Air drainage
Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"
Meaning
Air drainage is the flow of cold air down hill slopes at night. On clear, calm nights the slopes lose heat quickly. The air touching them becomes cold and heavy, and it slides downhill under gravity, like water. It collects in the valley bottom, below warmer air. This creates a valley temperature inversion, where the air gets warmer with height instead of colder. So valley floors become frost pockets. The middle slopes stay warmer and form a "thermal belt". Air drainage protects the slopes from frost, but not the valley floor.
Example
In the Himalayan valleys of north India, apple orchards and tea gardens are planted on the slopes, not on the valley floors. Villages also prefer the mid-slopes, away from the cold air that collects below.
Don't confuse with
- Temperature inversion: this is the general case where air gets warmer with height. Air drainage is one way an inversion forms, in valleys.