How mountains wring air dry and build deserts on their far side
Mountains do not just block the wind; they strip it of water. Moist air forced up a range cools and dumps its rain on the windward slopes, then sinks, warms and dries out beyond the peaks, leaving deserts such as the Gobi and the Thar in its wake.
The mechanism is called orographic lifting. Prevailing winds carry moisture evaporated from oceans and big lakes inland until high ground forces the air upward. Lower pressure at altitude lets it expand and cool adiabatically until it reaches its adiabatic dew point, a different figure from the dew point quoted in weather forecasts, and the moisture condenses and falls on the summit and windward side. Descending the leeward slope, the air is compressed and heats up, as in foehn winds, so it can absorb even more moisture and parches the land below.
Which side ends up dry depends on the planet's wind belts. Trade winds dominate between roughly 30 degrees north and south, blowing mostly from the northeast in the Northern Hemisphere and the southeast in the Southern, while westerlies rule the middle latitudes between 30 and 60 degrees. The resulting landscapes range from shrub-steppe and xeric shrubland to full desert.
Ranges whose peaks exceed 4,000 meters, including the Atlas Mountains, catch Atlantic moisture and leave the Sahara even drier, and the Ethiopian Highlands parch the Danakil Desert. In South Africa's Western Cape, the wettest parts of the Cape Mountains can get 1,500 millimetres of rain while Worcester, in their shadow, gets roughly 200 and is fit only for grazing. Central Myanmar, sheltered by the Arakan Mountains, receives just 750 millimetres, a sliver of what falls on the Rakhine coast.
Islands show the effect on a small scale. Madagascar's east coast, facing onshore easterlies, is wet and tropical, while the west and south, behind the central highlands, hold thorn forests and deserts. On remote Tristan da Cunha, Sandy Point on the east coast stays warmer and drier than Edinburgh of the Seven Seas, the wet and wind-battered settlement on the west side. Even Tokyo's Kantō Plain gets markedly less winter precipitation because the Japanese Alps block winds from Siberia.
Source: Rain shadow
