Chapter
19
Air Pressure and
Wind
19.1 Understanding Air Pressure
Air Pressure Defined
 Air pressure is the pressure exerted by the
weight of air.
 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.
 Watch this can crushing example of air
pressure. air pressure is a monster
19.1 Understanding Air Pressure
Measuring Air Pressure
 A barometer is a 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.
 This guy can show you how a barometer
works and has a cool voice.
A Mercury Barometer
19.1 Understanding Air Pressure
Factors Affecting Wind

If you’d rather HEAR this information continue to the next slide.
 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
Click the link to hear it rather than
read it!
19.1 Understanding Air Pressure
Factors Affecting Wind
 Pressure Differences
• A pressure gradient is the amount of pressure
change occurring over a given distance.
• Closely spaced isobars—lines on a map that
connect places of equal air pressure—indicate a
steep pressure gradient and high winds. Widely
spaced isobars indicate a weak pressure
gradient and light winds.
Isobars
-Complete the activities on the following few slides.
-Answer the questions on the back of your notes.
-You must exit out of presentation mode to draw the lines/arrows.
-Once you are donE with the activities show Ms. Joseph. DO NOT SAVE!
Pressure Activity
Introduction:
Though you may not realize it, the air has weight. All the air molecules in the atmosphere exert a force, or pressure, on our
bodies. Atmospheric pressure is the force exerted by the weight of the air above an object or surface. Variations in pressure
generate winds, which play a significant role in day to day weather conditions. The purpose of this activity is to introduce
characteristics of pressure, high and low pressure centers, and a brief analysis of an idealized pressure field. Key words
throughout this activity link directly to helper resources that provide useful information for answering the questions.
Characteristics of Pressure:
•What are the different units of pressure? Which unit is used most by meteorologists?
•Circle the correct response in the following sentence: Pressure (increases / decreases) with height. Please explain why
pressure changes this way with height.
Analysis of a Pressure Field
3) The diagram below is an idealized pressure field resembling those commonly found on surface weather maps. The
numbers along each contour indicate the pressure value in millibars for that particular contour. Use the diagram below to
answer the following questions.
•What are the green contours? What do they represent?
•Draw an arrow (on the diagram above) to indicate the direction in which the pressure gradient force is pointing.
Examine the Current Pressure Field:
•Using what you know about pressure gradients and wind… label the windiest areas with a W then use a C to label areas
with calm winds.
DRAWING ISOBARS ON A WEATHER MAP
•Surface weather maps have isobars drawn around high and low pressure systems. (Isobars are lines of equal pressure).
Here are the rules.
Try to figure out the isobars for the map below with data from Nov 28, 2000, when 5-11 inches of snow fell in the FargoMoorhead area due to a low pressure center Southeast of town. Drawing the isobars should reveal the location of the lowpressure center as well as regions of higher pressure. Label your isobars as 1000mb, 1004mb, 1008mb, 1012mb, 1016mb,
1020mb, and 1024mb (4millibar isobaric interval).
How can drawing isobars and isotherms be useful in the real world?
Drawing isobars (lines of equal pressure) enable a person to identify locations of high and low pressure. Also, areas where
there is steep pressure gradient (resulting in high wind speeds) can be located. This information is obviously useful to
meteorologists for the purpose of forecasting cloud cover, wind speed, etc.
19.1 Understanding Air Pressure
Factors Affecting Wind
 Coriolis Effect
• The Coriolis effect describes how Earth’s rotation affects
moving objects. In the Northern Hemisphere, all freemoving objects or fluids, including the wind, are deflected
to the right of their path of motion. In the Southern
Hemisphere, they are deflected to the left.
• The coriolis effect can be a difficult concept to understand.
To get more information about the coriolis effect, click on
the link below.
• Understand the Coriolis Effect
The Coriolis Effect
Use the cards, scissors, markers, etc. at your lab station.
Follow Dr. C as he helps you understand Coriolis Effect.
19.1 Understanding Air Pressure
Factors Affecting Wind
 Friction
• Friction acts to slow air movement, which
changes wind direction.
• Jet streams are fast-moving rivers of air that
travel between 120 and 240 kilometers per hour
in a west-to-east direction.
How do Jet Streams Work???
More about jet streams
Even more about jet streams in the United States
Effect of Friction
19.2 Pressure Centers and Winds
Highs and Lows
 Cyclones are centers of low pressure.
 Anticyclones are centers of high pressure.
 In cyclones, the pressure decreases from
the outer isobars toward the center. In
anticyclones, just the opposite is the case—
the values of the isobars increase from the
outside toward the center.
19.2 Pressure Centers and Winds
Highs and Lows
 Cyclonic and Anticyclonic Winds
• When the pressure gradient and the Coriolis
effect are applied to pressure centers in the
Northern Hemisphere, winds blow
counterclockwise around a low. Around a high,
they blow clockwise.
• In either hemisphere, friction causes a net flow
of air inward around a cyclone and a net flow of
air outward around an anticyclone.
Cyclonic and Anticyclonic Winds
19.2 Pressure Centers and Winds
Highs and Lows
 Weather and Air Pressure
• Rising air is associated with cloud formation and
precipitation, whereas sinking air produces clear
skies.
 Weather Forecasting
• Weather reports emphasize the locations and
possible paths of cyclones and anticyclones.
• Low-pressure centers can produce bad weather
in any season.
Airflow Patterns, Surface and Aloft
LOW =
Convergence
HIGH =
Divergence
• Lows move west to east.
• Their paths can be somewhat unpredictable, making accurate estimation of their
movement difficult.
• It is important to understand total atmosphere circulation….
19.2 Pressure Centers and Winds
Global Winds
 The atmosphere balances these differences
by acting as a giant heat-transfer system.
This system moves warm air toward high
latitudes and cool air toward the equator.
 Non-Rotating Earth Model
• On a hypothetical non-rotating planet with a
smooth surface of either all land or all water, two
large thermally produced cells would form.
Circulation on a Non-Rotating Earth
• Heated air at the
equator would rise until
it reached the
Tropopause.
• Eventually the upperlevel airflow would
reach the poles, sink,
spread out in all
directions at the
surface, and move
back toward the
equator.
19.2 Pressure Centers and Winds
Global Winds
 Rotating Earth Model
• If the effect of rotation were added to the global
circulation model, the two-cell convection system
would break down into smaller cells.
• Near the equator, rising air produces a pressure
Zone known as the equatorial low – a region
characterized by abundant precipitation.
• 20-30 degrees, north or south latitude, the sinking
air and its associated heating due to compression
produce hot, arid conditions – the center of this
zone of sinking dry air is the subtropical high.
• Trade winds are two belts of winds that blow
almost constantly from easterly directions and are
located on the north and south sides of the
subtropical highs.
• Westerlies are the dominant west-to-east motion
of the atmosphere that characterizes the regions
on the poleward side of the subtropical highs.
• Polar easterlies are winds that blow from the
polar high toward the subpolar low. These winds
are not constant like the trade winds.
• A polar front is a stormy frontal zone separating
cold air masses of polar origin from warm air
masses of tropical origin.
Circulation on a Rotating Earth
global air circulation
19.2 Pressure Centers and Winds
Global Winds
 Influence of Continents
• The only truly continuous pressure belt is the
subpolar low in the Southern Hemisphere. In the
Northern Hemisphere, where land masses break
up the ocean surface, large seasonal temperature
differences disrupt the pressure pattern.
• Monsoons are the seasonal reversal of wind
direction associated with large continents,
especially Asia. In winter, the wind blows from land
to sea. In summer, the wind blows from sea to
land.
Surface Pressure
The Intertropical Convergence Zone (ITCZ) is a zone of
unstable air that forms along the equator where the northeast
and southeast trade winds converge.
What are centers of low pressure called?
a. Air masses
b. Anticyclones
c. Cyclones
d. Jet streams
Variations in air pressure from place to
place are the principal caused of
a. clouds.
b. Lows.
c. Hail.
d. Wind.
In the winter, large landmasses often
develop a seasonal
a. high-pressure system
b. Low-pressure system
c. Typhoon
d. Trade wind
What is the pressure zone that is associated
with rising air near the equator?
a. Equatorial low
b. Equatorial high
c. Subtropical low
d. Subtropical high
In what stormy region do the westerlies and
polar easterlies converge?
a. Equatorial low
b. Subpolar high
c. Polar front
d. Subtropical high
19.3 Regional Wind Systems
Local Winds
 The local winds are caused either by
topographic effects or by variations in
surface composition—land and water—in
the immediate area.
 Land and Sea Breezes
• In coastal areas during the warm summer months, the
land surface is heated more intensely during the
daylight hours than an adjacent body of water is
heated. As a result, the air above the land surface
heats, expands, and rises, creating an area of lower
pressure. At night the reverse takes place.
Sea and Land Breezes
19.3 Regional Wind Systems
Local Winds
 Valley and Mountain Breezes
• In mountainous regions during daylight hours,
the air along the slopes of the mountains is
heated more intensely than the air at the same
elevation over the valley floor. Because this
warmer air on the mountain slopes is less
dense, it glides up along the slope and
generates a valley breeze. After sunset the
pattern may reverse.
Valley and Mountain Breezes
19.3 Regional Wind Systems
How Wind Is Measured
 Wind Direction
• The prevailing wind is the wind that 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.
19.3 Regional Wind Systems
How Wind Is Measured
 Wind Speed
• An anemometer is an instrument that resembles
a cup and is commonly used to measure wind
speed.
19.3 Regional Wind Systems
El Niño and La Niña
 El Niño
• El Niño is the name given to the periodic
warming of the ocean that occurs in the central
and eastern Pacific.
• At irregular intervals of three to seven years,
these warm countercurrents become unusually
strong and replace normally cold offshore waters
with warm equatorial waters.
• A major El Niño episode can cause extreme
weather in many parts of the world.
Normal Conditions
El Niño Conditions
19.3 Regional Wind Systems
El Niño and La Niña
 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.
19.3 Regional Wind Systems
Global Distribution of Precipitation
 Global precipitation can be explained if
knowledge of global winds and pressure
systems are applied.
Valley and mountain breezes
are examples of ____.
a.
b.
c.
d.
global winds
trade winds
local winds
jet streams
A sea breeze usually originates
during the ____.
a.
b.
c.
d.
evening and flows toward the land
day and flows toward the land
evening and flows toward water
day and flows toward the water
A land breeze usually originates
during the ____.
a.
b.
c.
d.
evening and flows toward the land
day and flows toward the land
evening and flows toward the water
day and flows toward the water
A wind that consistently blows
more often from one direction
than from any other is called a
____.
a.
b.
c.
d.
local wind
prevailing wind
trade wind
jet stream
Winds are labeled according to
which of the following?
a.
b.
c.
d.
whether they originate on land or water
the direction from which they blow
the region in which they can be found
none of the above
Which phenomenon is associated with
surface temperatures in the eastern
Pacific that are colder than average?
a.
b.
c.
d.
La Niña
El Niño
global warming
local winds
Which of the following is a warm
countercurrent that periodically flows
southward along the coasts of Ecuador
and Peru?
a.
b.
c.
d.
mountain breeze
La Niña
El Niño
none of the above