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558 Chapter 20

20.1 Air Masses

Reading StrategyBuilding Vocabulary Copy the table. Asyou read this section, write a definition foreach of the terms in the table. Refer to thetable as you read the rest of the chapter.

Key ConceptsWhat is an air mass?

What happens as an airmass moves over an area?

How are air massesclassified?

Which air massesinfluence much of theweather in NorthAmerica?

Why do continentaltropical air masses havelittle effect on weather inNorth America?

Vocabulary◆ air mass

Severe storms are among nature’s mostdestructive forces. Every spring, for example,newspapers and newscasts report the damagecaused by tornadoes, which are short but vio-lent windstorms that move quickly over land.The forces associated with these storms canbe incredibly strong, as you can see from thedamage shown in Figure 1. During latesummer and early fall, you have probablyheard reports about severe storms known ashurricanes. Unlike tornadoes, hurricanesform over Earth’s tropical oceans. As theymove toward land, the strong winds andheavy rains produced by these storms candestroy anything in their paths. You are prob-ably most familiar with a type of severe stormknown as a thunderstorm. Thunderstormsare a type of severe weather that produces

heavy rains, loud noises you know as thunder, and flashes of light calledlightning. Before learning more about these different types of violentweather, you will learn about the atmospheric conditions that mostoften affect the day-to-day weather.

Term Definition

Air mass a.

Source region b.

Polar air mass c.

Tropical air mass d.

Continental air mass e.

Maritime air mass f. ?

?

?

?

?

?

Figure 1 Tornado Damage inKansas The force of the windduring a tornado was strongenough to drive a piece of metalinto the utility pole.

558 Chapter 20

FOCUS

Section Objectives20.1 Define air mass.20.2 Explain how air masses are

classified.20.3 Explain the characteristic

features of each air mass class.20.4 Explain the influence of

continental polar and maritimetropical air masses on themajority of North America.

Build VocabularyParaphrase Ask students to write thevocabulary words on a sheet of paper.Instruct students to write a definition, intheir own words, for each term as theyencounter the term while going throughthe chapter. After writing their owndefinition, they should also write acomplete sentence using the term.

Reading Strategya. an immense body of air characterizedby similar temperatures and amounts ofmoisture at any given altitudeb. area over which an air mass gets itscharacteristic properties of temperatureand moisturec. cold air mass that forms at highlatitudes toward Earth’s polesd. warm air mass that forms at lowlatitudese. dry air mass that forms over landf. humid air mass that forms over water

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Reading Focus

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Section 20.1

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Weather Patterns and Severe Storms 559

Air Masses and WeatherFor the many people who live in the middle latitudes, which includemuch of the United States, summer heat waves and winter cold spellsare familiar experiences. During summer heat waves, several days ofhigh temperatures and high humidity often end when a series ofstorms pass through the area. This stormy weather is followed by a fewdays of relatively cool weather. By contrast, winter cold spells are oftencharacterized by periods of frigid temperatures under clear skies. Thesebitter cold periods are usually followed by cloudy, snowy, relativelywarm days that seem mild when compared to those just a day earlier.In both of these situations, periods of fairly constant weather condi-tions are followed by a short period of changes in the weather. What doyou think causes these changes?

Air Masses The weather patterns just described result from move-ments of large bodies of air called air masses. An air mass is animmense body of air that is characterized by similar temperaturesand amounts of moisture at any given altitude. Anair mass can be 1600 kilometers or more across andseveral kilometers thick. Because of its size, it maytake several days for an air mass to move over anarea. This causes the area to experience fairly con-stant weather, a situation often called air-massweather. Some day-to-day variations may occur,but the events will be very unlike those in an adja-cent air mass.

Movement of Air Masses When an airmass moves out of the region over which it formed,it carries its temperature and moisture conditionswith it. An example of the influence of a moving airmass is shown in Figure 2. A cold, dry air mass fromnorthern Canada is shown moving southward. Theinitial temperature of the air mass is �46°C. Itwarms 13 degrees by the time it reaches Winnipeg.The air mass continues to warm as it moves south-ward through the Great Plains and into Mexico.Throughout its southward journey, the air massbecomes warmer. But it also brings some of thecoldest weather of the winter to the places in its path. As it moves,the characteristics of an air mass change and so does the weather inthe area over which the air mass moves.

What is an air mass, and what happens asit moves over an area?

Winnipeg

Sioux FallsOmaha

WichitaOklahoma CityDallas

Houston

Tampico

Cold, dryair mass

–46°C

–33°C

–29°C–23°C

–18°C–15°C

–9°C–4°C

10°C

Figure 2 As a frigid Canadian airmass moves southward, it bringscolder weather to the area overwhich it moves. Computing How much warmerwas the air mass when it reachedTampico, Mexico, than when it formed?

INSTRUCT

Air Masses andWeather

Air Masses in a BottlePurpose Students will observe whatoccurs when hot and cold air massescollide.

Materials 2 wide-mouthed jars of thesame size, matches, small pan, hotwater, ice cubes, flashlight

Procedure Place the hot water in thepan. Place one of the jars in the pan.Using the matches, fill both jars withsmoke. Place the second jar on top ofthe one in the pan. Place the ice cubeson top of the second jar. Darken theclassroom and use the flashlight toobserve the movement of the smokewithin the jars.

Expected Outcomes The hot air willrise in the first jar; the cold air will sink inthe second jarKinesthetic, Visual

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Customize for English Language Learners

Direct students to Figure 3 on p. 560. Beforethey read, have them use the figure to make alist of the four types of air masses impacting

North America. Students should adddefinitions for the terms to the glossary as theyread the section.

Answer to . . .

Figure 2 56°C warmer

An air mass is animmense body of air

characterized by similar temperaturesand amounts of moisture at any givenaltitude. As it moves, the characteristicsof an air mass change and the weatherin the area over which the air massmoves also changes.


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Classifying Air MassesThe area over which an air mass gets its characteristic properties of tem-perature and moisture is called its source region. The source regionsthat produce air masses that influence the weather in North America areshown in Figure 3. Air masses are named according to their sourceregion. Polar (P) air masses form at high latitudes toward Earth’s poles.Air masses that form at low latitudes are tropical (T) air masses. Theterms polar and tropical describe the temperature characteristics of anair mass. Polar air masses are cold, while tropical air masses are warm.

In addition to their overall temperature, air masses are clas-sified according to the surface over which they form. Continental (c)air masses form over land. Maritime (m) air masses form over water.The terms continental and maritime describe the moisture character-istics of the air mass. Continental air masses are likely to be dry.Maritime air masses are humid.

Using this classification scheme, there are four basic types of airmasses. A continental polar (cP) air mass is dry and cool. A continentaltropical (cT) air mass is dry and warm or hot. Maritime polar (mP) andmaritime tropical (mT) air masses both form over water. But a maritimepolar air mass is much colder than a maritime tropical air mass.

MaritimepolarmP

ContinentalpolarcP

MaritimepolarmP

ContinentalpolarcP

Maritimetropical

mT

Maritimetropical

mT

Continentaltropical

cT

Figure 3 Air masses are classifiedby the region over which they form.Interpreting Maps What kindsof air masses influence theweather patterns along the westcoast of the United States?

For: Links on air masses

Visit: www.SciLinks.org

Web Code: cjn-6201

560 Chapter 20

Classifying Air MassesUse VisualsFigure 3 Direct students’ attention tothe map in Figure 3. Ask: What type ofair mass influence the weather in thenortheast? (maritime polar) Thesoutheast? (maritime tropical)Visual

Use CommunityResourcesInvite a meteorologist to speak to theclass about the role that air masses playin the weather in North America. Havestudents ask about the dominant airmasses that influence your local area.Interpersonal

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Section 20.1 (continued)

Maritime polar air originates over cold oceancurrents or high-latitude ocean waters. This airdoes not have as much moisture content asmT air, yet it can produce widespread rain orsnow. This air mass is notorious for producingfog, drizzle, cloudy weather, and long-lastinglight-to-moderate rain. Maritime polar airchanges as it moves over elevated terrain. Onthe windward side of mountain ranges, mP air

can produce an abundance of rain and snow.Once on the lee side of mountains, the mP airmass modifies into a continental air mass.These air masses produce cold fronts, but theair is not as cold as polar or arctic fronts. Theyare often referred to as “Pacific fronts” or“back-door cold fronts.” Maritime polar airoccurs frequently in the Pacific Northwestand to a lesser degree in New England.

Facts and Figures

Download a worksheet on airmasses for students to complete,and find additional teacher supportfrom NSTA SciLinks.



KeyThunder Bay

MN

IO

WI

MI

Marquette

ONTARIO

Rochester

Buffalo

NY

PAOH

IN

IL

Snowbelts

0 100 300 km200

Chardon

Lake-Effect Snowstorms

Figure 4

Location Marquette,Michigan, is southeastof Thunder Bay, Ontario.Identify What type ofair mass influences theweather of these two cities?Infer Which of these citiesreceives more snow in anaverage winter? Why?

Weather in North AmericaMuch of the weather in North America, especially weather east

of the Rocky Mountains, is influenced by continental polar (cP) andmaritime tropical (mT) air masses. The cP air masses begin in north-ern Canada, the interior of Alaska, and the Arctic areas. The mT airmasses most often begin over the warm waters of the Gulf of Mexico,the Caribbean Sea, or the adjacent Atlantic Ocean.

Continental Polar Air Masses Continental polar air massesare uniformly cold and dry in winter and cool and dry in summer. Insummer, cP air masses may bring a few days of relatively coolerweather. In winter, this continental polar air brings the clear skies andcold temperatures you associate with a cold wave.

Continental polar air masses are not, as a rule, associated withheavy precipitation. However, those that cross the Great Lakes duringlate autumn and winter sometimes bring snow to the leeward shores,as shown in Figure 4. These localized storms, which are known as lake-effect snows, make Buffalo and Rochester, New York, among thesnowiest cities in the United States. What causes lake-effect snow?During late autumn and early winter, the difference in temperaturebetween the lakes and adjacent land areas can be large. The tempera-ture contrast can be especially great when a very cold cP air masspushes southward across the lakes. When this occurs, the air gets largequantities of heat and moisture from the relatively warm lake surface.By the time it reaches the opposite shore, the air mass is humid andunstable. Heavy snow, like that shown in Figure 5, is possible.

What causes large amounts of snow to fall on thesouthern and eastern shores of the Great Lakes?

Figure 5 A six-day lake-effectsnowstorm in November 1996dropped a record 175 cm (69 in.)of snow on Chardon, Ohio.

Weather in NorthAmericaBuild Reading LiteracyRefer to p. 334D in Chapter 12, whichprovides the guidelines for outlining.

Outline Have students read thesection. Then, have students use theheadings as major divisions in anoutline. Allow students to refer to theiroutlines when answering the questionsin Section 20.1 Assessment.Visual

AnswersIdentify Continental polar air massesinfluence the weather in this region.

Infer Because it is on the downwindside of Lake Superior, Marquettereceives more snow than ThunderBay does.

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On November 20–23, 2000, Buffalo, NY, andthe surrounding area were hit with a 60-hourlake-effect snowstorm. During the period, thestorm dumped up to 79 cm of snow and wasthe most widespread and significant Novemberlake-effect storm since 1996, when a longerlasting storm dropped about a meter of snow.

The November 2000 storm had frequentlightning as snow showers grew heavy. Snow

fell at the rate of 5–10 cm per hour for severalhours. The timing of the most intense snowfallcould not have been worse. It hit just beforethe evening commute. Thousands werereported to have spent the night in their carsor to have taken shelter in stores and hotels.Many schoolchildren and school buses becametrapped. It was the most disruptive storm inthe Buffalo area since the blizzard of 1977.

Facts and Figures

Answer to . . .

Figure 3 maritime polar (mP) andmaritime tropical (mT) air masses

Continental polar airmasses, crossing the

Great Lakes, cause heavy lake-effectsnows.

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Maritime Tropical Air Masses Maritimetropical air masses also play a dominant role in theweather of North America. These air masses arewarm, loaded with moisture, and usually unstable.Maritime tropical air is the source of much, if notmost, of the precipitation received in the eastern twothirds of the United States. The heavy precipitationshown in Figure 6 is the result of maritime tropicalair masses moving through the area. In summer,when an mT air mass invades the central and east-ern United States, it brings the high temperaturesand oppressive humidity typically associated withits source region.

Maritime Polar Air Masses During the winter, maritimepolar air masses that affect weather in North America come from theNorth Pacific. Such air masses often begin as cP air masses in Siberia.The cold, dry continental polar air changes into relatively mild, humid,unstable maritime polar air during its long journey across the NorthPacific, as shown in Figure 7. As this maritime polar air arrives at thewestern shore of North America, it is often accompanied by low cloudsand showers. When this maritime polar air advances inland againstthe western mountains, uplift of the air produces heavy rain or snowon the windward slopes of the mountains.

Maritime polar air masses also originate in the North Atlantic offthe coast of eastern Canada. These air masses influence the weather ofthe northeastern United States. In winter, when New England is onthe northern or northwestern side of a passing low-pressure center,the counterclockwise winds draw in maritime polar air. The result isa storm characterized by snow and cold temperatures, known locallyas a nor’easter.

Figure 7 During winter, maritimepolar (mP) air masses in thenorthern Pacific Ocean usuallybegin as continental polar (cP) air masses in Siberia. Inferring What happens to themP air masses as they cross the Pacific?

mPCool, moist,

unstable

cPCold, dry,

stable

Modified cPCold, dry, stable

Figure 6 Rain Storm overFlorida Bay in the Florida Keys

562 Chapter 20

Build Science SkillsUsing Tables and Graphs Havestudents create a table to compareand contrast the four basic types ofair masses.Intrapersonal, Verbal

Build Reading LiteracyRefer to p. 124D in Chapter 5, whichprovides the guidelines for this strategy.

Summarize Have students summarizewhat they have learned in this sectionby listing the characteristics of each typeof air mass. Ask them to create a two-column chart with the column headings“Air mass type” and “Characteristics.”(You may alternatively create a chart onthe board to make this an interactiveclass activity.) Make sure studentsdescribe four air masses: cP, cT, mP,and mT.Portfolio, Group

Use CommunityResourcesInvite students to find out what types ofair masses commonly affect their region.Encourage them to consult periodicalsat their local library. If their sources donot explicitly mention a specific type ofair mass, have them record temperatureand precipitation data. Then lead adiscussion about what air masses arelikely to cause such conditions.Verbal, Group

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Section 20.1 (continued)

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Section 20.1 Assessment

Reviewing Concepts1. What is an air mass?

2. What happens as an air mass moves overan area?

3. How are air masses classified?

4. Which types of air masses have thegreatest effect on weather in North America?

5. Why do continental tropical air masseshave little effect on weather in North America?

Critical Thinking6. Comparing and Contrasting Compare

and contrast the four types of air masses.

7. Explaining Explain which type of air masscould offer relief from a scorching summerto the Midwestern United States. Justifyyour choice.

8. Applying Concepts How can continentalpolar air be responsible for lake-effectsnowstorms in the Great Lakes region?

9. Identifying Look again at Figure 3. Whatkinds of air masses influence the weatherpatterns over Florida?

10. Synthesizing What kind of weather couldbe expected in southern Canada if an mT airmass was to invade the region in mid-July?


Explanatory Paragraph Pick one of theair masses shown in Figure 3 that affectsthe weather in your area. Write a para-graph that explains the weather typicallyassociated with the air mass in both thesummer and the winter.

Continental Tropical Air Masses Continental tropical airmasses have the least influence on the weather of North America.These hot, dry air masses begin in the southwestern United States andMexico during the summer. Only occasionally do cT air massesaffect the weather outside their source regions. However, when a cTair mass does move from its source region, it can cause extremely hot,droughtlike conditions in the Great Plains in the summer. Movementof such air masses in the fall results in mild weather in the Great Lakesregion, often called Indian summer. Conditions during Indian summerare unseasonably warm and mild, as shown in Figure 8.

Figure 8 A cT air mass producesa few days of warm weather amidthe cool days of fall in the GreatLakes region.

ASSESSEvaluateUnderstandingHave each student write a paragraphexplaining the term air-mass weather.(Answers should include the fact thatweather is a result of moving air masses.Because air masses are so huge it usuallytakes several days for them to move overan area. This causes fairly constantweather, known as air mass weather.)

ReteachUse Figure 3 to review the classificationof air masses.

Answers will depend on students’choices of air masses. Continental polarair masses bring clear skies and coldtemperatures in winter and relativelycool, dry days in summer. Maritimetropical air masses bring high temper-atures and much humidity in summerand much precipitation year round.Continental tropical air masses affectonly the southwestern United States andresult in dry, warm weather in theirsource region. Maritime polar massesoften bring low clouds and showersin summer and snow and coldtemperatures in winter.

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5. Such air masses rarely move from theirsource regions.6. They are similar in that each influencesweather in North America. They differ inthat continental air masses form over landand thus are dry. Maritime air masses formover water and thus are wet. Polar airmasses are cold, while tropical air massesare warm.7. A continental polar (cP) air mass is cooland dry and is usually associated with highpressure and clear skies. Such an air mass

Section 20.1 Assessment

1. An air mass is an immense body of aircharacterized by similar temperatures andamounts of moisture at any given altitude.2. The air mass changes the weather in thearea over which it moves.3. Air masses are classified by temperature(polar or tropical) and the surface (conti-nental or maritime) over which they form.4. continental polar and maritime tropical airmasses

Answer to . . .

Figure 7 The cP air mass acquiresmoisture as it slowly moves over theocean to become an mP air mass.

would offer relief from hot summer weather.8. Although cP air masses are cold and dry,they acquire moisture as they cross the rela-tively warm lakes. The addition of moistureand the increase in temperature make theair masses unstable, causing snow to falldownwind of the lakes.9. maritime tropical10. Oppressively hot and humid weathertypical of the source region of the air masswould occur in southern Canada.


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