Today we’re going to 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 an ocean. The sun causes ocean water evaporation that evaporated ocean water rises and becomes clouds. Then the wind blows those clouds over the mountain, and then 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 there’s a strikingly different landscape 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 Sierra Nevada Mountain Range in California. And that rainshadow southwest of Manua Kea on the Big Island of Hawaii.
Ok, 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?
We can illustrate this by observing the effect of the prevailing wind and a mountain range on the level of rainfall in the region. Let’s begin at the ocean, where the water evaporates, and it is held in the air as water vapor and prevailing. 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. And 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 around the world 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 air pressure, making the rising air expand and cool. 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 continues to rise, condensation, clouds, and possibly rain or snow will occur.
Windward side effect
The mountainside where the wind is blowing up the mountain is called the windward side. Many world areas experience large amounts of rain or snow where the air is forced up mountains. The wind is blowing 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 as that air mass expands, the temperature of that air mass begins to drop. It is referred to as adiabatic cooling. Also, adiabatic means that there’s 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 or the lee side of the mountain, it starts to warm. The warming is caused by the air getting squeezed together as it moves downward into higher air pressure. Warmer air can hold more water vapor, so condensation does not occur when air is sinking. 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 of the mountain. 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 and are known as the rain shadow area. The mountain’s leeward side remains dry because the air not only 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. Bend, Oregon, on the mountains’ lee side, receives only about 12 inches of rainfall a year.
The air and temperature difference between the mountain’s windward and leeward sides is known as the orographic effect. 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 so many clues about the Ice Age Floods, the Columbia River Basalts, and other wonders of our geologic past.
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.