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 .
Apparent Temperature = 1.03T + T (exp((DP-59)/17)-1)/19-3where T = Observed Temperature (degree F) DP = Dew Point (degree F)
|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.
||Marginally Unstable - Thunderstorms possible|
||Unstable - thunderstorms probable|
||Very Unstable - heavy to strong thunderstorm potential|
Cross Totals (CT) =850 mb dew point - 500 mb temperature
When the two are combined, you have the following formula:
The following table shows what these relationships typically mean east of the Rockies:
|Isolated or few thunderstorms|
|Scattered thunderstorms, isolated severe|
|Scattered thunderstorms, few severe, isolated tornadoes|
|Scattered numerous thunderstorms, few to scattered severe, few tornadoes|
|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.
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:
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.
|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.|
All heights are above Mean Sea Level. Forecast winds are also prepared for 3,000 feet. wind directions are true directions.