Two slopes of the same mountain ridge, only a few kilometres apart, can host entirely different worlds. On one side: misty forests, moss-covered rocks and rushing streams. On the other: scorched earth, sparse shrubs and rainfall that barely reaches a few millimetres a year. Behind this striking contrast lies a single mechanism: moist air being forced to climb over a mountain barrier and unloading its cargo along the way. Geographers call this phenomenon the "rain shadow".
Orographic Precipitation: Moist Air on the Rise
Water vapour evaporated from oceans and large bodies of water is carried inland by prevailing winds. When this moisture-laden air mass runs into a mountain range, it has nowhere else to go and is forced to climb the slope. Precipitation produced by air being lifted over an obstacle in this way is called orographic precipitation (or relief rainfall). The word "orographic" comes from the Greek "oros", meaning mountain, and describes the influence of landforms on the atmosphere.
As the rising air experiences lower atmospheric pressure above it, it expands. Expanding gas cools, so the air mass drops in temperature as it climbs. As long as condensation has not yet begun, this cooling proceeds at the "dry adiabatic" rate of roughly 1 °C per 100 metres.
Condensation and the Birth of Clouds
As air cools, its capacity to hold water vapour falls sharply. When the temperature reaches the dew point, the vapour begins to condense into tiny water droplets; this is how clouds form. Because latent heat released during condensation warms the air slightly, cooling from this point on continues more slowly, at the "moist adiabatic" rate.
As the ascent continues, droplets merge, grow heavier and fall as rain or snow. This is why precipitation concentrates on the windward (wind-facing) side of mountains and usually increases with elevation. By the time the air mass reaches the summit, it has already shed most of its moisture.
Why Is the Windward Slope Lush and Green?
Constantly bathed in cloud, fog and rain, the windward slope becomes a water-rich ecosystem. Dense forests, ferns, deep soil cover and year-round streams are typical of this face. In some regions annual rainfall reaches several thousand millimetres; the windward hills of Kauai in Hawaii are counted among the wettest places on Earth. Because this face also stores a thick snowpack, it acts as a natural reservoir that feeds rivers through the dry summer.
The Leeward Slope and the Rain Shadow
The air that crosses the summit is now dry. As it descends the leeward (downwind) face of the mountain, rising pressure compresses and warms it. Warmer air can hold more moisture, but because it has none left to give, its relative humidity drops and producing rain becomes almost impossible. This dry zone is the rain shadow, and it often corresponds to semi-arid steppe or true desert.
The contrast between the two slopes can be dramatic: in Norway the western face of the Scandinavian Mountains receives more than 2,000 millimetres a year, while the area around Oslo, just to the east, barely reaches a third of that. Same longitude band, same latitude, an entirely different climate.
Foehn: The Warming, Drying Descent Wind
This warm, dry airflow sinking down the leeward slope is known in many languages as the foehn (foehn in the Alps, chinook in North America, zonda in Argentina). On the way up the air cools slowly at the moist adiabatic rate, but on the way down it warms quickly at the dry adiabatic rate. Because of this asymmetry, the air mass reaches the plain warmer and far drier than when it began its climb. Foehn winds can melt snow rapidly, crash the humidity within hours and raise the risk of wildfire.
Examples From Around the World
Rain shadows appear wherever mountains block moist winds. The Atacama Desert in Chile, sheltered by the eastern flank of the Andes, is regarded as the driest non-polar place on Earth. The Tibetan Plateau north of the Himalayas and the steppes of Central Asia are arid because monsoon moisture cannot clear the mountains. In North America, Death Valley in the shadow of the Sierra Nevada and the steppe of Eastern Washington behind the Cascade Range are products of the same process. In southern South America, where the Andes intercept winds from the opposite direction, the rain shadow falls over Argentine Patagonia. In every case one rule holds: whichever slope the mountain leaves its moisture on, the other slope runs dry.