From Glossary of Meteorology
- An elevation band along mountain and other terrain slopes where nighttime surface temperatures remain relatively mild compared with temperatures above and below.
Drainage winds carry the coldest air down the slopes to the bottom of the valley. The belt of warmer air (thermal belt) lies above this pool of cold air. Above the warm belt, temperature exhibits its normal decline with elevation, augmented by increased radiation loss from lower air density and lower moisture content at higher altitudes. The impact of this milder slope climate is a longer growing season, an earlier leafing out and blossoming of trees and other vegetation, and the ability to grow crops that could not survive at lower or higher elevations (e.g., vineyards). Geiger (1965) suggests that this effect influenced early settlement locations: "In Germany this area was preferred for the earliest villages, monasteries, and country houses."
Geiger, R. 1965. The Climate Near the Ground. Harvard University Press, Cambridge, Mass., . p. 437.