Troposphere

The troposphere is the lowest layer of the atmosphere. This is the layer where most weather takes place. Most thunderstorms don't go much above the top of the troposphere (about 10 km) . In this layer, pressure and density rapidly decrease with height, and temperature generally decreases with height at a constant rate. The change of temperature with height is known as the lapse rate. The standard lapse rate for the troposphere is a decrease of about 6.5 degrees Celsius (C) per kilometer (km) (or about 12 degrees F). Near the surface, the lapse rate changes dramatically from hour to hour on clear days and nights. Sometimes the temperature does not decrease with height, but increases. Such a situation is known as a temperature inversion. Persistent temperature inversion conditions, which represent a stable layer, can lead to air pollution episodes.

The other main characteristic of the troposphere is that it is well-mixed. The name troposphere is derived from the Greek tropein, which means to turn or change. Air molecules can travel to the top of the troposphere (about 10 km up) and back down again in a just a few days. This mixing encourages changing weather.

The troposphere is bounded above by the tropopause, a boundary marked as the point where the temperature stops decreasing with height and becomes constant with height. Any layer where temperature is constant with height is called isothermal. The tropopause has an average height of about 10 km (it is higher in equatorial regions and lower in polar regions). This height corresponds to about 7 miles, or at approximately the 200 mb (20.0 kPa) pressure level. Above the troposphere is the stratosphere.