WEB TUTORIAL 24.2 Rain Shadows

Text Sections Section 24.4 Earth's Physical Environment, p. 428 Introduction Atmospheric circulation patterns strongly influence the Earth's climate. Although there are distinct global patterns, local variations can be explained by factors such as the presence of absence of mountain ranges. In this tutorial we will examine the effects on climate of a mountain range like the Andes of South America. Learning Objectives • Understand the effects that topography can have on climate. • Know what a rain shadow is. Narration

Rain Shadows Why might the communities at a certain latitude in South America differ from those at a similar latitude in Africa? For example, how does the distribution of deserts on the western side of South America differ from the distribution seen in Africa? What might account for this difference?

Unlike the deserts of Africa, the Atacama Desert in Chile is a result of topography. The Andes mountain chain extends the length of South America and has a pro- nounced influence on climate, disrupting the tidy latitudinal patterns that we see in Africa.

Let's look at the effects on climate of a mountain range like the Andes.

The prevailing winds—which, in the Andes, come from the southeast—reach the foot of the mountains carrying warm, moist air. As the air mass moves up the wind- ward side of the range, it expands because of the reduced pressure of the column of air above it. The rising air mass cools and can no longer hold as much water vapor. The water vapor condenses into clouds and results in precipitation in the form of rain and snow, which fall on the windward slope.

As the air mass continues down the other side of the mountains, the pressure rises. The air is compressed and warms. But because this air mass has lost most of its moisture through precipitation on the windward side, it is now warm and dry, cre- ating desert conditions on the leeward side of the range. Dry regions leeward of mountain ranges are called rain shadows.

The red circles mark the general locations of deserts caused by rain shadows. The blue arrows indicate the movement of air masses that contribute to this pattern.

The deserts east of the Sierra Nevada, the Rockies, and the Cascades are all caused by rain shadows.

The Atacama Desert in Chile, as we have just seen, is caused by air masses mov- ing westward across the Andes.

The Patagonian Desert in southern Argentina is caused by air moving eastward across the southernmost part of the Andes. The western slopes in this area consti- tute one of only two temperate rain forests in the world. (The other is on the west- ern slopes of the Olympic Mountains in Washington State.)

The Gobi Desert is caused by air masses moving across the Himalaya. Moisture is dropped in India, causing monsoons.

You should now be able to… • Explain how the movement of winds around mountain ranges can cause rain shadows. • List 4 deserts that are caused by rain shadows.