Adiabatic process
Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"; Class 11, Ch 10 "Water in the Atmosphere"
Meaning
An adiabatic process is a change in the temperature of a body of air without any heat being taken in or given out. Rising air meets lower pressure, so it expands and cools. Unsaturated air cools by about 9.8 °C per km, which is the dry adiabatic rate. At the lifting condensation level, condensation begins and releases latent heat. This slows the cooling to about 4–7 °C per km, which is the saturated rate. Sinking air is squeezed and warms, so it becomes dry.
Example
The flat bases of cumulus clouds on a summer afternoon mark the level where rising air has cooled enough to start condensing. On the far side of a mountain, air sinks and warms, as in the warm, dry chinook winds east of the Rocky Mountains in North America.
Don't confuse with
- Cooling by contact or radiation: here air loses heat to a cold surface or to space. In an adiabatic process no heat leaves the air; it cools only because it expands.