

One of the parts of the atom having a negative charge. Indivisible particle with a charge of -1.

Electronegativity (electronegative)
Measure of a substances's ability to attract electrons.

Electrostatic Precipitator
A type of air pollution control system that uses high voltage fields to electrically charge and collect particulate matter. The charged particles approach an electrically grounded collection plate and accumulate as a dust layer, which is partially removed by mechanical rapping (hammers) on a routine basis.

Elementary Reaction
An individual reaction step in a mechanism.

A general term for any primary pollutant that is produced and placed into the atmosphere. Emissions can come from mobile or stationary sources.

Emissions Inventory
Emissions inventories evaluate air quality by providing information about where pollutants are emitted from, as well as the levels and types of pollutants emitted.

Empirical formula
Formula showing the simplest ratio of elements in a compound.

A measure of the quantity of internal energy (or heat) that is in a given amount of material.

Enthalpy of Vaporization
Represents the amount of internal energy required to change the phase of a substance from liquid to vapor and vice versa.

The process in which material, such as water droplets or particulate matter, is picked up and carried along by a gas or liquid stream.

Entrainment Zone
The zone where mixing of environmental air into a pre-existing air current or cloud takes place, so that the environmental air becomes part of the current or cloud.

Entry Loss
The loss of pressure that occurs when airflow moves into a system.

Environmental Modeling Center (EMC, formerly the Development Division)
This is one of 9 centers that comprises the National Centers for Environmental Prediction (NCEP, formerly the National Meteorological Center). This center improves numerical weather, marine and climate predictions at the , through a broad program of research in data assimilation and modeling. In support of the NCEP operational forecasting mission, the EMC develops, improves and monitors data assimilation systems and models of the atmosphere, ocean and coupled system, using advanced methods developed internally as well as cooperatively with scientists from Universities, NOAA Laboratories and other government agencies, and the international scientific community.

Equation of State
See Ideal Gas Law

A steady state condition. The amount of mass transferred in one direction is exactly balanced by the amount of mass transferred in the reverse direction.

Equilibrium Level (EL)
It is the height in the upper troposphere where a parcel of saturated air, rising because of its positive buoyancy, becomes equal in temperature to the surrounding environment. Beyond this point, the parcel become colder than its environment. As a result, it will be heavier than the surrounding air and it will begin to fall. Under the right conditions, severe thunderstorm tops can overshoot the EL by a considerable distance without reaching the tropopause. Conversely, non-severe thunderstorm tops can rise above the tropopause without overshooting the EL. Consequently, the EL provides more meaningful information than the tropopause for evaluating the strength of convective updrafts.

Equivalent Potential Temperature
The potential temperature that a saturated parcel of air would have if raise moist adiabatically to the top of the atmosphere (where P=0 mb).

Equivalent Weight
The amount of substance (in grams) that supplies one gram mole of reacting units. The equivalent weight is calculated by dividing the molecular weight of a compound by the change in the oxidation number it experiences during a chemical reaction.

ETA Model
One of the operational numerical forecast models run at NCEP. The ETA is run twice daily, with forecast output out to 48 hours. The ETA model is a newer model, which is very similar to the NGM model and forecasts the same atmospheric variables. The main difference is the ETA model has better resolution (29 kilometers). In other words, the grid box is much smaller than in the NGM. This allows the ETA to give a more detailed forecast over the USA. The ETA model was named after the ETA coordinate system, which is a mathematical coordinate system that takes into account topographical features such as mountains. As a result of using this coordinate system and the higher resolution, the ETA model has a much more accurate picture of the terrain across the USA.  By the way, the name, ETA is the seventh letter of the Greek alphabet.

Evaporative Cooling Tower
Equipment used to reduce the temperature of a gas stream. Fine droplets, injected into a vessel, are evaporated as they absorb heat from the gas stream.

The combined processes of evaporation and transpiration.

Explicit Mechanisms
Mechanisms which treat all chemical species and their reactions individually, and do not groups specific types of reactions together.

Extinction coefficient
A measure of the visibility of the atmosphere (represented by bext value).

Extratropical Cyclones
A cyclonic storm that most often forms along a front in middle and high latitudes. It is not a tropical storm or a hurricane.

Break Line

Families of elements
Groupings of elements with closely related properties.

Fan Drive
The way in which the motor shaft is linked to the fan wheel to transmit power and control speed.

NWSRFS Forecast Program to produce operational forecasts

Four-dimensional data assimilation (FDDA, 4DDA)
A technique used in atmospheric modeling, primarily at the pre-processing state, where data is distributed over space and time.

Fine Particles
EPA classification of particles having aerodynamic diameters greater than 0.1 micrometer and less than or equal to 2.5 micrometers.

Flow Rate (Actual)
The volume of gas moving through a system or stack per unit time expressed at actual conditions of temperature and pressure.

Flow Rate (Standard)
The volume of gas moving through a system or stack per unit time expressed at standard conditions of temperature and pressure.

Flow Rate (Volumetric)
The volume of gas moving through a ventilation system, stack, or air pollution control system per unit time.

Flue Gas Desulfurization (FSD)
The process by which sulfur is removed from combustion exhaust gas.

Fly Ash
Uncombusted particulate matter in the combustion gases resulting from the burning of coal and other material.

A visible aggregate of minute water particle (droplets) which are based at the Earth's surface and reduces horizontal visibility to less than 5/8 statue mile, and unlike drizzle, it does not fall to the ground. It occurs most frequently in coastal regions because of the great water vapor content of the air. However, it can occur anywhere. The rapidity with which fog can form makes it especially hazardous. It forms by any atmospheric process that does one of the following: 1) Cools the air to its point 2) Raises the dew point to the air temperature. Names given to fog types identify their methods of formation. The principle types are radiational fog, ice fog, advection fog, upslope fog, rain induced fog, and steam fog. These types of fog are called "dense" when the surface visibility is equal to or less than 1/4 miles. A Dense Fog Advisory will be issued when the dense fog becomes widespread.

Forcast Model
Forecasters use numerical weather models to make their forecasts. These numerical models are classified into four main classes. The first is global models, which focus on the entire northern hemisphere. The second is national models, which focus on the USA. The third is regional models. These regional models have a finer grid than national models and are run out for smaller periods of time. The final class of models is relocatable models, which do not focus on any permanent geographical location. Relocatable models are very limited on the size of the geographical area for which they can forecast, but these models have very high resolutions, or very small forecast grid boxes.

An influence on a physical object that causes a change in movement and/or shape.

A general term referring to the production of some event.

Free Atmosphere
The layer of air directly above the planetary boundary layer, where frictional forces are minimal.

Free Radical
An atom or group of atoms with at least one unpaired electron, making the species unstable and highly reactive.

The number of times a data value occurs in a data set.

Frequency Table
A table used to show a list of unique data values and their corresponding frequencies.

The process by which turbulence is created by the movement of winds over rough surfaces.

Friction Layer
The atmospheric layer near the surface usually extending up to 1 km (3,300 ft) where the wind is influenced by friction of the earth's surface and objects on it.

A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

Frontal Inversion
A transition zone between 2 different air masses. The temperature curve is the basic reference for locating frontal zones aloft. If the front were a sharp discontinuity, the temperature curve should show a clear cut inversion separating the lapse rates typical of cold and warm air masses. However, a shallow isothermal or relatively stable layer is more usual indication of a well-defined front. Frequently, the frontal boundary is so weak or distorted by other discontinuities that frontal identification is difficult. In an ideal case, the dew point curve through the frontal zone will show an inversion or sharp change associated with that of the temperature curve. The frontal surface is considered to be located at the top of the inversion.

Fuel NOx
Nitrogen oxides generated from the fuel or waste during combustion.

Fugitive Emissions
Emissions that escape from industrial processes and equipment.

Functional group
A specific combination of atoms, typically a carbon multiple bond and/or heteroatom bond, that reacts in a characteristic way no matter in which molecule it occurs.

Fundamental particle
the electron

G Break Line

Gauge Pressure
The relative pressure inside a vessel or container (the difference between the inside pressure and atmospheric pressure).

Refers to the normal distribution; phenomena whose events are "normally" distributed are "Gaussian" distributed. This is the most common distribution encountered in physical processes.

Geostationary Orbiting Environmental Sattellite (GOES)
Satellites orbiting at 22,370 miles above the Earth's surface with the same rotational velocity as the Earth; therefore, the satellite remains over the same location on the Earth 24 hours a day. Besides sending back satellite pictures to earth, it also relays the DCPs river and rainfall data back to the ground receiving located at Wallops Island, Virginia.

Geostrophic Balance
The balance between the Coriolis Force and the pressure gradient force (PGF).

Geostrophic Wind
The horizontal wind for which the coriolis acceleration (caused by the Earth's rotation) exactly balances the horizontal pressure force. In practice it is assumed that this marks the upper limit of frictional influence of the Earth's surface. The geostrophic wind blows along the contours on a constant pressure surface. The speed of the geostrophic wind is dependent upon how close your pressure contour are together. Thus, when your pressure contours are close together, you will see a strong geostrophic wind. The opposite occurs when your pressure contours are far apart.

Greenhouse Effect
The heating effect caused by gases in the atmosphere absorbing (solar radiation) instead of letting it escape back into space. There are 2 types: 1) Natural: It is what keeps the Earth's average temperature at 59oF instead of 0oF. In this case, the most abundant greenhouse gas is water vapor. 2) Anthropogenic: Additional warming caused by having too much carbon dioxide (CO2). In the first case, the Greenhouse Effect is good; however, if the second case is occurring it will be bad.

Greenhouse Gases
Gases in the atmosphere such as CO2 which absorb large amounts of infrared radiation.

Ground Fog
Fog of little vertical extent (usually 20 feet or less).

Ground Reflected Radiation
Any radiation that strikes the surface of the earth and is reflected back into the atmosphere.

Vertical columns in the periodic table.

A rapid fluctuation of wind speed with variations of 10 knots or more between peaks and lulls.

Break Line

Precipitation in the form of balls or lumps usually consisting of concentric layers of ice. A thunderstorm is classified as severe when it produces hail 3/4 of an inch or larger in diameter.

Hail Index (HI)
This WSR-88D radar product displays an indication of whether the thunderstorm structure of each identified by the storm series algorithm is conducive to the production of hail. A green triangle indicates areas where the algorithm thinks that there is hail. It will also display an estimated hail size. This does not mean that there is actually hail occurring or the hail is the size that the radar is indicating, but it does attract the radar operator's attention to check out the thunderstorm more closely.

The amount of time it takes for half an initial amount to disintegrate.

Hazardous Air Pollutants
See Air Toxics.

A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. The helicity is the area on a hodograph that is enclosed by a line from the tip of the storm motion vector to the surface wind vector, then following the hodograph curve to 3 km level, then back to storm motion vector. This value allows the forecaster to determine the rotational tendency of a thunderstorm. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2. It is dependent on the local environmental wind profile in which a thunderstorm develops and the thunderstorm motion.

Heat Flux
The amount of heat that is transferred across a surface of unit area in a unit of time.

Heterogeneous Nucleation
The accumulation of material from the vapor phase onto an existing particle.

A region of high pressure, marked as "H" on a weather map. A high is usually associated with fair weather.

A polar coordinate graph which shows the vertical wind profile of the lowest 7000 meters of the atmosphere. These plots are used to determine the advection patterns aloft, whether a thunderstorm will rotate, and the type of thunderstorms that you will likely see that day. On the graph, only the tip of wind vectors are plotted on this graph. The tips are denoted by a dot. As the distance between the dot and the center of the graph increases, the magnitude of the wind will also increase. These dots are sequentially connected together by a line beginning with the first wind reported and ending with last wind reported. This is similar to a dot to dot book. Normally, they are plotted every 500 meters from the surface to 7000 meters. Another interesting feature of this graph is that the axes are rotated 180 degrees. This means that 180o is located on the top of the graph, 270o is located on the right side of the graph, 0o is located on the bottom of the graph, and finally 90o is located on the left side of the graph. Therefore, if you are going to plot a west wind, it would be located to the right of the center of the graph. Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc.).

Homogeneous Nucleation
The formation of a particle from the vapor phase involving only one compound.

Horizontal Vortex System
The two vertical vortices on the lee side of the obstruction.

Horseshoe Vortex
Another name for the horizontal vortex system.

Hot Spots
Areas where pollution concentrations are especially high.

Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

An organic compound that contains only H and C atoms.

Hydrogen bonding (hydrogen bond)
Strong type of intermolecular dipole-dipole atttraction. Occurs between hydrogen and small highly electronegative atoms such as F, O or N.

Hydrologic Cycle
The constant movement of water above, on, and below the Earth's surface. Processes such as precipitation, evaporation, condensation, infiltration, and runoff comprise the cycle. Within the cycle, water changes forms in response to the Earth's climatic conditions.

Hydrologic Equation
The water inventory equation (Inflow = Outflow + Change in Storage) which expresses the basic principle that during a given time interval the total inflow to an area must equal the total outflow plus the net change in storage.

Hydrologic Model
A conceptual or physically-based procedure for numerically simulating a process or processes which occur in a watershed.

Any liquid or solid ice particle that is formed in a cloud. Hydrometeors may or may not fall to the ground int he form of precipitation.

An instrument for measuring the specific weight of a liquid by utilizing the principle of buoyancy.

The region that includes all the earth's liquid water, frozen water, floating ice, frozen upper layer of soil, and the small amounts of water vapor in the earth's atmosphere.

Hydrostatic equilibrium
The state of the atmosphere when there is a balance between the vertical pressure gradient force and the downward pull of gravity.

Hydroxide Ion (OH-)
A substance that can donate a pair of electrons to form a new bond

An instrument used to measure the air's water vapor content.

Break Line

Ideal Gas Law (Ideal Gas Equation)
An equation expressing the relationships among volume, pressure, temperature, and number of moles of an ideal gas: PV = nRT.

Independent variable
The variable, or value, that is changed. As an example: ozone concentration depends on the amount of emissions put into the atmosphere, so ozone concentration is a dependent variable, and emissions amount is the independent variable

The process by which a substance, usually water, enters into a porous substance, such as soil.

Infrared (IR) Satellite Imagery
This satellite imagery senses surface and cloud top temperatures by measuring the wavelength of electromagnetic radiation emitted from these objects. This energy is called "infrared". High clouds are very cold, so they appear white. Mid-level clouds are somewhat warmer, so they will be a light gray shade. Low cloud are warmer still, so they appear as a dark shade of gray or black. Often, low clouds are the same temperature as the surrounding terrain and cannot be distinguished at all. The satellite picks up this infrared energy between 10.5 and 12.6 micrometer (um) channels. This imagery can be used both during the day and night. .

A term that applies to compounds that contain no carbon-hydrogen bonds.

Incoming solar radiation. Solar heating; sunshine.

Instability (Unstable Air)
A state of atmosphere in which the vertical distribution of temperature allows rising, warm air to continue to rise and accelerate. This kind of motion is conducive for thunderstorm development. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.

The process whereby a particle moving in a gas stream is offset slightly from directly impacting a moving or stationary obstacle or target. As the particle tries to move past the obstacle, the obstacle intercepts (and collects) the particle. Compare with Inertial Impaction.

Generally, a departure from the usual increase or decrease in an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs. This occurs when warm air sits over cold air, possibly trapping moisture and pollutants in the surface air layer. An inversion is present in the lower part of a cap.

Removing or adding electrons to an atom creates an ion (a charged object very similar to an atom).

Ionic bond
When two oppositely charged ions share at least one pair of electrons but the electrons spend more time near one of the atoms than the other.

Ionic Solution
A solution containing at least 15% charged dissolved species by weight.

Ionization energy
Energy required to remove an electron from a specific atom.

Lines of equal barometric pressure as shown on a weather map.

A line on a weather map connecting points receiving equal sunlight.

Same as a contour depicting vertical height of some surface above a datum plane.

A line on a weather map connecting points receiving equal precipitation amounts.

A compound having the same percentage composition and molecular weight as another compound but differing in chemical or physical properties. Isomers may differ in the manner in which constituent atoms are linked or are arranged in space.

A line on a weather map connecting constant thickness (layer ofatmosphere).

Isoprene (C5H8)
A biogenic hydrocarbon that is emitted from deciduous plants.

A line on a weather map connecting points of equal temperature.

Of equal or constant temperature with respect to either space or time.

Isothermal Atmosphere
An atmosphere in hydrostatic equilibrium in which the temperature is constant with altitude and in which, the pressure decreases exponentially upward.

Isothermal Layer
Any layer where the temperature is constant with altitude, such that the temperature lapse rate is zero. Specifically, the approximately isothermal region of the atmosphere immediately above the tropopause.

Isothermal Process
Any thermodynamic change of state of a system that takes place at constant temperature.

Having the same characteristics in all directions, as with isotropic antennas. Directional or focused antennas are not isotropic.

An atom with the same number of protons as an element but with an excess of neutrons.

Break Line

Joules per Kilogram. These are the units for CAPE and CIN.

Jet Streak
JA concentrated region within the jet stream where the wind speeds arethe strongest. It sets up unique wind currents in its vicinity which either enhance ordiminish the likelihood of clouds and precipitation. It will propagate downstream along the jet stream axis.

Jet Stream
A narrow band of strong winds in the atmosphere that controls the movement of high and low pressure systems and associated fronts. Jet Streams meander from time to time. Wind speeds can reach 200 mph or higher in certain cases. It is usually found at 30,000 to 40,000 feet above the earth's surface. It owes its existence to the large temperature contrast between the polar and equatorial regions. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low-level jet.

Junge distributions
A statistical term that describes the distribution of particles in cloud formation processes. The distribution depends on the total concentration of particles.

Break Line

It is a measure of the thunderstorm potential based on vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer. The temperature difference between 850 mb and 500 mb is used to parameterize the vertical temperature lapse rate. The 850 dew point provides information on the moisture content of the lower atmosphere. The vertical extent of the moist layer is represented by the difference of the 700 mb temperature and 700 mb dew point. This is called the 700 mb temperature-dew point depression. The index is derived arithmetically and does not require a plotted sounding. K-index = (850 mb temperature - 500 mb temperature) + 850 mb dew point - 700 dew point depression The K-index favors non-severe convection, especially heavy rain producing convection. Threshold values vary with season, location, and synoptic situation. The following table shows what various K-indices mean.

K-index values vs. Airmass Thunderstorm Probability East of the Rocky Mountains
K-index value Thunderstorm Probability
Less than 20 None
20 to 25 Isolated thunderstorms
26 to 30 Widely scattered thunderstorms
31 to 35 Scattered thunderstorms
Above 35 Numerous thunderstorms

K-indices are also used to determine the potential of flooding. When your K-index is high (above 35), it means that you will likely see numerous thunderstorms develop. If these thunderstorms track across the same area, you may have a various serious flooding situation on your hands.

Another term for downslope winds.

Kinetic Molecular Theory (Kinetic Theory)
The model that explains gas behavior in terms of particles in random motion whose volumes and interactions are negligible.
Break Line
Lagrangian Time Scale
A measure of how quickly a variable becomes uncorrelated with itself.

Lake Breeze
A wind blowing onshore from the surface of a lake.

Land breeze
A coastal breeze that blows from land to sea, usually at night.

Lapse Rate
The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring. The global average rate of temperature change with height in the atmosphere is 6.5 degrees C/km. The adiabatic lapse rate (or dry adiabatic lapse rate) is the normal rate of change (9.8 degrees C/km) for a dry parcel of air that is moved up or down and cools or warms as the pressure changes. The wet (moist) adiabatic lapse rate (4.9 degrees C/km) is the rate at which satuarted air cools as it ascends.

Latent heat
Energy transferred from the earth's surface to the atmosphere through the evaporation and condensation processes.

A measurement of distance from the equator, with positive values going north and negative values going south.

Local Climatological Data (LCD)
This National Climatic Data Center (NCDC) publication is produced monthly and annually for some 270 United States cities and it's territories. The LCD summarizes temperature, relative humidity, precipitation, cloudiness, wind speed and direction observation. Most monthly publications also contain the 3-hourly weather observations for that month and an hourly summary of precipitation. Annual LCD publications contain a summary of the past calendar year as well as historical averages and extremes.The LCD contains 3-hourly, daily, and monthly values. The annual issue contains the year in review plus normals, means and extremes.

Level of Free Convection (LFC)
It is the height at which a parcel of air lifted dry-adiabatically until saturated (this is the Lifting Condensation Level) and moist-adiabatically thereafter would first become warmer (less dense) than the surrounding air. At this point, the buoyancy of the parcel would accelerate upward without further need for forced lift.

Lifted Index (LI)
It is a stability index used to determine thunderstorm potential. The LI is calculated by taking a representative low level air parcel and lifting it adiabatically to 500 mb. The algebraic difference between this air parcel and the sounding temperature at 500 mb (around 18,000 feet) denotes the LI. Since the LI accounts for moisture below 850 mb, it provides more reliable stability information than the "Showalter Index (SWI)". The greater negative values of LI indicate energy available for parcel ascent. The following table shows what LI values mean:

Lifted Index (LI) versus Thunderstorm Indications
Lifted Index (LI)
Thunderstorm Indication
0 to -2
Marginally Unstable - Thunderstorms possible
-3 to -5
Unstable - thunderstorms probable
less than -5
Very Unstable - heavy to strong thunderstorm potential

Lifting Condensation Level (LCL)
It is the height at which a parcel of air becomes saturated when lifted dry-adiabatically.

Lognormal Distribution
A distribution of frequencies that is symmetric (i.e. bell shaped) when plotted on a logarithmic abscissa.

Lone pairs
Electron pairs that are part of an atom's valence shell but not involved in covalent bonding.

Lowest Achievable Emission Rate (LAER)
This represents the most stringent control technology achieved in practice regardless of cost. LAER is used to determine emission limits for the NSR program.

Lumped Mechanisms
Grouping chemical species by certain charactaristics (such as bonding or reaction rate), in order to simplify the determination of mechanisms and reaction rates

Break Line

Large scale, characteristic of weather systems several hundred to several thousand kilometers in diameter.

Main group
Shorthand is used to refer to a large grouping of elements that all have easily predicted properties. Main group elements include all of the elements in Groups 1 and 2, Groups 13 through 18, and the element hydrogen.

An instrument used for measuring the pressure of liquids and gases. An open-tube manometer consists of a U-shaped tube that usually contains a liquid such as mercury or water. One end of the tube is open to the atmosphere and the other end of the tube is connected to the container where the pressure is to be measured.

The measure of the magnitude of a physical object that is related directly to the atoms in the object.

Mass Emission Rate
See Mass Flow Rate.

Mass Flow Rate
The quantity of mass per unit time moving through a process and/or air pollution control system.

Mass Number (A)
The number of protons and neutrons in an atom.

Mass Transfer
A natural phenomenon in which a component travels from a region of high concentration to one of low concentration in order to minimize concentration differences in a mixture.

Maximum Achievable Control Technology (MACT)
EPA standards mandated by the 1990 CAAA for the control of toxic emissions from various industries.

The arithmetic average of a distribution.

Mechanical Turbulence
Any turbulence caused by obstructions such as terrain, buildings, mountains, etc.

The middle value in a distribution above and below which lie an equal number of values.

Medium Range Forecast (MRF) model
One of the operational forecast models run at NCEP. The MRF is one of the main models forecasters use for the medium range time period beyond 48 hours into the future. It is run twice daily (0000 UTC and 1200 UTC). The MRF model forecasts for the entire northern hemisphere, unlike the national models, which only forecast for North America. The resolution of the MRF model is about 150 km, which is far less than the national models. The MRF is primarily used for the medium range time period from 60 to 240 hours (10 days) into the future. The MRF, like the previous models, has its own set of Model Output Statistics (MOS) equations known as MRF MOS.

The study of weather and all of its effects.

Methane (CH4)
One of the most abundent VOC’s in the atmosphere, methane acts not only as a precurser to ozone, but also as a “greenhouse gas” which traps heat at the earths surface.

Method 9
EPA reference method whereby qualified observers determine the opacity of plume-shaped visible emissions from stationary sources.

Method 22
EPA reference method whereby qualified observers determine the opacity of visible emissions not emitted directly from a stationary source.

A unit of measurement for mass. One microgram equals 10-6 grams.

The layer of the planetary boundary layer closest to the surface of the Earth.

The study of boundary layer meteorology

Mid-latitude Cyclones
Another name for extratropical cyclones

Millibar (mb)
Unit of atmospheric pressure. It is equal to 0.03 inches of mercury. One thousand millibars equals 29.55 inches of mercury on a barometer.

A unit of measurement for mass. One milligram equals 10-3 grams.

Mixed Layer
The unstable atmospheric layer that extends from the surface up to the base of an inversion. Within this layer, the air is well-mixed.

Mixing Height
The vertical extent of the mixing layer. (also known as the Mixing Depth)

Mobile Monitoring
Emissions monitoring from a movable monitoring station, this type of monitoring is used for measuring “hot spots” and taking measurements in between stations.

The value that occurs the most frequently in a distribution.

Model Output Satistics (MOS)
The Hydrometeorological Center of the National Environmental Prediction Centers (formerly National Meteorological Center) produces a short range (6 to 60 hours) MOS (Model Output Statistics) guidance package generated from the NGM (Nested Grid Model) for over 300 individual stations in the continental United States. These alphanumeric messages are made available at approximately 0400 and 1600 UTC for the 0000 and 1200 UTC forecast cycles, respectively. Model Output Statistics are a set of statistical equations that use model output to forecast the probability of precipitation, high and low temperature, cloud cover, and precipitation amount for many cities across the USA. The statistical equations were specifically tailored for each location, taking into account factors such as each location's climate. To indicate snow and precipitation type forecasts, the message varies between the cold (September 16 through May 15) and warm (May 16 through September 15) seasons. Snow and precipitation type forecasts are never issued for certain Florida and California stations.

Moist Adiabatic Lapse Rate
The rate of change of temperature in a rising or descending parcel of saturated air. The rate of cooling or warming varies but a common value of 6 degC per km (3.3 degF/1000ft) is used.

Moist Adiabatic Process
The process by which temperature changes in a saturated parcel of air.

Moist Adiabats
They show how the air temperature would change inside a rising parcel of saturated air.

Moist Advection
Transfer of moisture by horizontal winds.

Molar Flow Rate
The number of moles per unit time moving through a process and/or air pollution control system.

Molar Mass
The mass of one mole of entities (atoms, molecules, or formula units) of a substance, in units of g/mol.

The concentration of a solution given in gram moles of solute per liter of solution. A solution containing exactly one-gram mole of solute per liter of solution is referred to as "molar."

The mass of a material equal to the combined mass of 6.023 1023 atoms or molecules of the material.

Mole Fraction
An expression of the number of moles of a compound divided by the total number of moles of all compounds present.

Molecular formula
Shows the number of atoms of each element present in a molecule as a subscript to the symbol of the element.

Molecular Viscosity
The small-scale internal fluid friction taht is due to the random motion of the molecules within a smooth-flowing fluid, such as air.

The general chemical term for two or more atoms that are bonded together.

The product of the mass of an object times its velocity.

Monin_Obukhov Length
A rough measure of the height at which turbulence is generated more by buoyancy than by wind shear. In the daytime over land, L is typically between 1 to 50 meters.

Mountain Breeze
A local wind system of a mountain in which cooling air descends down the side of a mountain,typically at night.

Break Line

NOx refes to an indeterminate mix of Nitrogen Oxide and Nitrogen Dioxide. NOx contribute to the formation of ozone and are a component of photochemical smog.

NOy encompasses the general class of reactive nitrogen species which includes NOx, as well as PAN, HNO3 and others. NOy often serve as reservoir species for NOx’s.

A unit of measurement for mass. One nanogram equals 10-9 grams.

National Ambient Air Quality Standards (NAAQS)
The National Ambient Air Quality Standards are primary standards and secondary standards for pollution emissions established by the Clean Air Act Ammendments. non-point source pollution- pollution which comes from spatially undefined source (ex. automobiles, decomposition of organic matter)

National Weather Service (NWS)
The Federal agency that provides a wide variety of meteorological products and services to meteorologists and the general public.

Natural Logarithm (Napierian)
The exponent applied to the base number of 2.718 to equal a given value. For example, 2 is the natural logarithm that is equal to a value of 7.389. (2.718)2 = 7.389

National Centers for Environmental Predition (NCEP)
The National Oceanic and Atmospheric Administration created the National Centers for Environmental Prediction (NCEP) to take advantage of improving technology and better serve the public and modernized National Weather Service. The NCEP's goal is to protect life and property, as well as mitigate economic loss, by providing accurate forecasts and forecast guidance products to weather service field offices. The NCEP prepares and makes available national forecasts and outlooks of weather and climate. Meteorologists currently generate weather forecasts for three to five days, soon to extend to seven days. Climate predictions are made for two weeks out to a year.

National Oceanographic and Atmospheric Administration (NOAA)
NOAA warns of dangerous weather, charts our seas and skies, guides our use and protection of ocean and coastal resources, and conducts research to improve our understanding and stewardship of the environment which sustains us all. NOAA is an organization of the Department of Commerce. NOAA is composed of the National Ocean Service, National Weather Service, National Marine Fisheries Service, National Environmental Satellite Data, and Information Service, and Office of Oceanic and Atmospheric Research.

Negative Pressure
A relative pressure that is below atmospheric pressure.

Nested grid
A modeling technique used to define the modeling area. Nested grids have smaller, higher resolution grids inside of (nested) larger, less higher resolution grids.

Nested Grid Model (NGM)
A well-known (but soon to be phased-out) numerical weather prediction (NWP) model that uses a nested-grid scheme as the foundation of its domain and resolution practice.

Net Ionic Equation
An equation in which spaectator ions have been eliminated so that the actual chemical change is seen.

Neutralization (Neutralization Reaction)
The reaction of an acid and a base. In classical terms, the products are a salt and water; in Bronsted-Lowry terms, the products are a conjugate base and acid; in Lewis terms, the product is an adduct with a new covalent bond.

A particle found in the nucleus of an atom. It is almost identical in mass to a proton, but carries no electric charge.

Nested Grid Model (NGM)
One of the operational forecast models run at NCEP. The NGM is run twice daily, with forecast output out to 48 hours. The NGM is a short range model that forecasts variables such as temperature at various levels of the atmosphere, amount of precipitation, position of upper level troughs and ridges, and the position of surface high and low pressure areas. In the nested grid model and thers like it, the atmosphere is divided into squares, or a grid, for various levels of the atmosphere. Grids with smaller squares are "nested" inside larger ones for areas forecasters are particularly interested in, such as North America. The smaller the grids, the more calculations that have to be made and the bigger the computer needed. The resolution of the NGM is about 80 kilometers. The NGM produces forecasts every 6 hours from 0 hours to 48 hours into the future.

An acronym that stands for NEXt generation of weather RADar. It is a technologically-advanced weather radar deployed to replace WSR-57 and WSR-74 units. NEXRAD is a high-resolution Doppler radar with increased emphasis on automation, including use of algorithms and automated volume scans. NEXRAD units are known as WSR-88D.

Any nitrogen compound that contains three or more oxygen atoms.

Nitrogen Oxides
The family of chemicals that contain one nitrogren atom and some number of oxygen atom. Nitrogen oxides are an important class of primary pollutants.

The legal definition of a community that has not met the National Ambient Air Quality Standards (NAAQS) for one or more pollutants.

Non-Attainment Areas
A geographic area that does not meet one or more of the federal air quality standards (NAAQS) for the criteria pollutants.

Non-polar covalent bonds
Covalent bonds between atoms of such similar electronegativity that the bonding pair is shared equally.

The concentration of a solution expressed as the number of gram equivalent weights of solute per liter of solution. A solution containing one gram equivalent weight of solute per liter of solution is referred to as "normal."

To form a central part around which other parts can attach.

A particle of the atomic nucleus, a proton or a neutron.

Null Cycle
The primary cycle which governs the production and destruction of ozone.

Number of Gram Equivalent Weights
The number calculated by dividing the mass of a substance (in grams) by the equivalent weight of the substance.

Numerical instability
A condition of numerical models in which a numerical solution to an equation diverges (moves away) from an exact solution (due to a variety of error accumulation), resulting in the generation of unrealistic values.

Numerical method
As pertains to computational science, a general type of mathematical "recipe" that is used to solve mathematical equations that cannot be solved in closed form, or analytically. Numerical methods use a variety of approximations to compute non-closed form equations.

Numerical Weather Prediction (NWP)
The use of mathematical representations of the physical behaviors of the atmosphere, solved by high-performance computers (typically supercomputers).

Break Line

Occluded Front (Occlusion)
A complex frontal system that ideally forms when a cold front overtakes a warm front. When the air is colder than the air ahead of it, the front is called a Cold Occlusion. When the air behind the front is milder than the air ahead of it, it is called a Warm Occlusion. These processes lead to the dissipation of the front in which there is no gradient in temperature and moisture.

Ohm's Law
I = E / R, where I is current (amperes), E is electromotive force (i.e., voltage) and R is resistance (ohms)

Onshore Breeze
A breeze that blows from the water onto the land. Opposite of an offshore breeze.

An energy state in the atomic model which describes where an electron will likely be.

The Y-axis on a graph.

A term that applies to compounds that contain carbon bonded to hydrogen and a number of other elements including oxygen, nitrogen, sulfur and halogens.

Organic chemistry
The study of carbon compounds

Related to, or caused by, physical geography 9such as mountains or sloping terrain.

Orgographic Lifting (Upslope Flow)
Occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.

Orgographic Precipitation
Precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture.

Orographic Uplifting
The lifting of a parcel of air or layer of air as a result of topography, such as mountain slope.

Oxidation-reduction Reaction (Redox Reaction)
A process in which electrons are transferred from one reactant (reducing agent) to another (oxidizing agent).

A nearly colorless (but faintly blue) gaseous form of oxygen, with a characteristic odor like that of weak chlorine. Its chemical formula is O3. It is usually found in trace amounts in the atmosphere, but it is primarily found at 30,000 to 150,000 feet above the ground. Its production results from photochemical process involving ultraviolet radiation. Because it absorbs harmful radiation at those heights, it is a very beneficial gas. However, photochemical processes involving industrial/vehicle emissions can produce ozone near the ground. In this case, it can be harmful to people with respiratory or heart problems.

Ozone Action Day
A "heads-up message issued by the Department of Natural Resources (DNR) through the National Weather Service when ozone levels may reach dangerous levels the next day. This message encourages residents to prevent air pollution by postponing the use of lawn mowing, motor vehicles, boats, as well as filling their vehicle gas tanks.

Ozone Advisory
It is issued by the Department of Natural Resorces (DNR) through the National Weather Service when ozone levels reach 100. Ozone levels above 100 are unhealthy for people with heat and/ or respiratory ailments.

Ozone Season
The ozone season from May 1 to September 30 (Northern hemisphere) when ground-level ozone formation concentrations tend to be highest.

Break Line

PAN (Peroxyacyl Nitrates)
PAN’s are one of the main components in photochemical smog and are of the general structure RC(O)OONO2. The main formation reaction for PAN comes from reaction between aldehydes, NOx, and the OH radical.

Partial Pressure
The fraction of the total static pressure in a gas stream caused by one of the constituents of the mixture.

Parallel processor
An computer platform (hardware, or architecture) that contains more than one CPU, or processor.

Particulate matter
Any solid particles and liquid droplets that are small enough to remain suspended in air. PM-10 is the term that refers to particulate matter that has a diameter of 10 micrometers.

The ratio of the amount of pollutants leaving an air pollution control device versus the amount of pollutants entering the device.

Periodic Table
Grouping of the known elements by the number of protons in the atom of the element.

Horizontal rows of the periodic table.

Permanent Gases
Any one of the gases that exist in a constant ratio to other gases in the atmosphere near the surface. The permanent gases are: nitrogen (N2), oxygen (O2), argon (Ar), neon (Ne), helium (He), hydrogen (H2), and xenon (Xe)

The negative logarithm of [H3O+]; pH = -log[H+] = -log[H3O+].

Photochemical Reaction
A chemical change triggered by the radiant energy of the sun or other light source.

Photochemical Smog
Photochemical smog arises from the combination of ozone, VOC’s and Nox’s found primarily in urban areas. Smog has a number of deleterious effects of plant and animal life, including damage to respiratory function, oxidation of foliage, and many other problems.

A chemical process in which molecules are broken down into smaller units through the absorption of light.

Physical Attrition
The act of wearing or grinding down by friction.

Planetary Boundary Layer
The layer of the atmosphere closest to the surface of the Earth, roughly 1 to 2 km in height above the surface. The study of the PBL is known as micrometeorology.

EPA defines PM2.5 as particulate matter with a diameter of 2.5 micrometers collected with 50% efficiency by a PM2.5 sampling collection device. However, for convenience in this reference material, the term PM2.5 includes all particles having an aerodynamic diameter of less than or equal to 2.5 micrometers.

The U.S. EPA defines PM10 as particulate matter with a diameter of 10 micrometers collected with 50% efficiency by a PM10 sampling collection device. However, for convenience in this reference material, the term PM10 includes all particles having an aerodynamic diameter of less than or equal to 10 micrometers.

Point Source Pollution
Pollution which comes from a well-defined space (ex. factory smokestack).

Poisson's Equation
this equation allows one to calculate changes in temperature as air rises up through the atmosphere or subsides downward under the conditions where there is no heat gain, e.g from solar radiation, or heat loss, e.g. from terrestrial radiation

Polar covalent bonds
Covalent bonds in which the electron pair is shared unequally, so the bond has partially negative and partially positive poles.

Polar Jet Stream
the jet stream situated about 10 km (33,000 ft) near the polar front, also known as the polar front jet stream. Found near the tropopause, this is also sometimes known as the tropopause jet.

Polar molecules
Molecules with a difference in electron density across a molecule

Any substance (solid, liquid, or gas) that contaminates the atmosphere and has the potential to produce adverse health effects on humans and other animals, damage plant life, or cause damage to physical structures.

Polyatomic ion
An ion that consists of two or more atoms are bonded covalently.

A set of measurements that accounts for every data value in a pre-defined group

Potential Temperature
The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.

Potential Temperature Gradient
the difference between to potential temperatures, which is the temperature an air parcel would have if it were movide dry adiabatically to a pressure of 1000 mb.

The total emissions that a facility would release by operating at maximum load for 24 hours per day and 365 days per year.

PPMV [or PPM(V/V)]
The part per million concentration that is determined by comparing the volume of one constituent with the total volume of the substance. Gas concentrations are always expressed in a ppm(v/v) format as opposed to the ppm(w/w) format often used for liquids. Throughout APTI courses, the term ppm when applied to gases means ppm(v/v).

The part per million concentration that is determined by comparing the volume of one constituent with the volume of the other constituents with the exception of moisture.

The part per million concentration that is determined by comparing the mass of one constituent with the total mass of the sample. Liquid concentrations are often expressed in a ppm(w/w) format as opposed to the ppm(v/v) format used for gases. Throughout APTI courses, the term ppm when applied to liquids means ppm(w/w). Note that the abbreviation "w/w" is used despite the fact that the ppm concentration is based on a ratio of masses.

Precipitable Water (PW)
It measures the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column usually extending from the surface to 300 mb.

any form of water particles, liquid or solid, that falls from the atmosphere and reaches the ground.

Precipitation Reactions
Reactions in which two soluble ionic compounds form an insoluble product, a precipitate.

Predominant Wind
It is the wind that in the forecasters judgment generates (or is expected to generate) the local component of the significant sea conditions across the forecast area.

The force exerted by the weight of the atmosphere, also known as atmospheric pressure. When measured on a barometer, it is referred to as barometric pressure and it is expressed in inches of mercury, millibars, or kiloPascals.

Pressure Gradient
The amount of pressure change occurring over a given distance.

Pressure Gradient Force
A three-dimensional force vector operating in the atmosphere that accelerates air parcels away from regions of high pressure and toward regions of low pressure in response to an air pressure gradient. Usually resolved into vertical and horizontal components.

Primary Pollutants
Primary pollutants are considered to be pollutants which are emitted directly into the atmosphere (rather than arising from reaction in the atmosphere).

Prime Meridian
The line that divides the earth into the Western and Eastern Hemispheres, running through Greenwich, England

Probability distribution diagram (electron density diagram)
The pictorial representation for a given energy sublevel of the quantity ,the probability that the electron lies within a particular tiny volume, as a function of r, the distance from the nucleus.

A substance formed in a chemical reaction.

To put a law into effect by formal public announcement.

Particle found in a nucleus of an atom with a positive charge. The number of protons gives atomic number.

change in a parcel of air as a result of loss of latent heat energy

Pseudoadiabatic Lapse Rate
synomymous with moist adiabatic lapse rate (MALR)

Break Line

Qualitative Techniques
Measuring techniques which measure the type of species present in a sample.

Quantitative Techniques
Measuring techniques which measure the amount of species present in a sample.

Describes a low or high pressure area or a font that is nearly stationary.
Break Line
Rankine is an absolute temperature scale often used in engineering. Using this scale, the freezing point of water is 492°R and the boiling point of water is 672°R.

Energy propagated in the form of electromagnetic waves. These waves do not need molecules to propagate them and in a vacuum they travel at nearly 300,000 km per sec (186,000 mi per sec)

Radiation Fog
Fog produced results from the air near the ground being cooled to saturation by contact with the cold ground. The cooling of the ground results from night time loss of heat from the Earth to space (terrestrial radiation). Favorable conditions for radiation fog are clear sky, little or no wind, and high relative humidity. It occurs in stable air and is primarily a night time or early morning phenomenon. As the Earth and the lower layers of the atmosphere warm during the day, air that was stable during the early morning hours may become unstable--at least in the lower levels. For this reason visibility usually improves as the temperature rises during the day. Mixing in the lower levels disperses the fog into a thicker layer, and eventually it evaporates into the warmer air. When cloud layers form aloft over a radiation fog and retard heating from the sun, visibility improvement is very slow. It is also known as Ground Fog and Valley Fog.

Radiation Inversion
It is thermally produced, surface-based inversion formed by rapid radiational cooling of the Earth's surface at night. It does not usually extend above the lower few hundred feet. Conditions which are favorable for this type of inversion are: long nights, clear skies, dry air, little or no wind, and cold or snow covered surface. It is also called a Noturnal Inversion.

Radiative Cooling
the process by which the earth's surface and adjacent air cool by emitting infrared radiation

Radical Species
A radical species is a chemical species with an unpaired electron in the outer (valence) shell.

A balloon-borne intrument that measures and transmits pressure, temperature, and humidity to a ground-based receiving station. These were originally tracked by theodolites, but they are now tracked by the Automatic Radiotheodolite Master Control Unit (ART).

Precipitaion, either in the form of drops larger than 0.02 inch(0.5 mm), or smaller drops, which in contrast to drizzle, are widely separated.

Rapid Update Cycle (RUC)
A numerical model run at NCEP that focuses on short-term (up to 12 h) forecasts and small-scale (mesoscale) weather features. Forecastes are prepared every 3 hours for the contiguous United States.

Rate Constant (k)
The proportionality constant that relates the rate of a reaction to the concentrations of components.

Rate Determining Step
The elementary reaction in a mechanism that is so much slower than the rest that it governs the rate of the overall reaction.

Rate Law (Rate Equation)
An equation that expresses the rate as a function of reactant concentrations.

a radiosonde that also is used to track wind speeds by virtue of computer calculations of height and distance

Rawinsonde Observation
A radiosonde observaton which includes wind data.

A starting substance in a chemical reaction.

Reaction Mechanism
A series of elementary reactions that is proposed to account for the rate law of an overall reaction.

Reaction Rate Constant
The proportionality constant found in the rate equation, the mathematical relationship between the reactant concentration and the reaction rate.

Regional Acid Deposition Model (RADM)
A reaction mechanism model which is generally used in urban and regional air pollution modeling and bases its classifications of the reactivity rate constants of different species

Reflected Radiation
radiation that is turned back into the atmosphere, usually by the surface

Relative humidity
A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present.

Relative Pressure
The difference in pressure between a point in a ventilation system or air pollution control system and the atmospheric pressure.

Relative Temperature
The difference between the measured temperature value and an arbitrarily selected value, such as the freezing point of water.

Remote Sensing
A monitoring system in which the sample is removed from the monitoring device, an example is the use of sattellites to monitor aerosol levels.

Residence Time
the duration a pollutant resides or is suspended in the atmosphere

Residual Layer
the layer in the planetary boundary layer that is formed between sunset and sunrise.

Reynolds Number (Gas)
A dimensionless number corresponding to the ratio of the fluid inertial force to the fluid viscous force in a flow system. It is used as an index for turbulence.

Reynolds Number (Particle)
A dimensionless number corresponding to the ratio of the inertial force of a particle to the viscous force of the surrounding fluid in a flow system. It is used as an index for turbulence.

turbulent eddies that form downwind of a mountain chain, creating hazardous flying conditions

any movement of water as a result of gravity. Runoff typically involves drainage of water from a high elevation down to a lower elevation body of water, such as a lake or a river

Break Line

A set of measurements that are collected to create a subset of data that represents the actual population

Santa Ana-Foehn Wind
A warm, dry wind that blows into southern California from the east off the elevated desert plateau. Its warmth is derived from compressional heating. Over the Rocky Mountains, this wind is called a Chinook wind; over the Alps, it is known as a Foehn wind.

A reaction mechanism model developed for use in urban and regional air pollution modeling which bases its classifications of reaction rates as well as the “base mechanism” of the reaction


Saturated hydrocarbon
A hydrocarbon in which each C is bonded to four other atoms.

Lifted Index (LI) calculated using surfaced based parcel.

Standard Cubic Feet per Minute (SCFM)
The gas flow rate at standard conditions expressed in the American Engineering system of units.

Scattered Radiation
any radiation that is caused by small particles in the atmosphere deflecting radiation from its path in a different direction.

Scientific Visualization
The technique by which raw numerical data from a numerical simulation is "rendered" into a two- or three-dimensional still or animated image. Scientfic visualization is an integral part of the overall computational science discipline, and takes advantage of new mathematical techniques and improved graphics capabilities of modern computers.

Sea Breeze
a coastal local wind that blows from the ocean onto the land.

Sea Breeze Front
the leading edge of a sea breeze.

Sea Level Pressure
The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.

Secondary pollutant
Any pollutant that is formed in the atmosphere from chemical transformation, such as the formation of ozone through the photochemical transformation of nitrogen oxides and volatile organic compounds.

Sedimentation (Gravitational Settling)
The removal of particles whose diameter is between one and ten micrometers by the force of gravity.

Sensible Energy
A measure of the change in internal energy experienced by a substance during a temperature change that does not involve a phase change.

Sensible Heat
The excess radiative energy that has passed from the Earth's surface to the atmosphere through advection, conduction, and convection processes.

Sigma level
Another term for the vertical layer in the atmosphere. Many models, such as the Nested Grid Model (NGM), divide the atmosphere into some number of vertical layers, otherwise known as sigma levels.

A reaction where two or more reactants combine to form two or more compunds.

SI Unit
A unit composed of one or more of the base units of the Systeme International d'Unites, a revised metric system.

Slice Method
a method of determining atmospheric stability that compensates for the effects of the non-static environment. The slice method looks at a slice of the atmosphere with several saturated parcels rising through it. This method assumes that for every unit of mass being lifted, a unit of mass of environmental air is sinking.

Solar Radiation
any heating of the atmosphere through solar energy.

Solar Zenith Angle
The angle of the sun with respect to the Earth, a varying value dependent on the latitude.

The substance distributed (i.e. dissolved or suspended) in a solvent.

A homogeneous mixture of substances dispersed at the molecular level and present in a single phase.

A substance (usually a liquid) that dissolves a solute to form a solution.

A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature, etc.

Source Regions
Regions where air masses originate and acquire their properties of temperature and moisture.

Specific Gravity
A ratio of the density of a liquid and the density of pure water at a specific temperature.

Specific Heat
A ratio of the density of a liquid and the density of pure water at a specific temperature.

Spectator Ion
An ion that is present as part of the reactant but not involved in the chemaical change.

Stable Layer
one of the layers in the planetary boundary layer, also known as the nocturnal boundary layer

Standard Conditions (EPA-defined)
EPA-defined standard conditions of temperature and pressure are 68°F (20°C) and 14.7 psia (760 mm Hg).

Standard Deviation
A measure of the dispersion of a set of numbers.

Standard Oxygen Level
The specific oxygen level selected by a regulatory agency to account for dilution of the gas stream by ambient air.

Standard Temperature and Pressure (STP)
The reference conditions for a gas: 0 oC (273.15 K) and 1 atm (760 torr)

State Implementation Plan (SIP)
A complex and voluminous document that contains comprehensive emission inventories, proposed control strategies, demonstration of modeling/calculation results, summaries of regulatory authority, description of monitoring programs, and enforcement procedures.

Stationary front
A front that is nearly stationary with winds blowing almost parallel and from opposite directions on each side of the front.

Stationary Monitoring
Emissions monitoring from a stationary point, monitoring stations are set in various locations based on emissions, climatological data, population, and diffusion models.

From the Latin word meaning "status" or "state." Provides a body of principles and methodology for designing the process of data collection, summarizing and interpreting the data and drawing conclusions or generalities.

Stiff Ordinary Differential Equations
Stiff ODEs are systems of ordinary differential equations that involve different magnitudes that make solution mathematically difficult. For example, image a system of two differential equations. If one of the ODEs deals with time steps in microseconds, and the other deals with time steps in years, these equations might be stiff, since there is such a discrepancy in the size of the time steps.

The study of the mass-mole relationships of chemical formulas and reactions.

The atmospheric layer just above the troposphere, which starts at approximately 7.5 miles (12 km) above the Earth and rises to approximately 31.1 miles (50 km). The beneficial ozone layer resides in the stratosphere.

Street Canyon Effects
High, gusty winds that flow between buildings. These winds are quite complex in speed and structure.

Structural formula
A formula that shows the actual number of atoms, their sequence, and the bonds between them.

1) The slow sinking of air usually associated with high pressure areas. It is usually over a broad area. 2) Sinking down of part of the earth's crust due to underground excavation, such as the removal of groundwater.

Subsidence Inversion
It is produced by adiabatic heating of air as it sinks and is associated with anticyclones (high pressure) and/or stable air masses. These inversions form between sinking heated air and air below and they are characterized by temperature increase with height through the inversion, while above the inversion, the temperature cools almost dry adiabatically. The dew point temperature, relative humidity, and mixing ratio values all decrease with height through the inversion.

Subtropical Jet Stream
The jet stream typically found between 20 degrees and 30 degrees latitude at altitudes between 12 and 14 km.

Inorganic salts of sulfuric acid (H2SO4) containing the divalent, negative ion .

Inorganic salts of sulfurous acid (H2SO3) containing the divalent, negative ion .

Sulfur Dioxide (SO2)
A gas that accounts from approxamitely 95% of sulfur emissions in the United States, sulfur dioxide can contribute both to acid rain, and act as an irritant of breathing problems and respiratory illness.

Sulfur Oxides
The family of chemical compounds that contain one sulfur atom (S) and some number of oxygen atoms. These chemicals are formed from a variety of industrial processes, such as paper and pulp mills.

Surface Layer
The layer in the planetary boundary layer closest to the surface, with typically turbulent conditions due to the frictional forces of the surface.

Surface Roughness

Surface Winds
A general term for any wind near the surface, most often used in discussions of air pollutant disperion processes.

Synoptic Scale (or Large Scale)
The typical weather map scale that shows features such as high and low pressure areas and fronts over a distance spanning a continent. Also called a cyclonic scale

T Break Line

Temperature Inversion
An increase in air temperature with height, often simply called an inversion.

A general term for land shapes and forms, such as hills, valleys, mountains, bodies of water, etc.

A small, rising parcel of warm air produced when the earth's surface is heated unevenly.

Thermal high
Areas of high pressure that are shallow in vertical extent and are produced primarily by cold surface temperatures.

Thermal Low
Areas of low pressure that are shallow in vertical extent and are produced primarily by warm surface temperatures.

Thermal NOx
Nitrogen oxides generated from atmospheric nitrogen during combustion.

Thermal Turbulence
Turbulent vertical motions that result from surface heating and the subsequent rising and sinking of air.

Thermal Wind
Analagous to the geostrophic wind, a theoretical wind that blows parallel to the thickness lines for the layer considered. The closer the thickness isopleths, the stronger the thermal winds.

Theta-e (or Equivalent Potential Temperature)
The temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, itis useful in diagnosing atmospheric instability.

TKE (Turbulent Kinetic Energy)
An overall measure of the intensity of the turbulence of the atmosphere.

Total Totals (TT)
This index estimates the potential for severe convection. It combines the effects of vertical temperature lapse rate (Vertical Totals or VT) and low level moisture (Cross Totals or CT) in a given environment. The following formulas are used to create the Total Totals index:

Vertical Totals (VT) = 850 mb temperature - 500 mb temperature

Cross Totals (CT) =850 mb dew point - 500 mb temperature

When the two are combined, you have the following formula:

Total Totals (TT) = Vertical Totals (VT) + Cross Totals (CT)

The following table shows what these relationships typically mean east of the Rockies:

Cross Totals
Vertical Totals
Total Totals
26 or more
Isolated or few thunderstorms
26 or more
Scattered thunderstorms
26 or more
Scattered thunderstorms, isolated severe
26 or more
Scattered thunderstorms, few severe, isolated tornadoes
26 or more
Scattered numerous thunderstorms, few to scattered severe, few tornadoes
26 or more
Numerous thunderstorms, scattered severe, scattered tornadoes

High lapse rates or a source of low level moisture will yield large values of TT. However, high lapse rates can produce large TT, with little supporting low level moisture. The sounding must be examined carefully to ascertain the validity of the TT for a given environment. Also as with any index, you must carefully examine your environment.

When the particle or particles ejected are protons, the isotope becomes a different element with a smaller number of protons.

Water discharged into the atmosphere from plant surfaces.

A general term meaning the movement of some substance from one place to another, frequently used in discussions

Transport Wind
The average wind over a specified period of time within a mixed layer near the surface of the earth.

Treatment Time (or Residence Time)
The length of time that pollutants reside in the air pollution control system. The treatment time is calculated by dividing the volume of the air pollution control device by the gas flow rate.

Tropical Cyclones
A warm-core low pressure system which is non-frontal. It originates over tropical and subtropical waters and has an organized clyclonic (counter-clockwise) surface wind circulation.

the upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height). It is also the boundary between the troposphere and the stratosphere.

Refering to anything that occurs in the troposphere.

The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers), vertical motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).

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Universal Gas Constant (R)
A proportinality constant relating the energy, amount of substance, and temperature of a system; R = 0.08206 atm L/mol K = 8.314 J/mol K.

Unsaturated hydrocarbons
A hydrocarbon with a carbon multiple bond; one in which C is bonded to fewer than four atoms.

An atmospheric state warm air below cold air. Since warm air naturally rises above cold air (due to warm air being less dense than cold air), vertical movement and mixing of air layers can occur.

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Valence electrons
The electrons in the outermost shell of an atom.

Van der Waal forces
Weak attraction forces (1 to 10 Kcal/gm-mole) between molecules such as in gases and liquids.

Valley Breezes
An uphill movement of air as it warms during the day and moves up the side of a mountain from a valley.

Vapor Pressure
In a closed system at a constant temperature the pressure exerted by gaseous molecules that are in equilibrium with molecules of the same kind in the liquid or solid state.

A measure of variability.

Velocity Pressure
The pressure required to accelerate air from zero velocity to a greater velocity. It is proportional to the kinetic energy of the air stream.

Vertical Temperature Gradient
The change in temperature as one goes higher into the atmosphere. Vertical temperature gradients are often used as meteorological predictors.

Vertical Wind Shear
The rate of change of wind speed or wind direction over a given distance vertically.

Any hydrometeor that evaporates before it reaches the surface of the Earth.

Virtual Temperature
A temperature tht includes the effects of water vapor on density, defined by Tv=T*(1+0.61*r), where r is the water-vapor mixing ratio.

The greatest distance an observer can see and identify prominent objects.

Volatile Organic Compounds (VOC)
Volatile Organic Compounds (VOC) are hydrocarbons which contribute to the formation of ozone and are a component of photochemical smog. VOC's can be emitted from both biogenic and anthropogenic sources.

Volume Percent
Percentage of the total volume of a gas sample that is comprised of the volume of a single gaseous constituent.

Break Line

A turbulent wind that forms downwind of an obstruction, such as a building or a mountain.

Warm Front
A front that moves in such a way that warm air replaces cold air.

Wet Deposition
Wet deposition is the removal of pollutants through rain or other forms of precipitation.

The horizontal motion of the air past a given point. Winds begin with differences in air pressures. pressure that's higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the "pressure gradient force." High and low pressure are relative. There's no set number that divides high and low pressure. Wind is used to describe the prevailing direction from which the wind is blowing with the speed given usually in miles per hour or knots. The following table gives descriptions of winds used in National Weather Service forecasts.

Sustained Wind Speed
Descriptive Term
0-5 mph
Light or light and variable wind
5-15 mph or 10-20 mph
15-25 mph
Breezy, Brisk, Blustery
20-30 mph
30-40 mph
Very Windy
40 mph or greater
Strong, dangerous, or damaging

Wind Chill
It is based on the rate of heat loss from exposed skin caused by the combined effect of wind and cold. As the wind increases, heat is carried away from the body at an accelerated rate, driving down the body temperature. Animals are also affected by wind chill. The term wind chill was coined by Paul Siple. He was a pioneer in determining the relationship between heat loss to wind and temperature. During the 2nd Byrd Expedition to Antarctica in 1939-40, siple exposed water-filled plastic cylinders to various temperatures and wind speeds. He then recorded the time that it took for the water to freeze in the cylinders. With this data and the assistance of his colleague charles Passel, he developed the following formula for calcuating wind chill:

Wind Chill = 0.0817 (3.71 v0.5 + 5.81 - 0.25v)(t - 91.4) + 91.4

where: t = air temperature in degrees Fahrenheit (oF) v = wind speed in miles per hour from this formula, the following wind chill table was developed by the National Weather Service:

Wind Chill Chart
Wind Speed
Temperature (oF)
Very Cold
Bitter Cold
Extreme Cold

In order to determine the wind chill using the above chart, one must know the wind speed and temperature. For example, if your temperature is 10 oF and your wind speed was 25 mph. One would follow down the 10 oF column until it intersects with the 25 mph row. Doing this, one gets a -29 oF Wind Chill.

Wind Speed
The rate at which air is moving horizontally past a given point. It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, wind gust, or squall). The following table is a way of estimating wind speed:

Beaufort Wind Speed Estimations on Land and Water
Visual Appearance
Visual Appearance
Calm: smoke rises vertically; sea like a mirror
Strong Breeze: Large branches in motion; whistling heard in overhead wires; umbrellas used with difficulty; large waves begin to form; the white foam crests are more extensive everywhere (probably some spray).
Light Air: Direction of wind shown by smoke drift not by wind vanes. Ripples resembling scales are formed on water, but without foam crests.
Near Gale: Whole trees in motion; inconveniences felt against wind; sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.
Light Breeze: Wind felt on face; leaves rustle; wind vanes moved by wind; small wavelets form on water, still short, but more pronounced; crests have glassy appearance.
Gale: Breaks twigs off trees; impedes progress; moderately high waves of greater length; edges of crests begin to break into spindrift; foam on water surface is blown in well-marked streaks along wind.
Gentle Breeze: Leaves and small twigs in constant motion; wind extends light flag; large wavelets on water; wave crests begin to break; water has a foam of glassy appearance; scattered whitecaps appear on lakes.
Strong Gale: Slight structural damage occurs; high waves; dense streaks of foamon water surface along wind; crest of waves begin to roll over; spray may affect visibility.
Moderate Breeze: Raises dust, loose paper; small branches moved; small waves, becoming longer; fairly frequent white caps appear on lakes.
Storm: Trees uprooted; considerable damage occurs; very high waves with long overhanging crests; foam on water, in great patches, is blown in dense white streaks along wind; sea takes on a white appearance; tumbling of sea becomes heavy and shocklike; visibility affected over water.
Fresh Breeze: Small trees in leaf begin to sway; crested wavelets from on inland waters; moderate waves, taking a more pronounced long form; many whitecaps appear on lakes.
Violent Storm: Widespread damage; exceptionally high waves (small and medium-sized ships might be for a time lost to view behind waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into froth; visibility affected.
*Note: MPH equals approximately 1.15 x Knots

Winds and Temperatures Aloft
This NWS aviation product contains winds aloft which are computer prepared and contain forecast wind direction and speed and as well at forecast temperatures. Forecast winds and temperatures aloft are prepared for:

6,000 feet
9,000 feet
12,000 feet
18,000 feet
24,000 feet
30,000 feet
34,000 feet
39,000 feet

All heights are above Mean Sea Level. Forecast winds are also prepared for 3,000 feet. wind directions are true directions.

Break Line

Break Line

Break Line

Zenith Sets
Information sets based on a cities latitude and longitide used to determine the reaction rates of photochemical reactions.

"Zulu" (Z) Time
For practical purposes, the same as Universal Coordinated Time (UTC). The notation formerly used to identify time Greenwich Mean Time. The word "Zulu" is notation in the phonetic alphabet corresponding to the letter "Z" assigned to the time zone on the Greenwich Prime Meridian.