Lapse Rate and Inversions

The atmosphere is primarily heated and cooled at the earths surface through conduction and convection and at higher levels in the atmosphere heat is lost to space through radiation. Therefore, we can expect temperatures to decrease with height. Another reason for a decrease in temperature with increasing height is that pressure decreases with height and decreases in pressure lead to decreases in temperature (see Temperature, Heat and the Atmosphere).

The average rate of decrease of temperature, or lapse rate, in the troposphere (approximately the lowest 7 miles of the atmosphere) is 3.5º F per 1000 ft or 6.3º C per kilometer. If solely the change in pressure were responsible for the decrease in temperature with altitude the lapse rate would be larger ( 5.5º F per 1000 ft or 9.8º C per kilometer ) and result in a more rapid drop in temperature with height; this is the adiabatic lapse rate. Lapse rates in the atmosphere can often deviate widely from the average value as different layers of air can have different temperatures as a result of horizontal motion aloft. Near the surface during the day, air at the ground can be substantially warmer than that just a few feet above the surface resulting in lapse rates far larger than the either the average or adiabatic lapse rate. This is often termed a superadiabatic lapse rate.

An inversion is a layer of the atmosphere where the air temperature increases with increasing height. At night when the earths surface is cooling the air in contact with it, the air temperature will actually increase with height over a shallow layer forming a night inversion. Marine inversions are common in coastal areas. Lapse rates and inversions are key components in determining atmospheric stability.

The above discussion focuses on how air temperature varies with height, however there are also horizontal variations in surface air temperature.