Lapse Rate

Lapse rates are the average changes in temperature at different levels in the atmosphere. Four different rates are environmental, normal, dry adiabatic, and wet adiabatic. Each is applied in specifically different situations. The changing temperatures affect the humidity and the climatic patterns. Types of lapse rate, their applications, their relationship to cloud formation and atmospheric lifting, and the environmental hazards are discussed here.

The environmental lapse rate is the actual change in air temperature with altitude at a specific place at a specified time. The temperature depends on current micro- and macro-meteorological conditions. In general, temperatures are expected to decrease with altitude through the troposphere, the lowest temperature zone of the atmosphere. However, temperature variations occur, and the actual situation makes the environmental lapse rate. For example, this might be measured with a thermometer as a balloon rises vertically from the surface through the air.

The normal lapse rate is the mean of the environmental lapse rates. Considered to be 6.5 °C/1,000 m (meters) or 3.5 °F/1,000 ft. (feet), this value is used both to estimate the temperature at a higher elevation, such as for an airplane flying overhead, and to compare temperatures between locations at different elevations, for example, San Francisco, California, near the coast, and Denver, Colorado, about a mile higher.

The normal lapse rate is closely related to the greenhouse effect. Because the troposphere is predominantly heated by the absorption of terrestrial radiation, more of this Earth radiation is trapped near the surface than reaches the higher levels. So temperatures near the surface are generally warmer than in layers aloft, and this average temperature differentiation is the normal lapse rate.

The dry adiabatic lapse rate applies to temperature changes in an unsaturated parcel of air that is physically changing elevations. The atmospheric pressure surrounding a parcel of rising air decreases, so it expands and cools. Thus, the rate of temperature change is greater than the normal lapse rate. The dry adiabatic lapse rate is about 10 °C/1,000 m, or 5.5 °F/1,000 ft.

Figure 1 Lapse rates example diagram

Source: Author.

As unsaturated air cools, the relative humidity rises, and the temperature approaches the dew point temperature. Once cooled to the dew point temperature, net condensation exceeds net evaporation, and latent heat stored by the moisture is released to the environment in a warming effect. Thus, a parcel of rising saturated air will not cool as rapidly as rising unsaturated air. The result is the wet adiabatic lapse rate of 5 °C/1,000 m or 3.0 °F/1,000 ft. This rate depends on the amount of condensing moisture.