Download Rain Shadows

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 pronounced 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 windward 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, creating 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 moving 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 constitute one of only two temperate rain forests in the world. (The other is on the western 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.