Download Continental and Local Winds

Continental and Local Winds
Because of the tilt of Earth, the relative position of the sun changes over the
course of the year, creating seasons. As Earth’s surface temperatures
change with the seasons, so do the global winds. These winds are affected
by large-scale shifts in temperature as well as by the positions of
continents. In fact, because of such seasonal changes, the highest
temperatures on Earth are not usually at the equator. For example, in the
Northern Hemisphere summer, the hottest temperatures are north of the
equator, where the sun causes air to heat, rise, and flow toward the poles.
19.4
KEY IDEA
Seasons, land masses, and
topography cause winds to vary
from the global patterns depicted
in the three-celled model.
KEY VOCABULARY
• monsoon
Effects of Seasons and Continents
In the summer, continents become hotter than the surrounding oceans as
they absorb more radiant heat. The hot land heats the air above it, causing
the air to become less dense, creating an area of low air pressure. In
general, you can expect the average air pressure over the continents in
summer to be low. Because ocean waters do not absorb radiant heat as
quickly as land, the ocean air will be cooler and denser in the summer,
creating an area of high air pressure.
Because winds spiral out of high-pressure areas and into low-pressure
areas, the resulting highs and lows determine the directions of the
prevailing winds at various locations. For example, in the Northern
Hemisphere summer an Atlantic high brings prevailing southerly winds to
the eastern United States.
With the coming of winter, the air over the continents becomes cold
and dense. The average air pressure is therefore high. In comparison, the
average air pressure over the oceans is lower.
Because air pressures change seasonally, the directions of winds also
change seasonally. Winds that change direction seasonally are called
monsoons (mahn-SOONZ). The most dramatic monsoons occur in
southern Asia, where seasonal changes in pressure caused by the heating
and cooling of the Asian continent produce a complete wind reversal.
High
Pressure
ITCZ
Low
Pressure
WINTER MONSOON Cold, dry air
blows from the high-pressure area over
land to the low pressure of the ITCZ
(intertropical convergence zone).
Low
pressure
High
pressure
ITC
Z
High
pressure
SUMMER MONSOON Warm, moist air
blows from the high-pressure area over
water to the low-pressure area over land.
As the air rises, it releases rain.
Chapter 19 The Atmosphere in Motion
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