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WIND
Factors Affecting Wind
 Wind is the result of horizontal differences in
air pressure. Air flows from areas of higher
pressure to areas of lower pressure.
 The unequal heating of Earth’s surface
generates pressure differences. Solar
radiation is the ultimate energy source for
most wind.
 Three factors combine to control wind:
pressure differences, the Coriolis effect, and
friction.
Factors Affecting Wind:
Pressure Differences
 Isobars: lines on a map
that connect places of
equal air pressure.
 Closely spaced isobars
mean a steep pressure
gradient and high winds.
 Widely spaced isobars
mean a weak pressure
gradient and light winds.
Pressure Gradient
Force (PGF):
The pressure
gradient points
toward low
pressure and away
from high pressure.
Wind:
Wind happens
when air flows from
areas of high
pressure to areas of
low pressure.
Factors Affecting Wind:
Pressure Differences
Factors Affecting Wind: Friction
Jet streams are fastmoving rivers of air
that travel between
120 and 240
kilometers per hour
in a west-to-east
direction.
Friction slows air
movement and can
even change wind
direction.
Pressure
balances with
Coriolis and
wind travels
parallel to
isobars
Friction causes
surface winds
move across
isobars
Factors Affecting
Wind: FRICTION
Winds Aloft Winds
aloft blow parallel to
the pressure isobars.
Winds on the Ground
Winds on the ground
blow at about 45º to
the winds aloft. This is
due to friction with
the ground.
Factors Affecting Wind:
The Coriolis Effect
Coriolis Effect: If the Earth did
not turn, a wind from the north
would move along a straight path
to the south.
Because the Earth turns, a wind from
the north appears to veer to the right as
it moves south.
In the Northern Hemisphere winds veer to
the right, in the Southern Hemisphere to
the left. This is called the Coriolis Effect. It
is caused by the Earth's rotation and
curvature.
Air Circulation About Highs and Lows
The coriolis effect causes winds to blow counterclockwise
around a center of low pressure and clockwise around a
center of high pressure. In the Southern Hemisphere
winds blow in opposite directions.
Air Circulation About Highs and Lows
Airflow Associated with Highs and Lows
Lows: (also called
cyclones) have
winds that blow
inward and
counterclockwise.
Air rises upward
causing clouds.
Highs: (also called anticyclones) have winds that
blow outward and clockwise. Air descends in a high
leading to clear skies.
LOCAL WINDS
Local Winds: All winds
are produced by unequal
heating of the Earth's
surface which generates
pressure differences.
Local winds are produced
by a local pressure
gradient.
Valley Breezes: blows toward the mountains during the day.
In the day, air in the mountains is heated more intensely. It
rises up and draws more air up the valley, this generates a
valley breeze.
LOCAL WINDS
Mountain Breeze:
blows from the
mountains after
sunset. After dark,
cooling of the air
in the mountains
can result in a cool
air drainage into
the valley. This
generates a
mountain breeze.
LOCAL WINDS
Chinook winds:
warm, dry, gusty
winds that descend
down the leeward
side of mountain
slopes.
Sea and Land Breezes: The land heats up quickly in the day and
cools fast at night. This generates sea breezes in the day and land
breezes at night.
Questions:
1. Why doesn’t your table collapse under the great amount of
air pressure above it?
2. If the height of mercury in a mercury barometer is
decreasing, what is happening?
3. What three things affect wind?
4. What is the energy source for most winds?
5. When isobars are close together, how is wind affected?
6. Why do jet streams move parallel to isobars?