Adiabatic lapse rate
Also called: Dry and wet adiabatic lapse rate · Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Beyond NCERT
Meaning
The adiabatic lapse rate is how fast a rising parcel (bubble) of air cools as it goes up. Air pressure falls with height, so the parcel expands, and expanding air cools. "Adiabatic" means this happens without the parcel gaining heat from, or losing heat to, the air around it. Dry air cools by about 9.8°C per km. Saturated air (air full of water vapour) cools by only about 4-7°C per km. This is because condensation, the change of vapour into droplets, releases latent heat (heat stored in the vapour). Comparing this rate with the cooling of the surrounding air tells us whether the air is stable or unstable. That decides whether clouds and rain can form.
Example
Moist south-west monsoon winds rise up the Western Ghats of peninsular India. They cool and give heavy rain on the western side. As the air comes down the eastern side onto the Deccan, it warms and stays dry, so that side lies in a rain shadow.
Don't confuse with
- Normal lapse rate: it is the drop in temperature of still air as you go higher, about 6.5°C per 1,000 m. The adiabatic rate applies only to a parcel of air that is rising or sinking.
Related concepts
- Factors controlling temperature distribution
- Normal lapse rate
- Continentality
- Equable climate
- Microclimate
- Urban heat island