What is a rainshadow?
You’ve probably heard the term “rainshadow” before, but what does it actually mean?
A rainshadow is an area on the downwind side of a mountain range that gets very little rain. This happens because the mountains block the rain. On the windward side, moist air rises, cools, and drops its water as rain or snow. But once that air crosses over the mountain, it’s dry. As it descends, it warms up, soaking up any remaining moisture and creating a dry, arid environment.
This article explains the causes and consequences of the rainshadow effect, and gives some examples of where you can find them around the world.
How a rainshadow forms
Rainshadows form through a process called orographic lift. It sounds complicated, but it’s really not. Here’s how it works:
Orographic lift
When wind blows moist air toward a mountain range, the air is forced upward. As the air rises, it expands and cools. This is called adiabatic cooling.
As the air continues to cool, it eventually reaches its dew point, the temperature at which the water vapor in the air condenses and forms clouds. If there’s enough moisture, these clouds will produce rain or snow on the windward side of the mountain range.
Windward precipitation
The windward side of the mountain, the side facing the wind, receives a lot of precipitation. As the water vapor condenses, it releases latent heat, which warms the air slightly.
Leeward descent and drying
Once the air has passed over the mountain range, it begins to descend on the leeward side, the side sheltered from the wind. As the air descends, it’s compressed and warms up. This is called adiabatic warming.
Because the air has already lost much of its moisture on the windward side, and because it’s now warmer, it becomes drier. This dry air inhibits cloud formation and precipitation on the leeward side, creating a rainshadow.
What causes the rainshadow effect?
Several factors can affect how intense a rainshadow turns out to be. The most important are:
Height and width of the mountain range
Taller and wider mountain ranges create more intense rainshadows. A higher barrier forces more air to rise up, leading to more rain and snow on the windward side. This leaves the leeward side even drier.
Prevailing wind direction
The angle at which the wind blows toward the mountain range also matters. Winds that hit the range straight-on (perpendicular) create the most intense rainshadows. If the wind blows at an angle, the rainshadow effect will be less pronounced and may shift in location.
Moisture content of the air
Air that’s already full of moisture will produce more precipitation as it rises on the windward side of the mountains. This means even less moisture makes it over to the leeward side, making the rainshadow even drier.
Examples of rainshadows around the world
Rainshadows occur all over the globe wherever there are significant mountain ranges. Here are a few examples:
The Himalayas and the Tibetan Plateau
The Himalayas are so tall they prevent moisture from the Indian Ocean from traveling north, which creates a rainshadow effect on the Tibetan Plateau.
Because it gets so little precipitation, the Tibetan Plateau is characterized by sparse vegetation and a cold, dry climate.
The Andes Mountains and the Atacama Desert
The Andes Mountains in South America block moisture from the Amazon basin from reaching the western side of the mountains. This rainshadow effect is responsible for creating the Atacama Desert, the driest non-polar desert on Earth.
Even though it’s incredibly dry, life still manages to find a way in the Atacama. Certain specialized plants and microorganisms have evolved to be able to survive in these extreme conditions.
The Cascade Mountains and Eastern Washington/Oregon
In the Pacific Northwest, the Cascade Mountains block moisture coming in from the Pacific Ocean. This creates a rainshadow effect in eastern Washington and Oregon, resulting in a semi-arid climate.
Because the region gets so little rainfall, irrigation is essential for agriculture to thrive.
The Sierra Nevada and Owens Valley
The Sierra Nevada mountain range in California blocks moisture from the Pacific Ocean, creating a rainshadow effect in the Owens Valley.
Historically, water has been diverted from the Owens Valley to the city of Los Angeles, which has had huge environmental and social consequences for the region.
Ecological and human impacts of rainshadows
Rainshadows have a profound effect on the landscape and the life it supports.
Vegetation and biodiversity
In a rainshadow, you’re more likely to see drought-resistant plants like shrubs and grasses that can survive on very little water. The animal life in rainshadows has also adapted to arid conditions and can tolerate low water availability.
Agriculture and water resources
Farming in rainshadows can be tricky because of the lack of water. It often requires irrigation to grow crops. Rainshadows can also lead to water shortages and competition for what little water there is.
Human settlement and land use
Because resources are often limited in rainshadow regions, you’ll typically find lower population densities there. Land use tends to focus on things like grazing, growing drought-resistant crops, or attracting tourists to the unique landscape.
Rainshadows demonstrate how geographical features can shape ecosystems and influence how humans interact with their environment.
Frequently Asked Questions
Why is it called a rain shadow?
The term “rain shadow” perfectly describes the phenomenon. It’s the “shadow” cast by a mountain range in terms of rainfall. One side of the mountain gets drenched, while the other side, sheltered from the prevailing winds and moisture, receives significantly less precipitation. It’s as if the mountain is blocking the rain, creating a “shadow” of dryness.
What cities are in a rain shadow?
Many cities worldwide experience rain shadow effects. In North America, examples include Denver, Colorado (east of the Rocky Mountains), and Yakima and Tri-Cities in Washington State (east of the Cascade Mountains). Other examples can be found in the central valley of California as well as locations near the Andes mountains.
What is the biggest rain shadow in the world?
Pinpointing the “biggest” is tricky, as it depends on how you define “biggest.” However, a strong contender is the area east of the Himalayas, including the Tibetan Plateau and the Gobi Desert. The Himalayas block the moisture-laden monsoon winds from the Indian Ocean, creating an extensive arid region in their rain shadow. The Atacama Desert in South America is another very dry region that experiences rain shadow effects from both the Andes and the Chilean Coast Range.
Key Takeaways
The rainshadow effect is a powerful atmospheric process that has a tremendous effect on both the environment and the people who live in areas where it occurs. Mountains force air upward, causing precipitation on one side, while creating arid conditions on the other.
Understanding the rainshadow effect is essential for making good choices about land use, managing water resources effectively, and planning for the impact of climate change on affected areas.
More research is needed to determine how the rainshadow effect will change as global temperatures rise and what steps can be taken to lessen the impact of these changes.