At night, the air over the land becomes colder than the air over the
water. The resulting pressure gradient causes a cool land breeze to blow out
to sea. There, the air rises and flows inland. A land breeze starts long before
midnight and dies down after sunrise.
Warmer
air rises
Colder air
descends
LAND-BREEZE CIRCULATION
Temperature differences also cause mountain and valley breezes. Air
near a mountain can have a density different from that of the surrounding
air. For example, at night a mountain will cool the air next to it, making the
air more dense. The cool air sinks, flowing down into the valleys nearby.
The narrower the valley, the stronger the breeze. Because the breeze comes
from a mountain, it is called a mountain breeze. During the daytime,
mountain air becomes warmer and less dense than the surrounding air.
The mountain air rises, causing a valley breeze to blow up from nearby
valleys. The speed of an uphill valley breeze is generally much lower than
that of a downhill mountain breeze.
19.4 Section Review
1
How does average air pressure over the southwestern United States
in the winter compare to the average air pressure in the summer?
2
Explain how and why the location of the ITCZ (intertropical
convergence zone) varies from season to season.
3 Compare and contrast a sea-breeze circulation with a land-breeze
circulation.
4
Explain why a cool breeze may blow down a valley at night.
5 CRITICAL THINKING Explain why Southeast Asia experiences
heavy rains in the summertime. Why doesn’t Southeast Asia
experience heavy rains in the wintertime?
6
MATHEMATICS During the summer monsoon, parts of India may
receive as much as 75 centimeters of rain in one day. If you stood
outside in such a rain for 20 minutes, about how much water would
fall on your head? Express your answer in centimeters and as an
estimated volume.
Chapter 19 The Atmosphere in Motion
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