Chapter

19Air Pressure and Wind

Air Pressure Defined

19.1 Understanding Air Pressure

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

Measuring Air Pressure

19.1 Understanding 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

Factors Affecting Wind

19.1 Understanding Air Pressure

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!

Factors Affecting Wind

19.1 Understanding Air Pressure

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 Fargo-Moorhead area due to a low pressure center Southeast of town. Drawing the isobars should reveal the location of the low-pressure 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.

Factors Affecting Wind

19.1 Understanding Air Pressure

Coriolis Effect• The Coriolis effect describes how Earth’s rotation affects

moving objects. In the Northern Hemisphere, all free-moving 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.

Factors Affecting Wind

19.1 Understanding Air Pressure

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 streamsEven more about jet streams in the United States

Effect of Friction

Highs and Lows

19.2 Pressure Centers and Winds

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.

Cyclones are centers of low pressure. Anticyclones are centers of high pressure.

Highs and Lows

19.2 Pressure Centers and Winds

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

Highs and Lows

19.2 Pressure Centers and Winds

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

• 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….

LOW =Convergence

HIGH =Divergence

Global Winds

19.2 Pressure Centers and 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 upper-level airflow would reach the poles, sink, spread out in all directions at the surface, and move back toward the equator.

Global Winds

19.2 Pressure Centers and 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 pressureZone 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

Global Winds

19.2 Pressure Centers and 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 massesb. Anticyclonesc. Cyclonesd. 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 systemb. Low-pressure systemc. Typhoond. Trade wind

What is the pressure zone that is associated with rising air near the equator?

a. Equatorial lowb. Equatorial highc. Subtropical lowd. Subtropical high

In what stormy region do the westerlies and polar easterlies converge?

a. Equatorial lowb. Subpolar highc. Polar frontd. Subtropical high

Local Winds

19.3 Regional Wind Systems

The local winds are caused either by topographic effects or by variations in surface composition—land and water—in the immediate area.

• 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.

Land and Sea Breezes

Sea and Land Breezes

Local Winds

19.3 Regional Wind Systems

• 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

Valley and Mountain Breezes

How Wind Is Measured

19.3 Regional Wind Systems

• The prevailing wind is the wind that blows more often from one direction than from any other.

Wind Direction

• In the United States, the westerlies consistently move weather from west to east across the continent.

How Wind Is Measured

19.3 Regional Wind Systems

• An anemometer is an instrument that resembles a cup and is commonly used to measure wind speed.

Wind Speed

El Niño and La Niña

19.3 Regional Wind Systems

• At irregular intervals of three to seven years, these warm countercurrents become unusually strong and replace normally cold offshore waters with warm equatorial waters.

El Niño• El Niño is the name given to the periodic

warming of the ocean that occurs in the central and eastern Pacific.

• A major El Niño episode can cause extreme weather in many parts of the world.

Normal Conditions

El Niño Conditions

El Niño and La Niña

19.3 Regional Wind Systems

• 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.

La Niña

Global Distribution of Precipitation

19.3 Regional Wind Systems

Global precipitation can be explained if knowledge of global winds and pressure systems are applied.

Valley and mountain breezes are examples of ____.

a. global winds

b. trade winds

c. local winds

d. jet streams

A sea breeze usually originates during the ____.

a. evening and flows toward the land

b. day and flows toward the land

c. evening and flows toward water

d. day and flows toward the water

A land breeze usually originates during the ____.

a. evening and flows toward the land

b. day and flows toward the land

c. evening and flows toward the water

d. day and flows toward the water

A wind that consistently blows more often from one direction than from any other is called a ____.

a. local wind

b. prevailing wind

c. trade wind

d. jet stream

Winds are labeled according to which of the following?

a. whether they originate on land or water

b. the direction from which they blow

c. the region in which they can be found

d. none of the above

Which phenomenon is associated with surface temperatures in the eastern Pacific that are colder than average?

a. La Niña

b. El Niño

c. global warming

d. local winds

Which of the following is a warm countercurrent that periodically flows southward along the coasts of Ecuador and Peru?

a. mountain breeze

b. La Niña

c. El Niño

d. none of the above