The Atmosphere and Climate 221Windward side Leeward sidewMoist airDry airRain shadow
desert© Michael T. Sedam/CORBISa. A rain shadow refers to arid or semiarid land that occurs on the far side
(leeward side) of a mountain. Prevailing winds blow warm, moist air from the
windward side. Air temperature cools as it rises, releasing precipitation, so
dry air descends on the leeward side. Such a rain shadow exists east of the
Cascades.
b. Proxy Falls is in the Cascade Range,
which divides the states of Washington
and Oregon into a moist western region
and an arid region east of the mountains.WHAT A SCIENTIST SEES
Rain Shadow
- Geographic location. The rising air from the equa-
tor eventually descends to Earth near the Tropic
of Cancer and Tropic of Capricorn (latitudes 23.5
degrees north and 23.5 degrees south, respectively). By
then most of its moisture has precipitated, and the dry
air returns to the equator. Over land, this dry air pro-
duces some of the great tropical deserts, such as the Sa-
hara Desert. Air also dries out as it travels long distances
over landmasses. Near the windward coasts of conti-
nents (the side from which the wind blows), rainfall may
be heavy. However, in the temperate zones (the areas
between the tropics and the polar zones), continental
interiors are usually dry because they are far from the
ocean, which replenishes water in the air passing over it. - Topographic features. When flowing air encounters
mountains, it flows up and over them, cooling as it
gains altitude. Because cold air holds less moisture
than does warm air, clouds form and precipitation oc-
curs, primarily on the mountains’ windward slopes.
The air mass is warmed as it moves down on the other
side of the mountain, reducing the chance of precipi-
tation of the remaining moisture. This situation exists
on the West Coast of North America, where precipi-
tation falls on the western slopes of mountains close
to the coast (see What a Scientist Sees). The dry land
on the side of the mountains away from the prevail-
ing wind—in this case, east of the mountain range—
is called a rain shadow. The dry conditions of a rain
shadow often occur on a regional scale.- How do you distinguish between weather and
climate? - What effect does solar energy have on Earth’s
temperature? - What are some of the environmental factors
that produce areas of precipitation extremes,
such as rain forests and deserts?