The leeward side of a mountain, the side sheltered from the prevailing wind, gets less precipitation. In dramatic cases, the difference is enormous: the windward (wind-facing) slopes of Washington’s Olympic Mountains receive about 135 inches of rain per year, while the town of Sequim on the leeward side gets roughly 16 inches. That’s nearly a ninefold difference across a single mountain range.
Why One Side Stays Dry
The process behind this imbalance is called the orographic effect, and it works like a conveyor belt that wrings moisture out of the air before it can reach the other side. When wind pushes moist air toward a mountain range, the air has nowhere to go but up. As it rises, it expands and cools at a rate of about 7°C for every kilometer of altitude gained. Once the air cools enough to reach its dew point, water vapor condenses into clouds, and precipitation begins falling on the windward slope.
By the time the air crests the summit, it has already lost much of its moisture. As it descends the leeward slope, the process reverses: the sinking air compresses, warms, and its relative humidity drops sharply. This creates conditions that are not just dry but actively resist cloud formation. The result is a “rain shadow,” a zone of persistent dryness on the downwind side of the range.
A clear example of this plays out across California’s Sierra Nevada. Moist Pacific air rises over the range, cooling from around 12°C at 3,000 feet to below freezing above 14,000 feet. Moisture falls as rain at lower elevations and snow higher up. By the time the air sinks into the Owens Valley on the eastern side, it has warmed back to about 16°C with very little moisture left. That valley sits at nearly sea level, yet it’s arid.
Taller Mountains Create Drier Shadows
The height of a mountain range directly controls how severe the rain shadow becomes. When a range is tall enough relative to wind speed and atmospheric stability, it essentially blocks airflow rather than letting it pass smoothly over the top. Research published in the Journal of Geophysical Research found that when mountains exceed a critical height threshold, precipitation on the downwind side can vanish entirely, and cloud mass on that side drops by as much as 90%.
Lower mountain ranges still produce rain shadows, but the effect is more subtle. Air flows over them without much disruption, and some moisture survives the trip to the leeward side. This is why gentle hill ranges might show a modest difference in rainfall between their two sides, while massive ranges like the Andes or Himalayas create some of the driest places on the planet.
Rain Shadow Deserts Around the World
Some of the most extreme landscapes on Earth exist because they sit on the wrong side of a mountain range. The Atacama Desert in Chile, the driest non-Antarctic desert on the planet, lies in the rain shadow of the Andes. It’s doubly unlucky: the Andes block moisture from the east, while high-pressure systems over the Pacific prevent it from arriving from the west. Parts of the Atacama receive virtually no measurable rainfall in a typical year.
Patagonia, at the southern tip of South America, is another Andean rain shadow. Prevailing westerly winds dump their moisture on the Chilean side of the mountains, leaving Patagonia with sparse pastures, limited water, and scattered salt flats. Death Valley in California and Nevada, one of the hottest places on Earth, sits in the rain shadow of the Sierra Nevada. Rain shadow deserts generally receive less than 10 inches of precipitation per year.
The Landscape Tells the Story
You can often see the rain shadow effect without checking any weather data. The windward side of a mountain range tends to be lush, heavily forested, and green. The Hoh Rain Forest on the western slopes of Washington’s Olympic Mountains, which receives nearly 12 feet of rain annually, is a temperate rainforest thick with moss-draped trees and ferns. Drive to the northeastern side of the same peninsula, and you’ll find Sequim, which sits in open, dry grassland that looks nothing like the landscape just 50 miles away.
This vegetation shift is one of the most reliable visual indicators of a rain shadow. Forests give way to shrubland, then to grassland, then sometimes to outright desert. The transition can happen over a surprisingly short distance, especially when the mountain range is tall and steep.
When the Dry Side Shifts
The leeward side isn’t always the same side of the mountain. Which slope is “leeward” depends on prevailing wind direction, and that can change with the seasons. In mountain ranges influenced by monsoon patterns, the dominant moisture source may reverse during part of the year, temporarily wetting the normally dry slope. The Andes are a good example: the eastern slopes receive moisture from the Amazon basin, while the western slopes are influenced by the Pacific Ocean, and the balance between these sources shifts over time.
In most temperate regions, though, prevailing winds are consistent enough that one side of a range is reliably wetter year-round. If you’re looking at a mountain range and want to guess which side is drier, figure out which direction the dominant weather systems come from. The side facing away from those systems is the dry one.