Upland areas are cooler than lowlands of the same latitude, since temperatures fall by 1 °C/150 m. The heating of a valley floor may cause anabatic winds, and colder and heavier air at the peaks may spill down as a katabatic wind. When warm, moist air rises over mountain barriers, orographic rain falls; when this air has passed over the barrier, it descends and is adiabatically warmed, which may bring föhn winds. The warmed air can now hold the remaining moisture, so a ‘rain shadow’ of drier air develops in the lee of mountains. Lee depressions form downdraught of mountains, and major barriers such as the Himalayas affect the upper atmosphere. See C. D. Whiteman (2000).
Subjects: Earth Sciences and Geography.