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.
For example, this is why the east side of the Rocky Mountains in the United States is so dry compared to the west. Indeed, Washington state and Oregon both contain temperate rainforests, yet just beyond the mountains, you could find conditions verging on desert: arid conditions with little vegetation. Other examples of rain shadows include the Atacama Desert in Chile, which has the Andes on one side and the Pacific on the other, and the Gobi Desert in Mongolia, a shadow partly created by the Himalayas, the largest mountain range in the world.
A rain shadow is a dry area on one side of a mountain or mountain range. This is usually the opposite side to a sea or other large body of water. The body of water provides moist air, and wind currents that head towards the mountains. The water vapor in the moist air condenses to form clouds and finally rain as it rises up with the mountains. This rain falls on the near side of the mountain, causing the air that continues on the far side of the mountain to be dry. The rain that falls on the near side of the mountain is called relief rain, and the far side of the mountain tends to contain dry, arid conditions.
Examples of rain shadows include the east side of the Rocky Mountains in the United States, the Atacama Desert in Chile (caused by the Andes), and the Gobi desert in Mongolia (caused by the Himalayas).
Rain Shadow Review
|*A dry area on one side of a mountain range or mountain
*Caused by rising clouds that release their rain on one side of the mountain, called relief rain
*Examples include the Gobi Desert and the east side of the Rocky Mountains
When you are finished, you should be able to:
- Describe a rain shadow and how it is formed
- Explain why relief rain occurs
- Recall some of the geographical features around the world that have rain shadows