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Measuring Precipitation
The National Weather Service reports rainfall in hundredths of an inch.
Rainfall is measured by an instrument called a rain gauge. The recorded,
and reported, measurement represents what the depth of water would be
if the rain did not soak into the ground, flow away, or evaporate.
Snowfall is measured with a measuring stick. But because a dry snow
is deeper than an equal weight of wet snow, the depth of snow is not an
accurate measure of how much water it contains. The rain equivalent of the
snowfall is determined by melting the snow.
Where Does Precipitation Occur?
Precipitation occurs in every part of the world. In some places there may be
no precipitation for years at a time, while in other places it may rain almost
every day. One of the main causes of precipitation is the rising and cooling
of moist air. The warmer the air before it rises, the more moisture it can
contain. Furthermore, the higher the air rises, the more precipitation it can
release. Therefore, areas that receive the most precipitation are those
where warm, moist air rises high in large quantities. Following are
descriptions of the kinds of areas where these weather conditions exist.
• Heat from the sun produces high land temperatures near the equator,
and these in turn cause the air near the surface to become very warm
and to rise. The result is almost daily thunderstorms. Because of the
heavy rain, the land around the equator is home to dense tropical
forests.
• In storm areas of all kinds, including hurricanes and many low-pressure
areas and fronts, air rises and cools to produce precipitation.
CLASSZONE.COM
Which Way Does the Wind Blow?
Determine the direction of prevailing
winds by examining precipitation and
vegetation patterns on mountains.
Keycode: ES1806
• In areas where moist air often blows across a mountain range, the
windward side of the mountains—that is, the side the wind reaches
first—may receive large amounts of precipitation. When wind reaches
the mountain range, it must rise up the side. Because the air cools as
it rises, some of its moisture condenses and may fall as rain or snow.
One range with a rainy windward slope is the Cascade Range in the
northwestern United States.
MOUNTAIN RANGES Air is forced to
rise on the windward side of mountain
ranges. As the air rises, it cools. Clouds
form, and precipitation falls.
WINDWARD Rising air cools,
water vapor condenses, and
precipitation falls.
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Unit 5 Atmosphere and Weather
LEEWARD Now drier, the
sinking air compresses
and warms.