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The Rain Shadow Effect: Definition & Explanation

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  • 0:00 What Is a Rain Shadow?
  • 0:50 Formation of a Rain Shadow
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

You probably know how regular shadows form. But what about rain shadows? Learn why one side of mountains tends to be so dry, and why we call it the rain shadow effect. Test your knowledge with a quiz.

What is a Rain Shadow?

You're walking down the street on a sunny day. Without looking at the ground, can you predict where your shadow will be? Well, that probably depends on how closely you've been paying attention. If you're observant, you've probably noticed the shadow is always on the opposite side of you to the sun. That's because a shadow is just the sunlight being blocked by your body.

A rain shadow works in the same way: it's where moist air gets blocked by mountains. A rain shadow is a dry area on the side of a mountain opposite to the wind. We call this dry side of the mountain the leeward side. If wind is approaching from the west, the rain shadow is on the east. If the wind is approaching from the east, the rain shadow is on the west. Whichever side the wind is approaching from is called the windward side. Windward is the opposite of leeward.

But what exactly causes a rain shadow?

Formation of a Rain Shadow

A rain shadow forms when moist winds head towards a set of mountains and get forced upwards by them. This moist air often comes from the sea or from another large body of water. That's because where there is water, that water can evaporate. The more water evaporates, the more moist the air becomes.

Seas and oceans also tend to lead to sea breezes, which are winds that head from the sea to the land. This happens because the land changes temperature more easily than the sea. Overnight the sea gets cold and stays that way during the day. This causes the air above it to be colder than the land air.

Cold air is denser than warm air - the particles are closer together, and since air likes to spread out, this causes the cold air to rush toward the land. These sea breezes are part of why rain shadows tend to form when there are mountains alongside oceans.

Have you ever been hiking in the mountains? You might have noticed that the higher up you are the cooler it is. This is true for the air, as well. When the moist wind from the sea gets forced upwards by the mountains, the moisture in the wind gets cooler. This causes it to condense to form clouds. As more and more of the moisture condenses, the clouds get bigger and bigger until they get too heavy and start to rain.

We call this relief rain. This happens during the journey to the top of the mountain. The clouds rarely make it to the other side of the mountain, because they're not light enough to float over the top. By the time the air reaches the opposite side of the mountain, there is no longer any moisture inside it. This causes the far side of the mountain to be dry; it causes a rain shadow.

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