We will be talking about rain shadows, but before we talk about rain shadows, let’s take a moment to talk about where rain comes from. So rain comes from a few different places like the ocean. The sun causes ocean water evaporation that evaporates ocean water rising and becomes clouds. Then the wind blows those clouds over the mountain, and eventually, those clouds will rain.
Rain shadows are dry areas on the backsides or the downwind side of mountains. The mountain creates a shadow of dryness. Western Washington is famous for its rain. But a strikingly different landscape is on the other side of the Cascades. Washington’s the Evergreen State, but half of it’s Everbrown! There’s a rainshadow north of the Himalayas in Asia.
A rainshadow west of The Great Dividing Range in Australia and east of the Sierra Nevada Mountain Range in California. That rainshadow southwest of Manua Kea on the Big Island of Hawaii. Do you want to know about the rainshadow effect? You need three essential items: an ocean nearby, winds blowing steadily onshore, and a mountain range to block the traveling air mass. Here’s how it works.
What Is Rain Shadow Effect?
A rain shadow is formed when moist air encounters a mountain range, leading to a significant decrease in rainfall on the mountains’ leeward side (the side sheltered from the prevailing winds). The process can be explained as follows:
Windward Side: Moist air from an ocean or a large water body is carried by prevailing winds towards a mountain range. As the air encounters the mountains, it is forced to rise due to the topography.
Orographic Lift: As the air is forced to rise, it undergoes a process known as orographic lift. As the air ascends, it cools adiabatically (due to expansion) and reaches its dew point, resulting in condensation and the formation of clouds. This leads to precipitation on the windward side of the mountains, typically characterized by high rainfall.
Rain Shadow Effect: Once the air passes over the mountaintops and descends on the leeward side, it undergoes adiabatic warming. As the air descends, it compresses and warms, causing the moisture content to decrease. The descending air mass becomes drier, creating a rain shadow effect.
Reduced Precipitation: The descending, drier air on the leeward side inhibits the formation of clouds and rainfall. As a result, the area on the leeward side experiences significantly less precipitation than the windward side.
The rain shadow effect can lead to arid or semi-arid conditions on the leeward side of the mountain range, often resulting in desert or steppe-like environments. Some famous examples of rain shadows include the Atacama Desert in Chile (due to the Andes Mountains) and the Great Basin Desert in the western United States (due to the Sierra Nevada and Cascade Mountains).
We can illustrate this by observing the effect of the prevailing wind and a mountain range on the region’s rainfall level. Let’s begin at the ocean, where the water evaporates and is held in the air as water vapor. Why does wind carry all this moist air over the land? As the air rises from the mountain, it expands and cools because the cool air can carry less water vapor than the warm air.
Evaporation on the surface of the ocean creates moist air. Prevailing winds push the wet air inland until it hits the base of the mountains. The air is forced to rise. As the airlifts, it expands and cools. Cooler air can’t hold as much moisture, so clouds form, and it rains a bunch, resulting in a lush, green landscape.
Now dry air mass crosses the mountains and begins to sink on the leeward side of the range. It compresses and warms, promoting evaporation. Dry air warms 1 degree Celsius per 100 meters of elevation drop. Some of the driest places worldwide exist because of the rainshadow effect. Let’s look at the rain shadow effect diagram.
What causes a rain shadow? When the wind blows up a mountain, the air moves into an area with lower pressure, expanding and cooling the rising air. When air cools, its capacity to hold moisture in the form of water vapor decreases. Water vapor is the gaseous form of water. As the air continues to rise the mountain, it cools to the point it’s at its maximum capacity to hold water vapor.
When air is at its maximum capacity, it is at 100% relative humidity. Further cooling will cause excess water vapor to condense or turn into liquid water. This forms clouds. When moist air rises, it expands, cools to the dew point, condensation occurs, and clouds form. If enough condensation occurs, rain or snow will fall out of the clouds. As long as the air rises, condensation, clouds, and possibly rain or snow will occur.
Windward side effect
The windward side is the mountainside where the wind blows up the mountain. Many world areas experience large amounts of rain or snow where the air is forced up mountains. The wind blows up the mountain, and the air mass encounters less pressure. It turns out that as you move up in altitude.
There’s less air pressure, so as this air mass moves up. It begins to expand, and its temperature drops as that air mass expands. It is referred to as adiabatic cooling. Also, adiabatic means no addition or loss of heat going out here.
The western parts of Oregon and Washington states are good examples of this process. This side of the mountain can support a lot of plant life plants. It’s the windward side because it’s the side where the wind is blowing from the ocean, hitting the mountain, and going up.
Leeward side effect
As air flows down the other side of the lee side of the mountain, it starts to warm. The warming is caused by the air getting squeezed together and moving downward into higher air pressure. Warmer air can hold more water vapor, so condensation does not occur when air sinks. If there is no condensation, there are no clouds and, therefore, no rainfall.
There is not a lot of rain, and the temperature will increase. So this side of the mountain is usually hotter than the other side. Because the air is sinking here, and it’s totally dry. There’s no rain at all or very little rain. So this side of the mountain is usually like a desert.
Dry or desert conditions prevail on the leeward side, known as the rain shadow area. The mountain’s leeward side remains dry because the air passes through it and becomes warm. Eastern parts of Oregon and Washington are examples of where the rain shadow occurs. At the same time, Laurel Mountain in Western Oregon receives over 119 inches of yearly precipitation. On the mountains’ lee side, Bend, Oregon, receives only about 12 inches of yearly rainfall.
The orographic effect is known for the air and temperature difference between the mountain’s windward and leeward sides. You will experience significant differences in climate depending on which side of the hill you will be on.
The Rainshadow Effect has been a great help to geologists in the Pacific Northwest. The Cascades have cast a rainshadow on Washington’s Channeled Scablands, our desert landscape that has yielded many clues about the Ice Age Floods, the Columbia River Basalts, and other wonders of our geologic past.
Learn more:
What Is Coriolis Effect On Wind?
What Causes The Smell After Rain?
References:
Whiteman, C. David. Mountain Meteorology: Fundamentals and Applications. Oxford University Press. ISBN 0-19-513271-8.
Glossary of Meteorology. “trade winds.” Glossary of Meteorology. American Meteorological Society.
Rain Shadows by Don White. Australian Weather News. Willy Weather.
