HSES 1eTE C20.qxd 5/17/04 2:10 PM Page 558 Section 20.1 20.1...

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

L2

L2

Reading Focus

1

Section 20.1

HSES_1eTE_C20.qxd 5/17/04 2:10 PM Page 558

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

L2

2


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.


560 Chapter 20

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

L2

L1

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.

L1


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.

HSES_1eTE_C20.qxd 9/29/04 2:38 AM Page 561

562 Chapter 20

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

L2

L1

L2



Section 20.1 Assessment

Reviewing Concepts1. What is an air mass?2. What happens as an air mass moves over

an area?

3. How are air masses classified?4. Which types of air masses have the

greatest 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.

L1

L2

3


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


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.


20.2 Fronts

Reading StrategyOutlining As you read, make an outline likethe one below. Include information abouthow each of the weather fronts discussed inthis section forms and the weather associatedwith each.

Key ConceptsWhat happens when twoair masses meet?

How is a warm frontproduced?

What is a cold front?

What is a stationary front?

What are the stages in theformation of an occludedfront?

What is a middle-latitudecyclone?

What fuels a middle-latitude cyclone?

Vocabulary◆ front◆ warm front◆ cold front◆ stationary front◆ occluded front

Fronts

I. Warm front

A.

B.

II.Cold front

A.

B. ?

?

?

?

Figure 9 Precipitation from aStorm in South Africa

Formation of FrontsRecall that air masses have different temperatures and amounts ofmoisture, depending on their source regions. Recall also that theseproperties can change as an air mass moves over a region. What doyou think happens when two air masses meet? When two airmasses meet, they form a front, which is a boundary that separatestwo air masses. Fronts can form between any two contrasting airmasses. Fronts are often associated with some form of precipitation,such as that shown in Figure 9.

In contrast to the vast sizes of air masses, frontsare narrow. Most weather fronts are between about15 and 200 km wide. Above Earth’s surface, thefrontal surface slopes at a low angle so thatwarmer, less dense air overlies cooler, denser air.In the ideal case, the air masses on both sides of afront move in the same direction and at the samespeed. When this happens, the front acts simply asa barrier that travels with the air masses. In mostcases, however, the distribution of pressure acrossa front causes one air mass to move faster than theother. When this happens, one air mass advancesinto another, and some mixing of air occurs.

564 Chapter 20

564 Chapter 20

FOCUS

Section Objectives20.5 Describe the formation of

a front.20.6 Differentiate among the

formation of a warm front,cold front, stationary front,and occluded front.

20.7 Describe the weather patternsassociated with each type offront.

Build VocabularyWeb Diagram Have studentsconstruct a web diagram of thevocabulary words in this section. Themain concept (fronts) should be at thetop of the diagram. Ask students toprovide one descriptive statement foreach of the other vocabulary words.

Reading StrategyI. A. warm air moves into an area

formerly covered by cooler airB. light-to-moderate precipitation

over wide area for extended timeII. A. cold air moves into region

occupied by warmer airB. heavy downpours and winds

followed by drop in temperatureIII. Stationary front

A. flow of air parallel along frontB. gentle-to-moderate precipitation

IV. Occluded frontA. an active cold front overtakes an

active warm frontB. light precipitation

INSTRUCT

Formation of FrontsUse VisualsFigure 9 Direct students’ attention tothe photograph in the figure. Ask: Whattype of precipitation is falling? (a lightrain) What do you know about thegeneral weather conditions in thisarea based on the photograph?(The temperature is above freezing.)Visual

L1

2

L2

L2

Reading Focus

1

Section 20.2



Cool air

Warm air

Warmfront

Nimbostratus(Ns)

Altostratus(As)

Cirrostratus(Cs)

Cirrus(Ci)

Light to moderate precipitation

Figure 10 Formation of aWarm Front A warm front formswhen warm air glides up over acold, dense air mass. The affectedarea has warmer temperatures,and light to moderateprecipitation.

Types of FrontsFronts are often classified according to the temperature of the advanc-ing front. There are four types of fronts: warm fronts, cold fronts,stationary fronts, and occluded fronts.

Warm Fronts A warm front forms when warm air movesinto an area formerly covered by cooler air. On a weather map, thesurface position of a warm front is shown by a red line with red semi-circles that point toward the cooler air.

The slope of the warm front is very gradual, as shown in Figure 10.As warm air rises, it cools to produce clouds, and frequently precipi-tation. The sequence of clouds shown in Figure 10 typically comesbefore a warm front. The first sign of the approaching warm front isthe appearance of cirrus clouds. As the front nears, cirrus cloudschange into cirrostratus clouds, which blend into denser sheets of alto-stratus clouds. About 300 kilometers ahead of the front, thicker stratusand nimbostratus clouds appear, and rain or snow begins.

Because of their slow rate of movement and very low slope, warmfronts usually produce light-to-moderate precipitation over a large areafor an extended period. A gradual increase in temperature occurs withthe passage of a warm front. The increase is most apparent when a largetemperature difference exists between adjacent air masses. Also, a windshift from the east to the southwest is associated with a warm front.

What causes a warm front to form?

Types of FrontsUse VisualsFigure 10 Have students examine thediagram of a warm front in the figure.Ask: As a warm front approaches,what type of weather can you expect?(rainy weather) Describe how the skyconditions change as a warm frontapproaches. (It becomes increasinglycloudy.)Visual

Build Science SkillsApplying Concepts Students willprocess and remember informationabout fronts if they can connect thisinformation to their daily experiences.First, gather and display current weathermaps. (Many national newspapers havemaps that show notable air massesand fronts. Online resources, such asaccuweather.com, also have currentnational and regional weather maps.)Next, ask students to describe localweather conditions. Then discuss howtheir observations relate to the datapresented in the maps.Verbal, Visual

L2

L1


Customize for English Language Learners

Students who are learning English can benefitfrom real-life examples that relate to sciencecontent. Encourage students to think ofobservations they have made about theweather. For example, ask students if they

notice that after a severe, rather fast-movingthunderstorm, things become cooler the nextfew days. Explain that this is because a coldfront moved into the area, bringingthunderstorms and cooler weather.

Answer to . . .

When warm air movesinto an area formerly

covered by cooler air, a warm frontforms.


566 Chapter 20

Cumulonimbus (Cb)

Cold air

Heavyprecipitation

Warm air

Cold front

Cold Fronts A cold front forms when cold, dense air movesinto a region occupied by warmer air. On a weather map, the surfaceposition of a cold front is shown by a blue line edged with blue trian-gles that point toward the warmer air mass.

Figure 11 shows how a cold front develops. As this cold frontmoves, it becomes steeper. On average, cold fronts are about twice assteep as warm fronts and advance more rapidly than warm fronts do.These two differences—rate of movement and steepness of slope—account for the more violent weather associated with a cold front.

The forceful lifting of air along a cold front can lead to heavydownpours and gusty winds. As a cold front approaches, toweringclouds often can be seen in the distance. Once the cold front haspassed, temperatures drop and wind shifts. The weather behind a coldfront is dominated by a cold air mass. So, weather clears soon after acold front passes. When a cold front moves over a warm area, lowcumulus or stratocumulus clouds may form behind the front.

Stationary Fronts Occasionally, the flow of air on either side of afront is neither toward the cold air mass nor toward the warm air mass,but almost parallel to the line of the front. In such cases, the surfaceposition of the front does not move, and a stationary front forms. Ona weather map, stationary fronts are shown by blue triangles on one sideof the front and red semicircles on the other. Sometimes, gentle to mod-erate precipitation occurs along a stationary front.

How are cold fronts different from warm fronts?

Figure 11 Formation of a ColdFront A cold front forms whencold air moves into an areaoccupied by warmer air. Theaffected area experiencesthunderstorms if the warm air is unstable.

566 Chapter 20

Build Science SkillsUsing Models Providestudents with red andblue clay. The red claywill represent a warmair mass, and the blue clay will representa colder air mass. Ask students to usethe clay to model one of the four airmasses discussed in the text. Theyshould then present their model to theclass, explaining the motion of each airmass.Kinesthetic, Interpersonal

Students may think that the terms warmfront and cold front are actually referringto masses of warm air and cold air. It isimportant that they realize that warmand cold are comparative terms. Forexample, a warm front may pass thoughIowa in the middle of February. Thiswarm air mass may have a temperatureof �26°C when the air mass already inplace has a temperature of �32°C.Death Valley in the middle of July mayhave a cold front move through thatbrings temperatures of 46°C, while theair in place may have a temperature of49°C. Emphasize to students that awarm front brings warmer air to an areaand a cold front brings cooler air.Verbal

L2

L2




Occluded Fronts When an activecold front overtakes a warm front, anoccluded front forms. As you can see in Figure12, an occluded front develops as the advanc-ing cold air wedges the warm front upward.The weather associated with an occluded frontis generally complex. Most precipitation isassociated with the warm air’s being forcedupward. When conditions are suitable, how-ever, the newly formed front is capable ofmaking light precipitation of its own.

It is important to note that the descriptionsof weather associated with fronts are generaldescriptions. The weather along any individualfront may or may not conform to the idealizeddescriptions you’ve read about. Fronts, like allaspects of nature, do not always behave as wewould expect.

Middle-Latitude CyclonesNow that you know about air masses and whathappens when they meet, you’re ready to applythis information to understanding weatherpatterns in the United States. The mainweather producers in the country are middle-latitude cyclones. On weather maps, theselow-pressure areas are shown by the letter L.

Middle-latitude cyclones are largecenters of low pressure that generally travelfrom west to east and cause stormy weather.The air in these weather systems moves in acounterclockwise direction and in toward thecenter of the low. Most middle-latitudecyclones have a cold front, and frequently awarm front, extending from the central area.Forceful lifting causes the formation of cloudsthat drop abundant precipitation.

How do cyclones develop and form? Thefirst stage is the development of a front, whichis shown in Figure 14A on page 569. The frontforms as two air masses with different temper-atures move in opposite directions. Over time,the front takes on a wave shape, as shown inFigure 14B. The wave is usually hundreds ofkilometers long.

Warm air

Cold front Warm front

Occluded front

Occluded front

Cold air

Warm air

Cool air

Cold air

Warmair

Heavyrain

Cool airCold air

Light rain

Cool air

A cold front moves toward a warm front, forcing warmair aloft.

A cold front merges with the warm front to form anoccluded front that drops heavy rains.

Because occluded fronts often move slowly, lightprecipitation can fall for several days.

Formation of an Occluded Front

Figure 12 An occluded front forms when acold front overtakes a warm front, producinga complex weather pattern.

A

B

C

Middle-LatitudeCyclonesBuild Reading LiteracyRefer to p. 556D, which provides theguidelines for active comprehension.

Active Comprehension Instructstudents to read the first two paragraphsof Middle-Latitude Cyclones on p. 567.Ask: What more would you like toknow about middle-latitude cyclonesor Where does the weather in yourarea come from? You will need to makeconnections for students between theweather and their lives. For example,students may notice from weatherreports on television or on the Internetthat stormy weather seems to movefrom the west to the east. Write downseveral students’ responses on theboard. Have students continue readingthe section and examine Figure 14 onp. 569. While reading, have studentsconsider the questions they had aboutthe material. Have students discuss thesection content, making sure that eachquestion raised at the beginning isanswered or that students know whereto look for the answer.Verbal

L1


Answer to . . .

Cold fronts are muchsteeper than warm

fronts and advance more rapidly thanwarm fronts do, causing more violentweather to form.

One-Eyed Storms Have students considerthe word Cyclops. These giants of Greekmythology had a single eye in the middleof their foreheads. This word comes fromthe Greek word Kykl-ops, which means

“round-eyed.” The word cyclone has a similarmeaning. It comes from the Greek wordkykloein, which means “to circle around.”Students can think of cyclones as one-eyedmonsters.

Facts and Figures


568 Chapter 20

As the wave develops, warm air moves towards Earth’s poles. Thereit invades the area formerly occupied by colder air. Meanwhile, coldair moves toward the equator. This change in airflow near the surfaceis accompanied by a change in pressure. The result is airflow in a coun-terclockwise direction, as Figure 14C shows.

Recall that a cold front advancesfaster than a warm front. Whenthis occurs in the development ofa middle-latitude cyclone, the coldfront closes in and eventually liftsthe warm front, as Figure 14Dshows. This process, which isknown as occlusion, forms theoccluded front shown in Figure14E. As occlusion begins, thestorm often gets stronger. Pressureat the storm’s center falls, andwind speeds increase. In the winter,heavy snowfalls and blizzard-likeconditions are possible duringthis phase of the storm’s evolu-tion. A satellite view of this phaseof a mature cyclone is shown inFigure 13.

As more of the warm air is forced to rise, the amount of pressurechange weakens. In a day or two, the entire warm area is displaced.Only cold air surrounds the cyclone at low levels. The horizontal tem-perature difference that existed between the two air masses is gone. Atthis point, the cyclone has exhausted its source of energy. Frictionslows the airflow near the surface, and the once highly organizedcounterclockwise flow ceases to exist (Figure 14F).

The Role of Airflow AloftAirflow aloft plays an important role in maintaining cyclonic and anti-cyclonic circulation. In fact, these rotating surface wind systems areactually generated by upper-level flow.

Cyclones often exist for a week or longer. For this to happen, sur-face convergence must be offset by outflow somewhere higher in theatmosphere. As long as the spreading out of air high up is equal to orgreater than the surface inflow, the low-pressure system can be sus-tained. More often than not, air high up in the atmosphere fuelsa middle-latitude cyclone.

How do middle-latitude cyclones form and develop?

Figure 13 This is a satellite viewof a mature cyclone over theeastern United States.

568 Chapter 20

Getting to thePoint of FrontsPurpose Students will observe themotion of mid-latitude cyclones.

Materials pencil

Procedure This activity may beperformed as a demonstration or eachstudent can participate individually.Place a pencil between your hands.Explain that your hands representtwo different air masses and the gapbetween them is the front. Slowly moveyour right hand forward and your lefthand backward. This models themovement of the wind in a stationaryfront. Direct students’ attention to themovement of the pencil.

Expected Outcome The pencil shouldmove in a counterclockwise direction.This models the movement of a mid-latitude cyclone.Kinesthetic, Visual

Build Reading LiteracyRefer to p. 186D in Chapter 7, whichprovides the guidelines for this strategy.

Relate Text and Visuals The textdescribes Figure 13 as a “maturecyclone.” Ask students to describewhat has happened to form this maturecyclone. (A cold front has overtakenand lifted up a warm front to form anoccluded front.) Now ask students tocompare Figures 13 and 14. Ask:Which phase of Figure 14 is mostrepresentative of Figure 13? (Both Dand E are acceptable answers, althoughthe occluded front in Figure 13 seems tobe well-developed, making E the betteranswer.) Ask: What weather conditionswould this phase of a cycloneproduce? (high wind speeds andheavy precipitation)Verbal, Visual

L1

L2



The formation of a front sets thestage for a mid-latitude cyclone.

Over time, the front takes on awave shape.

Changes in air flow andpressure result in acounterclockwise flow of air.

Storm track

The cold front closes in on thewarm front to produce anoccluded front.

As the cold front lifts, anoccluded front forms.

Eventually, the cycloneweakens.

Cold

Warm

Cold

Cold

Cold

Cold

Warm

Warm

Warm

Low

Low

Cold

Cold

Cold

Warm

Low

Cold

Cold

Warm

Cold

LLLL

LL

LL

Middle-Latitude Cyclone Model

A B C

D E F


Figure 14 Cyclones have a fairly predictable life cycle.

The Role ofAirflow AloftUse VisualsFigure 14 Instruct students to studythe details of the cyclone in the figure.Ask: In what direction is the airmoving in this cyclone?(counterclockwise)Visual

IntegrateLanguage ArtsCyclones Ask students to studyFigure 14. The development of amiddle-latitude cyclone is complex.Engage students in a discussion aboutthe complexity of this diagram. So muchinformation is conveyed here that oftenstudents may feel that they have notgathered all the information they needfrom it. Challenge them to write adescriptive narrative, explaining theformation of a middle-latitude cyclonein their own words. Students shouldshare their narratives with small groupsto make sure that all the stepshave been covered.Verbal, Interpersonal

L2

L1


Answer to . . .

The first stage is thedevelopment of a front.

Over time, the front takes on a waveshape. Changes in air flow andpressure result in a counterclockwiseflow of air. The cold front eventuallycloses in on the warm front to producean occluded front. As the cold frontlifts, an occluded front forms.Eventually, the cyclone weakens.


570 Chapter 20


Reviewing Concepts1. What happens when two air masses meet?2. How does a warm front form?3. What is a cold front?4. What is a stationary front?5. What are the stages in the formation of an

occluded front?

6. What is a middle-latitude cyclone?7. What causes a middle-latitude cyclone to

sustain itself?

Critical Thinking8. Comparing and Contrasting Compare

and contrast warm fronts and cold fronts.

9. Synthesizing Use Figure 15 and what youknow about Earth’s atmosphere to describethe air movement and pressure conditionsassociated with both cyclones and anticyclones.

Because cyclones bring stormy weather, they have received far moreattention than anticyclones. However, a close relationship exists betweenthese two pressure systems. As shown in Figure 15, the surface air thatfeeds a cyclone generally originates as air flowing out of an anticyclone.As a result, cyclones and anticyclones typically are found next to eachother. Like a cyclone, an anticyclone depends on the flow of air high inthe atmosphere to maintain its circulation. In an anticyclone, air spread-ing out at the surface is balanced by air coming together from high up.

Explanatory Paragraph Write a para-graph to explain this statement: Theformation of an occluded front marksthe beginning of the end of a middle-latitude cyclone.

Jet stream

Convergence Divergence

Flow aloft

Anticyclonicflow

CyclonicflowHH L

Figure 15 Movements of air highin the atmosphere fuel thecyclones and anticyclones nearEarth’s surface. Comparing and ContrastingCompare and contrast themovement of air in cyclones andanticyclones.

570 Chapter 20

ASSESSEvaluateUnderstandingHave each student write a paragraphexplaining the relationship betweenfronts and precipitation. (Answers willvary, but students should mention thatprecipitation is often associated withfronts.)

ReteachReview the fact that the main weatherproducers in the United States aremiddle-latitude cyclones. Use Figure 14as a visual aid to discuss how middle-latitude cyclones work.

An occluded front forms in the latestage of development of a middle-latitude cyclone as fast-moving cold aircatches up to a warm front. The warmerair then is forced aloft and eventuallydissipates. When this warmer air is gone,there is little condensation and thereforelittle precipitation.

L1

L2

3


position of the front does not move, and astationary front forms.5. A cold front moves toward a warm front,forcing warmer air aloft. The cold front mergeswith the warm front to form an occludedfront that drops heavy rains. The heavy rainsare followed by periods of light precipitation.6. A middle-latitude cyclone is a large centerof low pressure that generally travels fromwest to east in the United States and causesstormy weather.7. Airflow aloft plays an important role insustaining a middle-latitude cyclone.


1. When two air masses meet, they form afront, which is a boundary that separatestwo air masses.2. A warm front forms when warmer air movesinto an area formerly covered by cooler air.3. A cold front forms when colder, denser airmoves into a region occupied by warmer air.4. Occasionally, the flow of air on either sideof a front is neither toward the cold air massnor toward the warm air mass, but almostparallel to the line of the front. The surface

8. Both fronts form when two air massesmeet. Warm fronts form when warmer airmoves into a region formerly occupied bycold air. Cold fronts form when colder airactively moves into a region occupied bywarmer air. Warm fronts move more slowlythan cold fronts and have more gradualslopes than cold fronts have.9. Cyclones are fueled by upward airmovement and reduced surface pressure.Anticyclones form as the result of the down-ward movement of air and increased surfacepressure.

Answer to . . .

Figure 15 Both result from changesin surface pressure. Both systemsdepend on the flow of air high in theatmosphere to maintain their circula-tion. Cyclones are fueled by upwardair movement and reduced surfacepressure. Anticyclones form as theresult of the downward movement ofair and increased surface pressure.



20.3 Severe Storms

Severe weather has a fascination that everyday weather does not pro-vide. For example, a thunderstorm with its jagged lightning andbooming thunder can be an awesome sight. The damage and destruc-tion caused by these storms, as well as other severe weather, can also befrightening. A single severe storm can cause billions of dollars in prop-erty damage as well as many deaths. This section discusses three typesof severe storms and their causes.

ThunderstormsHave you ever seen a small whirlwind carry dust or leaves upward on ahot day? Have you observed a bird glide effortlessly skyward on an invis-ible updraft of hot air? If so, you haveobserved the effects of the vertical movementsof relatively warm, unstable air. These exam-ples are caused by a similar thermal instabilitythat occurs during the development of athunderstorm. A thunderstorm is astorm that generates lightning and thunder.Thunderstorms frequently produce gustywinds, heavy rain, and hail. A thunderstormmay be produced by a single cumulonimbuscloud and influence only a small area. Or itmay be associated with clusters of cumu-lonimbus clouds that stretch for kilometersalong a cold front.

Reading StrategyIdentifying Cause and Effect Copy thetable and complete it as you read this section.

Severe Storms

Causes Effects

Thunderstorms a. b.

Tornadoes c. d.

Hurricanes e. f. ??

??

??

Vocabulary◆ thunderstorm◆ tornado◆ hurricane◆ eye wall◆ eye◆ storm surge

Key ConceptsWhat is a thunderstorm?

What causes athunderstorm to form?

What is a tornado?

How does a tornadoform?

What is a hurricane?

How does a hurricaneform?

Figure 16 Lightning is aspectacular and potentiallydangerous feature of athunderstorm.

FOCUS

Section Objectives20.8 Explain the formation of a

thunderstorm.20.9 Describe the conditions

needed for a tornado to form.20.10 Identify the conditions that

must exist for a hurricane toform.

Build VocabularyVenn Diagram Have students createa Venn diagram of hurricanes andtornadoes.

Reading Strategya. warm, humid air rising in an unstableenvironmentb. gusty winds, heavy rain, hailc. associated with thunderstorms andthe development of a mesocycloned. violent windstorm, isolated pathe. water temperatures warm enough toprovide heat and moisture to airf. widespread damage as winds canreach 300 km/h

INSTRUCT

ThunderstormsUse VisualsFigure 16 Ask students to look at thephotograph in Figure 16. Ask: Whattype of clouds is probably in the areain this photograph? (cumulonimbusclouds)Visual

L1

2

L2

L2

Reading Focus

1


Section 20.3


572 Chapter 20

Occurrence of Thunder-storms How common arethunderstorms? Consider thesenumbers.At any given time, thereare an estimated 2000 thunder-storms in progress on Earth. Asyou might expect, the greatestnumber occurs in the tropicswhere warmth, plentiful mois-ture, and instability are commonatmospheric conditions. About45,000 thunderstorms take placeeach day. More than 16 millionoccur annually around the world.The United States experiences

about 100,000 thunderstorms each year, most frequently in Florida andthe eastern Gulf Coast region. Most parts of the country have from 30 to100 storms each year. The western margin of the United States has littlethunderstorm activity because warm, moist, unstable maritime tropicalair seldom penetrates this region.

Development of Thunderstorms Thunderstormsform when warm, humid air rises in an unstable environment. Thedevelopment of a thunderstorm generally involves three stages. Duringthe cumulus stage, shown in Figure 17A, strong updrafts, or upwardmovements of air, supply moist air. Each new surge of warm air riseshigher than the last and causes the cloud to grow vertically.

Usually within an hour of the initial updraft, the mature stagebegins, as shown in Figure 17B. At this point in the development ofthe thunderstorm, the amount and size of the precipitation is too greatfor the updrafts to support. So, heavy precipitation is released fromthe cloud. The mature stage is the most active stage of a thunderstorm.Gusty winds, lightning, heavy precipitation, and sometimes hail areproduced during this stage.

Eventually, downdrafts, or downward movements of air, dominatethroughout the cloud, as shown in Figure 17C. This final stage is calledthe dissipating stage. During this stage, the cooling effect of the fallingprecipitation and the flowing in of colder air from high above cause thestorm to die down.

The life span of a single cumulonimbus cell within a thunderstormis only about an hour or two.As the storm moves, however, fresh suppliesof warm, humid air generate new cells to replace those that are scattering.

Describe the stages in the development of a thunderstorm.

(km

)

15

10

5

0

Cumulus stage

0°C

Mature stage

0°C

Heavy rain

Dissipating stage

Light rain

0°C

(km

)

15

10

5

0

Stages in the Development of a Thunderstorm

Figure 17 A During the cumulusstage, warm, moist air is suppliedto the cloud. B Heavy precipitationfalls during the mature stage. C The cloud begins to evaporateduring the dissipating stage.Observing How do the cloudsinvolved in the development of a thunderstorm vary?

A CB

572 Chapter 20

Students may have heard or noticedin media photographs that manytornadoes seem to hit trailer parks.It may even seem that trailers attracttornadoes. In reality, there are possiblyhundreds of very small tornadoes thattouch down in the United States everyyear, but are not recorded because theydo no damage. However, since a trailerflips over so easily in even the weakesttornado, trailers probably act as “minitornado detectors.” This makes it seemlike tornadoes are attracted to trailers,but that is because trailers are some ofthe only things that reveal the presenceof what would otherwise be anunrecorded event.Logical

L2


Customize for Inclusion Students

Behaviorally Disordered Have studentswork in pairs and use index cards to create aset of flashcards. Students can use the cards tosupport each other in small study groups. Eachcard should contain information about acharacteristic of one of the three types of

storms in this section. One side of the cardshould contain the term thunderstorm, tornado,or hurricane. The other side should have somefact about the formation of each storm, somefactors contributing to each storm, or damagedone by each storm.



Stronger winds

Weaker winds

Spinningalong

horizontalaxis

Thunderstormforming

Updraft

Air inflow

Anvil

Tornado

Clouds overshoottop of thunderstorm

Mesocyclone(3 to 10 kmdiameter)

Figure 18 A mesocyclone canoccur before the formation of atornado. A First, stronger windsaloft cause lower winds to roll.B Updrafts tilt the rolling air sothat it becomes nearly vertical.C When the rotating air iscompletely vertical, themesocyclone is established.

A

B

For: Links on fronts and severeweather

Visit: www.SciLinks.org

Web Code: cjn-6203

TornadoesTornadoes are violent windstorms that take the form of a rotat-

ing column of air called a vortex. The vortex extends downward froma cumulonimbus cloud. Some tornadoes consist of a single vortex.But within many stronger tornadoes, smaller vortexes rotate withinthe main funnel. These smaller vortexes have diameters of only about10 meters and rotate very rapidly. Smaller vortexes explain occasionalobservations of tornado damage in which one building is totallydestroyed, while another one, just 10 or 20 meters away, suffers littledamage.

Occurrence and Development of Tornadoes In theUnited States, about 770 tornadoes are reported each year. These severestorms can occur at any time during the year. However, the frequency oftornadoes is greatest from April through June. In December and January,tornadoes are far less frequent.

Most tornadoes form in association with severe thunder-storms. An important process in the formation of many tornadoes isthe development of a mesocyclone. A mesocyclone is a vertical cylin-der of rotating air that develops in the updraft of a thunderstorm. Theformation of this large vortex begins as strong winds high up in theatmosphere cause winds lower in the atmosphere to roll, as shown inFigure 18A. In Figure 18B, you can see that strong thunderstormupdrafts cause this rolling air to tilt. Once the air is completely verti-cal (Figure 18C), the mesocyclone is well established. The formation ofa mesocyclone does not necessarily mean that a tornado will follow.Few mesocyclones produce tornadoes like the one shown in Figure 19on page 574.

Formation of a Mesocyclone

C

Q What is the most destructivetornado on record?

A The Tri-State Tornado, whichoccurred on March 18, 1925,started in southeastern Missouriand remained on the groundover a distance of 352 kilome-ters, until it reached Indiana.Casualties included 695 peopledead and 2027 injured. Propertylosses were also great, with sev-eral small towns almost totallydestroyed.

Tornadoes

Homemade TornadoPurpose Students will observe a visualmodel of a tornado.

Materials piece of sturdy cardboard;glue; 2 transparency sheets; smallhand-held, battery-operated fan; smallplastic bowl; clear plastic plant dish,approximately 7" in diameter with ahole cut in the middle; water; dry ice

Procedure Glue the plastic bowl to thecenter of the cardboard. Glue half ofone of the transparency sheets to oneside of the bowl. Glue the rest of thesheet in a half circle around the bowlwithout touching the bowl. Glue thesecond sheet to the opposite side of thecup in the same manner. The two sheetsmust overlap but not touch. Pour abouthalf a cup of water in the cup. Usinggloves, add a few small pieces of dry iceto the water. Place the plant dish upsidedown on top of the transparencies. Turnon the fan and place it in the hole,facing up to draw air up.

Expected Outcome Students shouldsee the “smoke” from the dry ice formin a tornado pattern as it flows past thetransparency sheets with the air beingdrawn up.Kinesthetic, Visual

L2


The largest recorded tornado was in the highplains of the Texas panhandle near the town ofGruver on June 9, 1971. At times, the tornadowas nearly 4 km wide, with an average width

of about 2 km. This is probably close to themaximum size for tornadoes, but it is possiblethat larger, unrecorded tornadoes haveoccurred.

Facts and Figures

Answer to . . .

Figure 17 The clouds vary in height,the smallest being the cloud thatinitiates the storm.

During the cumulusstage, strong updrafts

supply moist air that causes the cloudto grow vertically. Usually within aboutan hour of the initial updraft, heavyprecipitation is released from the cloud.Gusty winds, lightning, and sometimeshail also are experienced during thisstage. Eventually, downdrafts domi-nate throughout the cloud and thestorm dies down.

Download a worksheet on fronts andsevere weather for students tocomplete, and find additionalteacher support from NSTA SciLinks.


574 Chapter 20

Tornado Intensity Pressures withinsome tornadoes have been estimated to beas much as 10 percent lower than pres-sures immediately outside the storm. Thelow pressure within a tornado causes airnear the ground to rush into a tornadofrom all directions. As the air streamsinward, it spirals upward around the core.Eventually, the air merges with the airflowof the cumulonimbus cloud that formedthe storm. Because of the tremendousamount of pressure change associatedwith a strong tornado, maximum windscan sometimes approach 480 kilometersper hour. One scale used to estimate tor-nado intensity is the Fujita tornado

intensity scale, shown in Table 1. Because tornado winds cannot bemeasured directly, a rating on this scale is determined by assessing theworst damage produced by a storm.

Tornado Safety The Storm Prediction Center (SPC) located inNorman, Oklahoma, monitors different kinds of severe weather. TheSPC’s mission is to provide timely and accurate forecasts and watchesfor severe thunderstorms and tornadoes. Tornado watches alert peopleto the possibility of tornadoes in a specified area for a particular timeperiod. A tornado warning is issued when a tornado has actually beensighted in an area or is indicated by weather radar.

Figure 19 The tornado shownhere descended from the lowerportion of a mesocyclone in theTexas Panhandle in May, 1996.

Table 1 Fujita Tornado Intensity Scale

Intensity Wind Speed Estimates Typical Damage(kph)

F0 < 116 Light damage. Some damage to chimneys; branches broken off trees;shallow-rooted trees pushed over; sign boards damaged.

F1 116–180 Moderate damage. Peels surface off roofs; mobile homes pushed offfoundations or overturned; moving cars blown off roads.

F2 181–253 Considerable damage. Roofs torn off frame houses; mobile homesdemolished; large trees snapped or uprooted; light-object missilesgenerated; cars lifted off ground.

F3 254–332 Severe damage. Roofs and some walls torn off well-constructed houses;trains overturned; most trees in forest uprooted; heavy cars lifted offthe ground and thrown.

F4 333–419 Devastating damage. Well-constructed houses leveled; structures withweak foundations blown some distance; cars thrown; large missiles generated.

F5 > 419 Incredible damage. Strong frame houses lifted off foundations andcarried away; automobile-sized missiles fly through the air in excessof 100 m; bark torn off trees.

574 Chapter 20

IntegrateSocial StudiesStorm Warnings Accurate stormpredictions and warnings can help tominimize the loss of property and of life.The National Weather Service has createda system to inform the public of thelikelihood of a storm event in theirarea. They use the terms watch andwarning to relay the imminent danger.A hurricane watch means that hurricanesare possible in the area within 36 hours.A hurricane warning means thathurricanes are expected in the areawithin 24 hours. Also a tornado watchmeans that conditions are favorablefor a tornado in the area. A tornadowarning means that a tornado has beensighted or has been seen on radar.Challenge students to devise a planof action that should be taken duringa hurricane watch and warning or atornado watch and warning. Ask themto present their findings to the class inthe form of a poster or an emergencybulletin.Interpersonal

L2


The deadliest tornado in the United Statesoccurred on March 18, 1925. The so-calledTri-State Tornado killed 695 people as it racedalong at 96–117 km/h in a 352 km-long trackacross parts of Missouri, Illinois, and Indiana,

producing F5 damage. This event also holdsthe known record for most tornado fatalitiesin a single city or town: at least 234 atMurphysboro, IL.

Facts and Figures



HurricanesIf you’ve ever been to the tropics or seen photographs of these regions,you know that warm breezes, steady temperatures, and heavy but brieftropical showers are the norm. It is ironic that these tranquil regionssometimes produce the most violent storms on Earth. Whirlingtropical cyclones that produce winds of at least 119 kilometers perhour are known in the United States as hurricanes. In other parts ofthe world, these severe tropical storms are called typhoons, cyclones,and tropical cyclones.

Regardless of the name used to describe them, hurricanes are themost powerful storms on Earth. At sea, they can generate 15-meterwaves capable of destruction hundreds of kilometers away. Should ahurricane hit land, strong winds and extensive flooding can cause bil-lions of dollars in damage and great loss of life. Hurricane Floyd, whichis shown in a satellite image in Figure 20, was one such storm. InSeptember 1999, Floyd brought flooding rains, high winds, and roughseas to a large portion of the Atlantic coast. More than 2.5 millionpeople evacuated their homes. Torrential rains caused devastatinginland flooding. Floyd was the deadliest hurricane to strike the U.S.mainland since Hurricane Agnes in 1972. Most of the deaths caused byHurricane Floyd were the result of drowning from floods.

Hurricanes are becoming a growing threat because more and morepeople are living and working near coasts. At the close of the twenti-eth century, more than 50 percent of the U.S. population lived within75 kilometers of a coast. This number is expected to increase evenmore in the early decades of this century. High population density nearshorelines means that hurricanes and other large storms place millionsof people at risk.

Figure 20 This satellite image ofHurricane Floyd shows its positionoff the coast of Florida a few daysbefore the hurricane moved ontoland. Floyd eventually madelandfall near Cape Fear, NorthCarolina.

Q Why are hurricanes givennames, and who picks thenames?

A Actually, the names are givenonce the storms reach tropical-storm status (winds between61–119 kilometers per hour).Tropical storms are named toprovide ease of communicationbetween forecasters and thegeneral public regarding fore-casts, watches, and warnings.Tropical storms and hurricanescan last a week or longer, andtwo or more storms can beoccurring in the same region atthe same time. Thus, names canreduce the confusion aboutwhat storm is being described.

The World MeteorologicalOrganization creates the lists ofnames. The names for Atlanticstorms are used again at the endof a six-year cycle unless a hurri-cane was particularly destructiveor otherwise noteworthy. Suchnames are retired to preventconfusion when the storms arediscussed in future years.

HurricanesUse VisualsFigure 20 Direct students’ attentionto the satellite image in the figure. Ask:What is the direction of air flow inHurricane Floyd? (counterclockwise)Visual

Use CommunityResourcesInvite students to gather first-handreports of any significant tornadoesor hurricanes in their area. Help themidentify good people to interview,and prepare questions in advance. Forexample: Have there been any especiallysevere storms in this area? When did thisevent occur? What damage did it do?Then have students look for quantitativerecords of the event’s intensity, in termsof the Fujita Scale for tornadoes and theSaffir-Simpson Scale for hurricanes.Interpersonal

L2

L1


Of all the hurricane-prone areas of the UnitedStates, Tampa Bay, FL, is considered one of themost vulnerable to severe flooding, damage,and loss of life in a major hurricane. Thereare several reasons for this. Tampa Bay islocated on a peninsula with long stretches ofwaterfront. This makes the area one of themost densely populated in Florida, and leadsto limited evacuation routes. There is also alarge population of elderly people in the

area. The evacuation of this segment of thepopulation could prove to be difficult foremergency workers. Tampa Bay’s geographycould also increase the effects of a stormsurge; the Gulf of Mexico has a broad, shallowcontinental shelf on which a storm surge couldbuild to heights great enough to destroy ordamage thousands of homes and businesses.The Tampa Bay area has not received a directhit from a major hurricane in several decades.

Facts and Figures


576 Chapter 20

Occurrence of Hurricanes Most hurricanes form betweenabout 5 and 20 degrees north and south latitude. The North Pacifichas the greatest number of storms, averaging 20 per year. The coastalregions of the southern and eastern United States experience fewerthan five hurricanes, on average, per year. Although many tropical dis-turbances develop each year, only a few reach hurricane status. A stormis a hurricane if the spiraling air has winds blowing at speeds of at least119 kilometers per hour.

Development of Hurricanes A hurricane is a heat engine thatis fueled by the energy given off when huge quantities of water vaporcondense. Hurricanes develop most often in the late summerwhen water temperatures are warm enough to provide the necessaryheat and moisture to the air. A hurricane begins as a tropical distur-bance that consists of disorganized clouds and thunderstorms. Lowpressures and little or no rotation are characteristic of these storms.

Occasionally, tropical disturbances become hurricanes. Figure 21shows a cross section of a well-developed hurricane. An inward rushof warm, moist surface air moves toward the core of the storm. The airthen turns upward and rises in a ring of cumulonimbus clouds. Thisdoughnut-shaped wall that surrounds the center of the storm is theeye wall. Here the greatest wind speeds and heaviest rainfall occur.Surrounding the eye wall are curved bands of clouds that trail away

from the center of thestorm. Notice thatnear the top of thehurricane, the risingair is carried awayfrom the storm center.This outflow providesroom for more inwardflow at the surface.

At the very centerof the storm is the eyeof the hurricane. Thiswell-known feature isa zone where precipi-tation ceases andwinds subside. The airwithin the eye gradu-ally descends andheats by compression,making it the warmestpart of the storm.

Figure 21 Cross Section of aHurricane The eye of thehurricane is a zone of relativecalm, unlike the eye wall regionwhere winds and rain are mostintense. Describing Describe the airflowin different parts of a hurricane.

Outflow

Eye

Subsidingair

Surface convergence

Spiralrain bands

576 Chapter 20

Build Science SkillsComparing and Contrasting Askstudents to explore the similarities anddifferences of tornadoes and hurricanesby making a chart. They should includein their chart information on location,associated storms, pressures associatedwith the storm, impact on society, andmaximum wind strength.Logical

Build Reading LiteracyRefer to p. 502D in Chapter 18, whichprovides the guidelines for visualizing.

Visualize Ask students to read thesection under Development ofHurricanes on p. 576. After the firstreading, instruct students to close theireyes and think of a hurricane. Havethem suppose they are flying throughthe clouds of a hurricane and note thechanges in wind velocity and pressure asthey travel from one side to the other.Have students refer to Figure 21 to helpthem visualize their trip.Intrapersonal

L1

L2


Hurricanes in the past were identified usingawkward latitude/longitude methods. Itbecame clear that the use of short, distinctivenames would be quicker and less subject toerror. These advantages are especiallyimportant in exchanging detailed storminformation between hundreds of widelyscattered stations, coastal bases, and ships at

sea. Since 1953, Atlantic tropical storms havebeen named from lists originated by theNational Hurricane Center. The lists featuredonly women’s names until 1979, when men’sand women’s names were alternated. If astorm is deadly or very costly, the name isnever used again.

Facts and Figures



Reviewing Concepts1. What is a thunderstorm?2. What causes a thunderstorm?3. What is a tornado?4. How does a tornado form?5. What is a hurricane?6. How does a hurricane form?

Critical Thinking7. Formulating Hypotheses What kind of

front is associated with the formation oftornadoes? Explain.

8. Synthesizing Explain why a hurricanequickly loses its strength as the storm movesonto land.

Hurricane Intensity The intensity of a hurricane is describedusing the Saffir-Simpson scale shown in Table 2. The most devastatingdamage from a hurricane is caused by storm surges. A storm surge is adome of water about 65 to 80 kilometers wide that sweeps across thecoast where a hurricane’s eye moves onto land.

A hurricane weakens when it moves over cool ocean waters thatcannot supply adequate heat and moisture. Intensity also drops whenstorms move over land because there is not sufficient moisture. Inaddition, friction with the rough land surface causes winds to subside.Finally, if a hurricane reaches a location where the airflow aloft is unfa-vorable, it will die out.


Explanatory Paragraph Examine Tables1 and 2 to contrast the damage caused bytornadoes and hurricanes. Use the data toexplain why even though hurricanes havelower wind speeds, they often cause moredamage than tornadoes do.

Table 2 Saffir-Simpson Hurricane Scale

Category Sustained Wind Speeds Typical Damage (kph)

1 119–153 Storm surge 1.2–1.5 meters; some damage to unanchored mobile homes, shrubbery, and trees; some coastal flooding; minor pier damage.

2 154–177 Storm surge 1.6–2.4 meters; some damage to buildings’ roofs, doors, and windows; considerable damage to mobile homes and piers; moderate coastal flooding.

3 178–209 Storm surge 2.5–3.6 meters; some structural damage to small buildings; some large trees blown over; mobile homes destroyed; some coastal and inland flooding.

4 210–249 Storm surge 3.7–5.4 meters; severe damage to trees and signs; complete destruction of mobile homes; extensive damage to doors and windows; severe flooding inland.

5 > 249 Storm surge >5.4 meters; complete roof failure on many buildings; some complete building failure; all treees and signs blown away; major inland flooding.

ASSESSEvaluateUnderstandingHave students make a game ofconcentration using the terms in thechapter and their definitions. Havegroups of students write each term onseparate index cards and the definitionof each term on a second set of indexcards. To play the game, studentsshould shuffle all the cards togetherand then lay them face down in a grid.Each student takes turns flipping overtwo index cards. If the cards match(definition matches the term), thestudent can remove the cards from thegrid. If the cards do not match, thestudent places the cards face down.After all of the cards are gone, thestudent who has removed the mostcards wins the match.

ReteachUse Figures 17, 18, and 21 as visual aidsto summarize the development of severestorms.

Hurricanes generally inflict moredamage because they are larger andlast longer than tornadoes do.

L1

L2

3


5. A hurricane is a whirling tropical cyclonethat produces wind that can reach 300 km/h.6. A hurricane develops when water tempera-tures are warm enough to provide the neces-sary heat and moisture to the air.7. Tornadoes often form in thunderstormsthat develop along cold fronts because airmasses on either side of the front have verydifferent temperature and moisture conditions.8. There is not sufficient moisture. The roughland surface causes winds to subside.


1. A thunderstorm is a severe storm that gen-erates lightning, thunder, gusty winds, heavyrain, and hail.2. A thunderstorm forms when relativelywarm, humid air rises in an unstableenvironment.3. A tornado is a violent rotating columnof air that extends downward from cumulo-nimbus clouds.4. Most tornadoes form in association withsevere thunderstorms.

Answer to . . .

Figure 21 Air entering the stormfrom the surface is moving counter-clockwise. Air within the eye region isalso moving counterclockwise. Outflow,however, is moving in a clockwisedirection.


Winds and StormsThe world’s atmosphere is forever on the move. Wind,or air in motion, occurs because solar radiation heatsup some parts of the sea and land more than others.Air above these hot spots becomes warmer and lighterthan the surrounding air and therefore rises. Elsewhere,cool air sinks because it is heavier. Winds blow becauseair squeezed out by sinking, cold air is sucked in underrising, warm air. Wind may move slowly as in a gentlebreeze. In extreme weather, wind moves rapidly, creat-ing terrifyingly destructive storms.

How the Earth Works

LIGHTNING AND THUNDERElectricity is discharged from a thundercloud in the form of lightning. A bolt of lightning can heat the air around it to a temperature fourtimes as hot as the sun. The heated air expandsviolently and sends out a rumbling shock wave

that we hear as thunder.

THUNDERSTORMSThunderclouds are formed by

powerful updrafts of air that occuralong cold fronts or over ground heated

very strongly by the sun. Ice crystals andwater droplets high in the cloud are torn apartand smashed together with such ferocity that

they become charged with electricity.Thunderstorms can unleash

thunder, lightning, wind, rain, and hail.

MONSOONSSeasonal winds called monsoons affectlarge areas of the tropics and subtropics.They occur in South Asia, southern NorthAmerica, eastern Australia, and other

regions of the world. In South Asia,southwest monsoons generally

bring desperately needed rainfrom May until October.

Rain clouds

Warm sea

Moistsouthwestmonsoonbringingrain

Himalayas

Rain clouds

Hot land

Indian Ocean

Warm sea

Moistsouthwestmonsoonbringingrain

Drynortheastmonsoon

Coolland Dry

northeastmonsoon

Warm sea

SouthwestMonsoonDuring the earlysummer, the hot,dry lands of Asiadraw in cooler,moist air fromthe Indian Ocean.

NortheastMonsoonThe cold, drywinter air fromCentral Asiabrings chilly,dusty conditionsto South Asia.

578 Chapter 20

578 Chapter 20

FOCUS

ObjectivesIn this feature, students will• explain what causes wind and what

factors affect wind speed.• describe the development of the

different types of storms.• summarize the impact of storms.

Build VocabularyKey Terms Write these key terms onthe board: wind, tornado, blizzard,tropical cyclone, typhoon, storm surge.Ask students to define them. Then havestudents explain what causes wind andwhat factors affect wind speed.

INSTRUCTBellringerHave students read the feature captionheads that name different types ofstorms. Ask: Do any of these kinds ofstorms occur in your region? If so,have students estimate how frequentlythe storms occur. Discuss how locationand regional climate account for thefrequency of storms.Verbal, Logical

L22

L2

Reading Focus

1

Customize for Inclusion Students

Gifted Explain to students that the termtropical cyclones is used to refer to certainhurricanes and typhoons. Have students findout how many tropical cyclones occurred last

year and make a chart with the name, dates,and location (ocean) of each storm. Post thechart in your classroom and discuss whetheror not there is a pattern in the chart.


TORNADOESTornadoes may strikewherever thunderstormsoccur. A tornado beginswhen a column of stronglyrising warm air is set spin-ning by high winds at acloud’s top. A funnel isformed and may touch theground. With winds that can rise above 419 kph, tornadoes can lift people,cars, and buildings highinto the air and thensmash them back tothe ground.

BLIZZARDSIn a blizzard, heavy snowfalland strong winds often makeit impossible to see. Winds pileup huge drifts of snow. Traveland communication can grindto a halt.

IMPACT OF TROPICAL STORMS Tropical storms are often devastating. The strongest winds, with gusts sometimes more than 249 kph,occur at the storm’s center, or eye. When a tropical storm strikes land, raging winds can uproot treesand destroy buildings. Vast areas may be swamped by torrential rain, and coastal regions may be overwhelmed by a storm surge, a wall of water some 8 m high sucked up by the storm’s eye.

A Pacific typhoon struck this ship off the coast of Taiwanin November 2000. Many of the crew members fell victimto the raging sea.

Stage 1: Thunderstorms developover the ocean.

Stage 2: Storms group to forma swirl of cloud.

Stage 3: Winds grow and a distinctcenter forms in the cloud swirl.

Stage 4: Eye forms. The hurricaneis now at its most dangerous.

Stage 5: Eye passes over land.The hurricane starts to weaken.

1. Key Terms Define (a) wind, (b) tornado, (c) blizzard, (d) tropicalcyclone, (e) typhoon, (f) storm surge.

2. Physical Processes How do thunder-storms come into being?

3. Economic Activities (a) How canstorms have a negative impact on economic activities? (b) How can monsoons benefit economic activities?

4. Natural Hazards How can a tropicalcyclone result in the loss of thousandsof lives?

5. Critical Thinking Developing aHypothesis Since 1991, theBangladeshi government has construct-ed hundreds of concrete storm sheltersin coastal regions of the country. (a) Whydo you think the government decidedon this policy? (b) How do you think thepolicy has benefited the country?

HOW TROPICAL STORMS DEVELOPTropical storms begin whenwater evaporates over an oceanin a hot tropical region to pro-duce huge clouds and thunder-storms. When the storms clustertogether and whirl around a low-pressure center, they form a tropical cyclone. Tropicalcyclones with winds of at least119 kph are called hurricanes insome regions and typhoons in other regions. The sequencebelow shows satellite images ofan Atlantic hurricane.

These women wade through the streets ofDhaka, Bangladesh, flooded by a tropicalcyclone. In 1991, a cyclone killed morethan 130,000 Bangladeshis.

579

Integrate Social StudiesAsk students to discuss which kind ofstorm seems most threatening to theircommunity and why. Then discuss thekinds of emergency services that are inplace to help in case of a storm.Logical

ASSESSEvaluateUnderstandingHave students compare and contrastthe development of the different typesof storms described in the feature(monsoons, thunderstorms, tornadoes,blizzards, and tropical cyclones). Thenhave students draw diagrams on theboard that show how these storms form.

ReteachHave students create flash cards forthe key terms wind, tornado, blizzard,tropical cyclone, typhoon, and stormsurge. Encourage them to use their flashcards to review the definitions ofdifferent types of storms.

L1

L2

3

L2


water droplets high in the cloud are tornapart and smashed together so energeticallythat they become charged with electricity.3. (a) During a severe storm, economicactivities may come to a halt. When proper-ty is destroyed, people spend money toreplace or rebuild it. (b) Monsoons bringrain to South Asia, and rain is needed togrow crops.4. Extremely strong winds can destroybuildings, bring torrential rain that resultsin floods, or cause a storm surge. Any ofthese can result in loss of life.

Assessment

1. (a) air in motion; (b) a spinning columnof air with high winds; (c) a storm withheavy snowfall and strong winds; (d) acluster of tropical thunderstorms that whirlaround a low-pressure center; (e) a tropicalcyclone that has winds moving at least 74mph (f) a wall of water sucked up by astorm’s eye2. Thunderstorms are formed by powerfulupdrafts of air that occur along cold frontsor over heated ground. Ice crystals and

5. (a) Bangladesh is located in a region thatis subject to tropical storms. The govern-ment hopes to offer shelter to the peoplewho are most likely to be affected by thestorms. (b) Sample answer: It has providedshelter for people who live in coastal areas.


HSES 1eTE C20.qxd 5/17/04 2:10 PM Page 558 Section 20.1 20.1...

Documents

Transcript of HSES 1eTE C20.qxd 5/17/04 2:10 PM Page 558 Section 20.1 20.1...