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Lapse Rate
From SKYbrary Wiki
| Article Information | ||
|---|---|---|
| Category: | Weather | 
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| Content source: | SKYbrary | 
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| Content control: | EUROCONTROL | 
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| WX | |
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| Tag(s) | Atmosphere |
Adiabatic lapse rate and atmospheric stability
Definition
The Lapse Rate is the rate at temperature changes with height in the Atmosphere.
Description
When air is forced to rise up in the atmosphere, the pressure reduces with height. For a given volume of gas, the pressure divided by the temperature remains constant (Boyle's Law). Therefore, as the air pressure reduces, so does the temperature. If no heat is exchanged with the surrounding air during this process, which is called “adiabatic cooling”, the rate at which the air cools, the Adiabatic Lapse Rate (ALR) is a constant. For unsaturated air, the lapse rate is 3°C276.15 K
37.4 °F
497.07 °R per 1000 feet304.8 m; this is called the Dry Adiabatic Lapse Rate (DALR). However, when the parcel of air reaches the Dew Point and becomes saturated, water vapour condenses to form cloud, latent heat is released during the condensation process, which warms the air, and the lapse rate reduces. The Saturated Adiabatic Lapse Rate (SALR) is therefore the rate at which saturated air cools with height and is, at low levels, 1.5°C274.65 K
34.7 °F
494.37 °R per thousand feet. At higher altitudes, where there is less water content in the air, and therefore less latent heat to release, the SALR is closer to 3°C276.15 K
37.4 °F
497.07 °R per thousand feet.
The ELR (Environmental Lapse Rate) is the actual rate at which the ambient temperature changes with height.
If the ELR is greater than the ALR, rising air will be warmer than the surrounding air and therefore keep rising; the atmosphere is then said to be unstable.
If the ELR is less than the ALR, then the rising air will be cooler than the surrounding air and will sink - the atmosphere is said to be stable.