Chapter 19: Air Pressure and Wind

Air pressure is simply the pressure exerted by
the weight of air above
Air pressure is exerted in all directions—
down, up, and sideways
The air pressure pushing down on an object
exactly balances the air pressure pushing up
on the object
Barometer – device
used for measuring
air pressure
When air pressure
increases, the
mercury in the tube
rises; when air
pressure decreases,
so does the height of
the mercury column
Scientists will often
use the more
portable aneroid
barometer to record
changes over time of
air pressure
Wind is the result of horizontal differences in air
Air flows from areas of higher pressure to areas of
lower pressure
Wind is nature’s way of balancing out pressure
The unequal heating of Earth’s surface generates
pressure differences
Solar radiation is the ultimate energy source for most
If Earth did not rotate, and if there were no friction
between moving air and Earth’s surface, air would
flow in a straight line from high to low pressure areas
Three factors combine to control wind: pressure
differences, the Coriolis effect, and friction
Wind is created from differences in pressure—the greater these
differences are, the greater the wind speed is
Over Earth’s surface, variations in air pressure are determined
from barometric readings taken at hundreds of weather stations
Pressure Gradient – the amount of pressure change occurring
over a given distance
Equal pressures are connected on a map using isobars
Closely spaced isobars indicate a steep pressure gradient and
high winds
Widely spaced isobars indicate a weak pressure gradient and
light winds
The pressure gradient is the driving force of wind
Coriolis Effect – the apparent
deflective force of earth’s
rotation on all free-moving
All free-moving objects or
fluids, including the wind,
are deflected to the right of
their path in the Northern
Hemisphere; in the Southern
Hemisphere, they are
deflected to the left
This deflection: 1) is always
directed at right angles to
the direction of airflow; 2)
affects only wind direction
and not wind speed; 3) is
affected by wind speed—the
stronger the wind, the
greater the deflection; and 4)
is strongest at the poles and
weakens toward the equator,
becoming nonexistent at the
The effect of friction on wind is
important only within a few
kilometers of Earth’s surface
Friction acts to slow air
movement, which changes wind
When air is above the friction
layer, the pressure gradient
causes air to move across isobars
The most prominent features of
airflow above the friction layer
are jet streams
Jet Streams – fast-moving rivers
of air that travel between 120 to
240 kilometers per hour in a
west-to-east direction
The roughness of the terrain
determines the angle of airflow
across the isobars; the smoother
the terrain, the smaller the angle
of airflow
Slower wind speeds caused by
friction decrease the Coriolis
Cyclones – centers of low
Anticyclones – centers of high
In cyclones, pressure decreases
from the outer isobars toward the
In anticyclones, the values of the
isobars increase from the outside
toward the center
When the pressure gradient and
the Coriolis effect are applied to
pressure centers in the Northern
Hemisphere, wind blows
counterclockwise around a low
and clockwise around a high
In either hemisphere, friction
causes a net flow of air inward
around a cyclone and a net flow
outward around an anticyclone
The usual “villain” in weather
reports is the low-pressure center
The underlying cause
of wind is the unequal
heating of Earth’s
The atmosphere
balances these
differences by acting
as a giant heattransfer system
The system
(atmosphere) moves
warm air toward high
latitudes and cool air
toward the equator
Trade Winds – two belts
of winds that blow
almost constantly from
easterly directions and
are located on the north
and south sides of
subtropical highs
Westerlies – dominant
west-to-east motion of
the atmosphere that
characterizes the regions
on the poleward side of
the subtropical highs
Polar Easterlies – winds
that blow from the polar
high toward the subpolar
Polar Front – stormy
frontal zone separating
cold air masses of polar
origin from warm air
masses of tropical origin
Where landmasses break
up the ocean surface, large
seasonal temperature
differences disrupt the
global pattern of pressure
zones in the atmosphere
Large landmasses can
become cold in the winter
when a seasonal highpressure system develops,
and the surface airflow will
be directed off the land
Monsoons – seasonal
reversals of wind direction
associated with large
continents, especially Asia;
in the winter, the wind
blows from land to sea,
and in the summer, the
wind blows from sea to
The local winds are caused either by topographic
effects or by variations in surface composition—
land and water—in the immediate area
During warm summer months, coastal land is
heated more intensely than the water, producing
an area of low pressure which the cooler ocean
air moves into fill, creating a breeze in the
afternoon (sea breeze); at night the reverse may
take place (land breeze)
The same happens in the mountains, with the air
from the valley coming up to replace the air from
the mountain slopes (valley breeze); at night the
reverse takes place (mountain breeze)
Sea and Land
Two basic wind
measurements—direction and
speed—are especially
important to the meteorologist
Winds are always labeled by
the direction from which they
The wind vane is the
instrument used to determine
wind direction
Prevailing Wind – when wind
consistently blows more often
from one direction than from
any other
In the United States, the
westerlies consistently move
weather from west to east
across the continent
Anemometer – device to
measure wind speed
At irregular intervals of three to seven years, the
warm countercurrents, along the coasts of Peru and
Ecuador, become unusually strong and replace
normally cold offshore waters with warm equatorial
waters – El Niño
The warm waters block the nutrients from reaching
the surface waters, causing many fish to die off, and
greatly affects the fishing industries of Peru and
Some inland areas that are normally arid get an
abnormal amount of rain, increasing their crop
These episodes mostly effect the eastern tropical
Pacific, but is a part of the global circulation and
affects the weather all over the world
The opposite of El Niño is an atmospheric
phenomenon known as La Niña
Researchers have come to recognize that when
surface temperatures in the eastern Pacific are
colder than average, a La Niña event is triggered
that has a distinctive set of weather patterns
A typical La Niña winter blows colder than normal
air over the Pacific Northwest (with more
precipitation) and the Northern Great Plains
It warms much of the rest of the United States
Areas dominated by the convergent Trade
winds (equatorial low) have mainly rain
forests and abundant precipitation
Areas dominated by the subtropical highpressure cells are regions of extensive
The interiors of large land masses commonly
experience decreased precipitation
You will be able to explain much about global
precipitation through your knowledge of
global winds and pressure systems
Read Chapter 19 (pg. 532-549)
Do Chapter 19 Assessment
◦ #1-29 (pg. 553-554)
◦ # 1-6 (pg. 555)

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