Chapter 12: Meteorology · WEATHER AND CLIMATE Atmospheric phenomena, shown in Figure 12-1,interact...

298

What You’ll Learn• What determines global

weather patterns.

• How air masses moveand change.

• How the strengths andweaknesses of weatherforecasts differ.

• How to create a weatherchart.

Why It’s ImportantFew aspects of the envi-ronment have as muchimpact on our everydaydecisions as weather does.A basic knowledge ofweather processes canmake those decisions easier and sometimes far safer.

MeteorologyMeteorology1212

To find out more aboutmeteorology, visit the Earth Science Web Site at earthgeu.com

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12.1 The Causes of Weather 299

An air mass is a large body of airthat takes on the characteristics of thearea over which it forms. You candemonstrate the formation of a coldair mass using simple materials.

1. Place a tray full of ice cubes on atable with a pencil underneatheach end of the tray so that the trayis slightly elevated.

2. Slide a liquid-crystal temperaturestrip underneath the ice-cube tray.

3. Rest another temperature strip on two pencils on top of the tray.

4. Record the temperature of eachstrip at one-minute intervals forabout five minutes.

CAUTION: Always wear protective clothing in the lab.

Observe In your science journal,make a graph showing the tempera-ture changes for each temperaturestrip. What happened to the temperature of the air beneaththe tray and the air abovethe tray? Explain how thismodel represents a coldair mass.

Model a Cold Air MassDiscovery LabDiscovery Lab

OBJECTIVES

• Compare and contrastweather and climate.

• Analyze how imbalancesin the heating of Earth’ssurface create weather.

• Describe how and whereair masses form.

VOCABULARY

meteorologyweatherclimateair massair mass modification

Have you ever sat back and watched the sky on a lazy summer after-noon? You might have noticed clouds of different shapes, or felt thewarmth of the sun against your face or an occasional puff of wind asit cooled your skin. All of these phenomena are part of a highly orga-nized sequence of events with specific causes. Those events and thefactors that cause them are all part of meteorology. Meteorology is thestudy of atmospheric phenomena. The root of the word meteorol-ogy—meteor—is the name given in modern times to a flaming rockfalling through space. But the ancient Greek meaning of meteor was“high in the air,” and it is this meaning that pertains to meteorology.

Clouds, raindrops, snowflakes, fog, dust, and rainbows are alltypes of atmospheric “meteors.” The primary types are clouddroplets and forms of precipitation that contain water in any phase;they are known as hydrometeors. Smoke, haze, dust, and other con-densation nuclei are called lithometeors. Thunder and lightning areexamples of electrometeors, which are visible or audible manifesta-tions of atmospheric electricity. These various phenomena are theobjects and events that meteorologists study.

The Causes of Weather12.112.1

WEATHER AND CLIMATEAtmospheric phenomena, shown in Figure 12-1, interact to affectthe environment and life on Earth. This is basically what we callweather. Weather is the current state of the atmosphere. When wespeak of weather, we are referring mainly to short-term variations inthe atmosphere. These variations can take place over minutes, hours,days, weeks, or months. Long-term variations in weather for a par-ticular area make up the climate of that area. Climate is usually aver-aged over the course of 30 years or more. You’ll learn more aboutclimate in Chapter 14. For now, simply recognize that meteorology,weather, and climate are related. Meteorology is the study of theatmosphere; weather is the current state of the atmosphere, includ-ing short-term variations that affect our lives; and climate describesthe average weather over a long period of time.

A QUESTION OF BALANCEAs you’ve learned, the Sun heats the surface of Earth, and Earth radi-ates back to space about as much energy as it receives over the courseof a year. In meteorology, a crucial question is how that radiation isdistributed around the planet. You know that the Sun feels hotterduring the afternoon, when its rays strike Earth more directly, thanit does in the early morning or evening, when its rays strike Earth at

a low angle. The Sun’s rays are more spread out when theystrike Earth at a low angle, as you’ll see in the MiniLab later inthis chapter. The same amount of energy is spread over alarger area. As shown in Figure 12-2, the solar radiation reach-ing Earth’s surface at the poles is therefore less intense. Thisexplains, in part, why the tropics are warmer than the poles.But why don’t the tropics become steadily warmer if the Sunis always directly overhead? How do regions manage to main-tain fairly constant average temperatures?

300 CHAPTER 12 Meteorology

C

A

B

Figure 12-1 Snowflakes(A), lightning (B), andfog (C) are types ofatmospheric phenomena.

Balancing the Budget The tropics and other places maintainfairly constant average temperatures because heat energy is redistrib-uted around the world. The continual motion of air and water reallo-cates heat energy among Earth’s surface, oceans, and atmosphere andbrings it into balance. Virtually everything that we consider to beweather—every atmospheric motion from the tiniest convection cur-rent to thunderstorms to large-scale weather systems—is part of thisconstant redistribution of Earth’s heat energy.

AIR MASSESIn Chapter 11, you learned that when air over a warm surface, suchas a parking lot, becomes warmer than the surrounding air becauseof conduction, the warm air rises. Now, imagine this same processtaking place over thousands of square kilometers. Imagine that thewarm air remains over this same area for days or weeks. The resultis the formation of an air mass. An air mass is a large body of airthat takes on the characteristics of the area over which it forms.Meteorologists call the region over which an air mass forms thesource region. Air masses form over land or water. Those that form


90°

66.5°

Sun‘srays

30°

0°

Equator

Figure 12-2 The Sun’s raysstrike Earth more directly atthe tropics than they do atthe poles. At the poles, thesame amount of solar radia-tion is spread over a largerarea than at the equator.Therefore, polar regions arenever very warm.

over land have less exposure to largeamounts of moisture, so they are drier thanthose that form over water. Air masses takeon the temperature of the source region, too.

Classifying Air Masses Air masses areclassified exactly as we have alreadydescribed them: according to their sourceregions. The main types of air masses,shown in Figure 12-3, are warm and drycontinental tropical (cT), warm and humidmaritime tropical (mT), cold and dry conti-nental polar (cP), cold and humid maritimepolar (mP), and arctic (A). An arctic airmass is basically the same as a continentalpolar air mass, but much colder. It’s the typeyou may have heard most about because itbrings the most frigid outbreaks of winter.Extremely cold arctic air masses are usuallyassociated with very high pressure as a resultof the massive sinking of cold air over alarge area.

Source Regions All five main types ofair masses can be found in North Americabecause of the continent’s proximity to thesource regions associated with each air mass.Maritime polar air forms over the coldwaters of the North Atlantic and NorthPacific. It primarily affects the West Coast,bringing occasionally heavy rains in winter.Continental polar air forms over the inte-rior of Canada and Alaska and can be quitefrigid in the winter, when nights are long. Inthe summer, however, cP air can bring pleas-ant relief from heat and humidity because ofits cool and relatively dry composition.

The origins of maritime tropical air aretropical and subtropical oceans, such as theCaribbean Sea and the Gulf of Mexico. In thesummer, mT air brings hot, oppressivelyhumid weather to the eastern two-thirds ofthe United States and Canada. The desertSouthwest and Mexico are the source regions


How does the angle of theSun’s rays differ?

Model the angle at which sunlight reachesEarth’s surface. This angle greatly affects the intensity of solar energy received in anyone place.

Procedure1. Hold a flashlight several centimeters

above a piece of paper and point theflashlight straight down.

2. Use a pencil to trace the outline of thelight on the paper. The outline modelshow the Sun’s rays strike the equator.

3. Keeping the flashlight at the same dis-tance above the paper, tilt the top of theflashlight to roughly a 30° angle.

4. Trace the new outline of the light. This issimilar to how the Sun’s rays are receivedat the poles.

Analyze and Conclude1. Describe how the outline of the light dif-

fered between step 1 and step 3. Explainwhy it differed.

2. How do you think the change in area cov-ered by the light affects the intensity oflight received at any one place?

3. The flashlight models solar radiation striking the surface of Earth. Knowingthis, compare how much heat energy is absorbed near the equator and near the poles.


Maritimepolar

(Pacific)air masses

Maritime tropical(Pacific)

air masses

Maritime tropical(Atlantic)air masses

Maritime tropical(Gulf)

air masses

Maritimepolar

(Atlantic)air masses

Arctic air masses

Continental polarair masses

Continental tropicalair masses

Cool,humid

Cool,humid

Warm,humid

Warm,humid

Warm,humid

Win

ter

on

ly

Dry

Dry, hot

Major Air Masses Over the United States

of continental tropical air, which is hot and dry, especially in summer.Arctic air develops over latitudes above 60°N in the ice- and snow-covered regions of Siberia and the Arctic Basin. During the winter, thisarea receives almost no solar radiation but continues to radiate heatout to space, so it can get very cold indeed. In addition to temperatureand humidity, another important characteristic of an air mass is itsstability. The stability of air is an important factor in its ability to pro-duce clouds and precipitation.

Air Mass Modification Like the warm air over a large city dur-ing a summer afternoon, air masses do not stay in one place indefi-nitely. Eventually, they move, transferring heat from one area toanother and thus establishing the heat balance discussed earlier. As anair mass moves, it may travel over land or water that has different

Figure 12-3 Each of themajor air masses thataffects weather in theUnited States has a similar temperature and moisturecontent as the area overwhich it formed.

characteristics from those of its source region. The air mass then startsto acquire some of the characteristics of the new surface beneath it.When this happens, it is said to undergo air mass modification,which is the exchange of heat or moisture with the surface over whichan air mass travels. Table 12-1 summarizes the characteristics of themain types of air masses before modification.

All air masses become modified to some extent as they move awayfrom their source regions. Eventually, an air mass becomes modifiedto such a degree that its characteristics are almost the same as thenew surface over which it is traveling. At this point, the air mass haslost its original identity and is now simply part of the air over thenew source region it has encountered.


Table 12-1 Air Mass Characteristics

Source Region Stability Characteristics

Air Mass Type Winter Summer Winter Summer

A Stable Bitter cold, dry

cP Stable Stable Very cold, dry Cool, dry

cT Unstable Unstable Warm, dry Hot, dry

mP (Pacific) Unstable Unstable Mild, humid Mild, humid

mP (Atlantic) Unstable Stable Cold, humid Cool, humid

mT (Pacific) Stable Stable Warm, humid Warm, humid

mT (Atlantic) Unstable Unstable Warm, humid Warm, humid

1. What is the difference between weatherand climate? How do both relate to thescience of meteorology?

2. What must happen to keep the polesfrom steadily cooling off and the tropicsfrom heating up over time?

3. What method of heat transfer plays the primary role in the formation of anair mass?

4. Describe how the moisture in a maritimepolar (mP) air mass that formed over theNorth Pacific Ocean would modify as itmoved inland over the western coast ofNorth America.

5. Thinking Critically Explain why an arcticair mass is usually more stable than a mar-itime tropical air mass. In other words,why does the arctic air resist rising morethan the tropical air does?

SKILL REVIEW

6. Predicting Which type of air mass wouldyou expect to become modified morequickly: an arctic air mass moving over theGulf of Mexico in winter or a maritimetropical air mass moving into the south-eastern United States in summer? Formore help, refer to the Skill Handbook.

earthgeu.com/self_check_quiz

http://earthgeu.com/self_check_quiz

12.2 Weather Systems 305

12.212.2 Weather Systems

OBJECTIVES

• Describe how the rota-tion of Earth affects themovement of air.

• Compare and contrastwind systems.

• Identify the varioustypes of fronts.

VOCABULARY

Coriolis effecttrade windsprevailing westerliespolar easterliesjet streamfront

Path ifEarth didnot rotate

Northernhemisphere

Southernhemisphere

Equator

Actual path

Rotation of Earth

A

Figure 12-4 The rotation of Earthcauses the Coriolis effect (A), which,along with the heat imbalance onEarth, creates the three major globalwind systems (B). The convectioncells show the movement of air ineach zone.

60°

60°

30°

30°

Doldrums

Horse latitudes

Sinking air

Equator

Sinkingair

Rising air

Risingair

Prevailingwesterlies

Northeasttrade winds

Southeasttrade winds

Prevailingwesterlies

Polar easterlies

Polar easterlies

Horse latitudes

0°

If Earth were either all land or all water and did not rotate on itsaxis, a large convection cell would form in each hemisphere with thecolder and denser air at the poles sinking to the surface and flowingtoward the tropics. There, it would force the warm air already at theequator to rise, and then it would cool and flow back toward thepoles. The problem with this proposal is that Earth does rotate fromwest to east. This rotation causes the Coriolis effect, wherein mov-ing particles such as air are deflected to the right in the northernhemisphere and to the left in the southern hemisphere. The Corioliseffect, illustrated in Figure 12-4A, combines with the heat imbalancefound on Earth to create distinct global wind systems that transportcolder air to warmer areas and warmer air to colder areas. The endresult is the balancing of heat energy on Earth.

GLOBAL WIND SYSTEMSThere are three basic zones, or wind systems, in each hemisphere, asshown in Figure 12-4B. The first, known as the trade winds, occursat 30° north and south latitude. There, air sinks, warms, and movestoward the equator in a westerly direction. When the air reaches theequator, it rises again and moves back toward latitude 30°, where itsinks and the process starts anew. The circulation pattern for the trade

B


winds closely fits a model proposed by English scientist GeorgeHadley in 1735; thus, this zone is sometimes known as the Hadley cell.

Around 30° latitude, the sinking air associated with the trade windscreates a belt of high pressure that in turn causes generally weak sur-face winds. Spanish sailors called this belt the horse latitudes becausetheir ships became stranded in these waters as a result of the near-calmwinds. According to legend, they could no longer feed or water theirhorses and were forced to throw them overboard.

Near the equator, the trade winds from both hemispheres movetogether from two different directions, as shown in Figure 12-4. Theair converges, is forced upward, and creates an area of low pressure.This process, called convergence, can occur on a small or large scale.Near the equator, it occurs over a large area called the intertropicalconvergence zone (ITCZ). As Figure 12-5 shows, the ITCZ migratessouth and north of the equator as the seasons change. In essence, it fol-lows the path of the Sun’s rays, which are directly over the equator inSeptember and March. Because the ITCZ is a region of rising air, it ischaracterized by a band of cloudiness and occasional showers that helpprovide the moisture for many of the world’s tropical rain forests. Notethat the ITCZ is also called the doldrums. As in the horse latitudes,sailing ships were often stranded in this belt of light winds.

Other Wind Zones The second wind system, the prevailingwesterlies, flows between 30° and 60° north and south latitude in acirculation pattern opposite that of the trade winds. In this zone, sur-face winds move toward the poles in a generally easterly direction, asshown in Figure 12-4. Note that wind is named for the direction

24°

20°

16°

12°

8°

4°

0°

4°

8°

12°

16°

20°

24°

Jan. Feb. Mar. Apr. May Jun. Jul. Aug.Sep. Sep.Oct. Nov. Dec.

Yearly Movement of ITCZ

Sou

th l

atit

ud

eN

ort

h l

atit

ud

e

Equator

ITCZ

Overhead positionof the Sun

Figure 12-5 The meanposition of the ITCZ basically follows the path of the Sun’s rays through-out the year. The two pathsdon’t match exactly becausethe ITCZ responds graduallyto changes in the Sun’s posi-tion, and thus lags behindthe Sun.


from which it blows, so that a wind blowing fromthe west toward the east is considered a westerlywind. The prevailing westerlies are responsible formuch of the movement of weather across the UnitedStates and Canada.

The last major wind zone, the polar easterlies, liesbetween 60° latitude and the poles. Similar to the trade winds,the polar easterlies flow from the northeast to the southwest in thenorthern hemisphere. Note that global wind direction reverses in thesouthern hemisphere. South of the equator, for instance, the polareasterlies flow from the southeast to the northwest. In both hemi-spheres, the polar easterlies are characterized by cold air.

JET STREAMSEarth’s weather is strongly influenced by atmospheric conditions andevents that occur at the boundaries between wind zones. On eitherside of these imaginary boundaries, both surface and upper-level airdiffers greatly in temperature and pressure. Remember that wind,temperature, and pressure are related. Differences in temperature andpressure cause wind. Therefore, a large temperature gradient inupper-level air should result in strong westerly winds, and indeed, thisis what happens. Narrow bands of fast, high-altitude, westerly windscalled jet streams flow at speeds up to 185 km/h at elevations of10.7 km to 12.2 km. Jet streams, shown in Figure 12-6, are so namedbecause they resemble jets of water. The most significant one, thepolar jet stream, separates the polar easterlies from the prevailingwesterlies. A second version, the subtropical jet stream, is locatedwhere the trade winds meet the prevailing westerlies.

30°

60°

90°

Subtropicaljet stream

Polarjet stream

B

Figure 12-6 This satellite imageshows the jet stream over theMiddle East (A). The polar jetstream ranges from roughly 40° to60° north latitude, and the subtropi-cal jet stream ranges from roughly20° to 30° north latitude (B).

A

Large-Scale Weather Systems Disturbances form along jetstreams and give rise to large-scale weather systems that transportsurface cold air toward the tropics and surface warm air toward thepoles. Keep in mind that the position of the jet stream varies. It candive almost directly south or north, instead of following its normalwesterly direction. It can also split into different branches and laterreform into a single stream. Whatever form or position it takes, thejet stream represents the strongest core of westerly winds. Together,these winds form a sort of atmospheric railroad track, with large-scale weather systems serving as the atmospheric trains. Weather sys-tems generally follow the path of these winds. The jet stream alsoaffects the intensity of weather systems by moving air of differenttemperatures from one region to another. Thus, despite its altitude,it has a significant impact on weather.

FRONTSThe different temperatures and pressures of air masses have otherconsequences apart from the jet stream. In the middle latitudes, airmasses with different characteristics sometimes collide, forming afront. A front is the narrow region separating two air masses of dif-ferent densities. The density differences are caused by differences intemperature, pressure, and humidity. Fronts can stretch over thou-sands of kilometers across Earth’s surface. The interaction betweenthe colliding air masses can bring dramatic changes in weather. Asshown in Figure 12-7, there are four main types of fronts: coldfronts, warm fronts, stationary fronts, and occluded fronts.


Frontal movement

Precipitation

Warmair

Cold air

Frontal movement

Precipitation

Warmair

Cold air

Figure 12-7 These dia-grams show the structuresof the four main types offronts. Symbols below eachdiagram indicate how thefronts are represented on aweather map. A cold frontoften moves quickly.Thunderstorms may formalong the front (A). In awarm front, precipitationoften occurs over a wideband. High-altitude cirrusclouds may form as watervapor condenses (B). Lightwind and precipitation aresometimes associated with astationary front (C). Strongwinds and heavy precipita-tion may occur along anoccluded front (D).

A B


Cold Fronts In a cold front, shown in Figure 12-7A, cold, dense airdisplaces warm air and forces the warm air up along a steep front. Asthe warm air rises, it cools and condenses. Clouds, showers, and some-times thunderstorms are associated with cold fronts. A cold front isrepresented on a weather map as a solid blue line with blue trianglesthat point in the direction of the front’s motion.

Warm Fronts In a warm front, advancing warm air displaces coldair, as shown in Figure 12-7B. Because the air ahead of a warm frontmoves more slowly than does an advancing cold air mass, the warm airencounters less friction with the ground and thus develops a gradualfrontal slope rather than a steep boundary. A warm front is character-ized by extensive cloudiness and precipitation. On a weather chart, awarm front appears as a solid red line with regularly spaced, solid redsemicircles pointing in the direction of the front’s motion.

Stationary Fronts Sometimes, two air masses meet and neitheradvances into the other’s territory. In this case, the boundarybetween the air masses stalls. This type of front, called a stationaryfront, frequently occurs when two air masses have become so modi-fied in their travels that the temperature and pressure gradientsbetween them are small. Stationary fronts seldom have extensivecloud and heavy precipitation patterns; any patterns that do occurare somewhat similar to those of a warm front. A stationary front isrepresented on a weather map by a combination of short segmentsof cold- and warm-front symbols as shown in Figure 12-7C.

Precipitation

Stalled front

Cold Warm

Precipitation

Cold Cold

Warm

Frontalmovement

DC

Topic: WeatherTo find out more aboutweather fronts, visit theEarth Science Web Site atearthgeu.com

Activity: Obtain a weathermap showing today’sweather. Label the weatherfronts.

http://earthgeu.com


Occluded Fronts Sometimes, a cold air mass moves so rapidlythat it overtakes a warm front. The advancing cold air wedges thewarm air upward, as shown in Figure 12-7D on page 309. Recall thata warm front involves warm air gliding over a cold air mass. Whenthe warm air is lifted, this cold air mass collides with the advancingcold front. The warm air is thus squeezed upward between the twocold air masses. This is called an occluded front and is represented ona weather map by a line with alternating purple triangles and semi-circles that point toward the direction of motion. Precipitation iscommon on both sides of an occluded front.

PRESSURE SYSTEMSYou have learned that at Earth’s surface, rising air is associated withlow pressure and sinking air is associated with high pressure. Risingor sinking air, combined with the Coriolis effect, results in the for-mation of rotating low- and high-pressure systems in the atmo-sphere. Air in these systems moves in a general circular motionaround either a high- or low-pressure center.

High-Pressure Systems In a surface high-pressure system, airsinks, so that when it reaches Earth’s surface, it spreads away from thecenter. The deflection of air to the right caused by the Coriolis effectmakes the overall circulation around a high-pressure center move ina clockwise direction in the northern hemisphere, as shown in Figure12-8A. Keep in mind that the Coriolis effect is reversed in the south-ern hemisphere; there, high-pressure systems rotate in a counter-clockwise direction. Some high-pressure systems are associated withcold air masses that move and modify; others, such as subtropicalhigh-pressure systems, are more stationary.

H L

A B

Figure 12-8 In the north-ern hemisphere, winds in ahigh-pressure system rotatein a clockwise direction (A),and winds in a low-pressuresystem rotate in a counter-clockwise direction (B).

Using NumbersMost weather occursin the tropospherebetween the surfaceof Earth and analtitude of 11 km.Suppose the temper-ature in an areadrops about 7°C foreach kilometer ofincrease in altitude.If the surface temper-ature is 15°C, what isthe temperature atan altitude of 3 km?


Figure 12-9 The counter-clockwise circulation is char-acteristic of a wave cyclone.

Low-Pressure Systems In surface low-pressuresystems, air rises. The rising air must be replaced byair from outside the system, so the net flow isinward toward the center and then upward. Incontrast to air in a high-pressure system, air in alow-pressure system in the northern hemispheremoves in a counterclockwise direction, as shownin Figure 12-8B. This movement is reversed in thesouthern hemisphere.

Recall from Chapter 11 that it’s difficult forclouds to form when air is sinking, as it does in high-pressure systems. Thus, high-pressure systems areusually associated with fair weather, while low-pressure systems are associated with clouds and pre-cipitation. In fact, most of Earth’s subtropical oceansare dominated by large high-pressure systems with generally pleasantconditions. One of the main producers of inclement weather in themiddle latitudes, meanwhile, is a specific type of low-pressure systemcalled a wave cyclone. A wave cyclone usually begins along a stationaryfront. Some imbalance in temperature, pressure, or density causes partof the front to move south as a cold front and another part of the frontto move north as a warm front. This sets up a counterclockwise orcyclonic circulation, as shown in Figure 12-9. Eventually, if upper-levelconditions are favorable, a fully developed low-pressure system forms.Pushed by the prevailing westerlies, this system may travel thousandsof kilometers affecting large areas in the middle latitudes.

Low

Cold

Warm

1. Describe the main global wind systems.Give characteristics of each.

2. Explain why most tropical rain forests arelocated near the equator.

3. How does the jet stream affect the move-ment of air masses?

4. What is the Coriolis effect? How does itaffect air in the northern and southernhemispheres?

5. Compare and contrast a low-pressure system and a high-pressure system.

6. Thinking Critically Based on what youknow about the three major zones ofglobal air circulation, form a hypothesis

about why most of the world’s deserts are located between 10° and 30° northand south latitudes.

SKILL REVIEW

7. Interpreting Scientific Illustrations Referto the illustrations of the four types offronts shown in Figure 12-7. Sketch thefronts in your science journal or use acomputer graphics program to make amodel of them. Label the warm and coldair masses. Indicate the direction of theirmovement and describe the type ofweather associated with each front. Formore help, refer to the Skill Handbook.



12.312.3 Gathering Weather Data


OBJECTIVES

• Recognize the impor-tance of accurate weatherdata.

• Describe the technologyused to collect weatherdata.

• Analyze the strengthsand weaknesses ofweather observation systems.

VOCABULARY

thermometerbarometeranemometerhygrometerceilometerradiosondeDoppler effect

C

When you visit a doctor, she or he first measures your temperatureand blood pressure to make an accurate diagnosis and prescribetreatment. If the doctor’s data are incomplete or inaccurate, the diag-nosis is likely to be inaccurate as well. The same principle applies tometeorology. Meteorologists measure the atmospheric variables oftemperature, air pressure, wind, and relative humidity to make accu-rate weather forecasts. The quality of the data is critical. In fact, twoof the most important factors in weather forecasting are the accuracyand the density of the data—density in this case refers to the amountof data available. Just as a doctor uses several different instrumentsto assess your health, meteorologists use several types of technologyto gather information about the atmosphere.

SURFACE DATAOne of the most common weather instruments is a thermometer, adevice used to measure temperature. Usually, thermometers containliquids such as mercury or alcohol, which expand when heated. Theheight of the liquid column indicates temperature. Another commonweather instrument, the barometer, also uses mercury to obtainweather data. Barometers measure air pressure. In a mercury barom-eter, changes in air pressure are indicated by changes in the height ofa column of mercury. An aneroid barometer contains a vacuum

Figure 12-10 A thermometer (A),barometer (B), anemometer (C),and hygrometer (D) are commonlyused weather instruments.

A

D

B

12.3 Gathering Weather Data 313

inside a metal chamber. The chamber contracts or expands withchanges in air pressure. A thermometer and a mercury barometer areshown in Figures 12-10A and 12-10B.

Other Surface Instruments An anemometer, shown in Figure12-10C, is used to measure wind speed. The simplest type ofanemometer has cupped arms that rotate as the wind blows. Ahygrometer measures relative humidity. One type of hygrometer,shown in Figure 12-10D, uses wet- and dry-bulb thermometers. Aswater evaporates from the wet bulb, the bulb cools, creating a tem-perature difference between the wet bulb and the dry bulb. This tem-perature difference is used in conjunction with a relative humiditychart to determine relative humidity. See Appendix F for an exampleof a relative humidity chart.

Automated Surface Observing System To make weatherforecasts, meteorologists analyze and interpret data gathered fromweather instruments. In this regard, timing is crucial. Data must begathered at the same time at many different locations. Why? It woulddo no good to analyze how temperature and air pressure are inter-acting in the atmosphere if the two variables were measured at dif-ferent times. Meteorologists need an accurate “snapshot” of theatmosphere at a particular moment in time to develop a reliable fore-cast. Thus, the National Weather Service in the United States hasestablished a surface observation network across the country. Madeup of some 1700 official sites, the network gathers data in a consis-tent manner at regular intervals—usually a minimumof once an hour. Most of these data are collected by theAutomated Surface Observing System (ASOS), shownin Figure 12-11. In addition to the weather instrumentsalready discussed, the ASOS uses a rain gauge for mea-suring rainfall, as well as a ceilometer, which measuresthe height of cloud layers and estimates the amount ofsky covered by clouds. You’ll learn more about theASOS in this chapter’s Science & Technology feature.

UPPER-LEVEL DATAWhile surface weather data are important, the weatherthat we experience is largely the result of changes thattake place high in the troposphere. To make accurateforecasts, meteorologists must gather atmospheric dataat heights of up to 30 000 m. This is a more formidabletask than gathering surface data, and therefore itrequires more sophisticated technology.

Figure 12-11 This auto-mated weather system measures surface data.

At present, the instrument of choice forgathering upper-level data is a balloon-bornepackage of sensors called a radiosonde,shown in Figure 12-12. The sensors on aradiosonde measure temperature, air pres-sure, and humidity. These readings are con-stantly sent back by radio signal to aground station that tracks the movementsof the radiosonde. Tracking is a crucialcomponent of upper-level observations—meteorologists can determine wind speedand direction by tracking how fast and inwhat direction the radiosonde is moving.The various data are plotted on a chart, giv-ing meteorologists a profile of the temper-ature, pressure, humidity, wind speed, andwind direction of a particular part of thetroposphere. Such charts are used to fore-cast atmospheric changes that affect sur-face weather. While radiosondes provideaccurate snapshots of atmospheric condi-tions, they are quite expensive. It is hopedthat in the future, data from satellites willreplace or greatly supplement radiosondeobservations.

WEATHER RADARThere are thousands of surface observation sites and 100 upper-levelobservation sites across the United States. Yet the data from thesesites cannot pinpoint where rain is falling at any given moment. Forthat purpose, a weather radar system is needed. The term radarstands for “radio detecting and ranging.” A radar system is made ofseveral parts. A transmitter generates electromagnetic waves, whichleave the transmitter through antennae. In weather radar systems,the waves are programmed to ignore small cloud droplets and tobounce off large raindrops. The large raindrops scatter some of theradio waves. These scattered waves, or echoes as they are often called,are received by other antennae. An amplifier increases the wave sig-nals of the scattered waves. A computer then processes the signalsand displays them on a screen. From this, meteorologists can com-pute the distance to the raindrops and the location of the rain rela-tive to the receiving antennae. The radar system rotates in a circle,allowing meteorologists to gauge where rain is falling within theradar’s range—usually an area with a diameter of about 400 km.


Figure 12-12 Radiosondesare used to gather upper-level weather data.

12.3 Gathering Weather Data 315

Figure 12-13 As the train approaches, the soundwaves ahead of it are compressed. These shortwaves have a high frequency, so the horn soundshigh. Behind the train, the sound waves arestretched out. These longer waves have a lower frequency, so the horn sounds lower.

Doppler Radar Many advanced weather radar systems take advan-tage of a phenomenon called the Doppler effect. The Doppler effectis the change in wave frequency that occurs in energy, such as soundor light, as that energy moves toward or away from an observer.You’ve probably noticed that sounds produced by a horn from anapproaching train change once the train has passed. Look at Figure12-13. As the train approaches, the frequency and pitch of the soundcoming from the horn are high. As the train passes, the frequencyand pitch lower. This is the Doppler effect in action. Meteorologistsuse Doppler radar, which is based on the Doppler effect, to plot thespeed at which raindrops move toward or away from a radar station.Because the motion of the moving raindrops is caused by wind,Doppler radar provides a good estimation of the wind speeds associ-ated with precipitation areas, including those that are experiencingsevere weather such as thunderstorms and tornados. The ability tomeasure wind speeds gives Doppler radar a distinct advantage overconventional weather radar systems.

WEATHER SATELLITESIn addition to communications, one of the main uses of satellites inorbit around Earth is to observe weather. Cameras mounted aboard aweather satellite take photos of Earth at regular intervals. These pho-tos are beamed back to ground stations and their data are plotted onmaps. Unlike weather radar, which tracks precipitation but not clouds,satellites track clouds but not necessarily precipitation. By combiningdata from the two types of technology, meteorologists can determinewhere both clouds and precipitation are occurring.

Infrared Imagery Weather satellites use both visi-ble light and invisible radiation to observe the atmos-phere. The satellites discussed thus far use camerasthat need visible light to take photos. When such asatellite is observing a portion of Earth that is indarkness, however, its cameras are useless. Thus,some satellites are designed to use infrared imagery.Infrared imagery detects differences in thermalenergy, which are used to map either cloud cover orsurface temperatures. In an infrared image, such asthe one shown in Figure 12-14, objects that radiatewarmth at slightly different frequencies show up asdifferent colors. As you learned in Chapter 11, differ-ent types of clouds form at different levels of theatmosphere, which are characterized by differenttemperatures. Infrared images allow meteorologiststo determine the temperature of a cloud. From this,they can infer what type it is and estimate its height.Infrared imagery is especially useful in detectingstrong thunderstorms that extend to great heights in

the atmosphere and consequently show up as very cold areas on aninfrared image. Because the strength of a thunderstorm is related toits height, infrared imagery can be used to establish a storm’s poten-tial to produce severe weather.


1. If your goal was to vastly improve thedensity of weather data in the UnitedStates, would you focus on gatheringmore surface data or more upper-leveldata? Explain.

2. What is the main advantage of Dopplerradar over conventional weather radar?

3. Compare and contrast infrared imageryand visible-light imagery.

4. What is the main disadvantage ofradiosondes?

5. Thinking Critically All else being equal,would you expect weather forecasts to bemore accurate for the state of Kansas or aCaribbean island? Why?

SKILL REVIEW

6. Concept Mapping Use the followingterms to construct a concept map aboutinstruments that gather surface weatherdata. For more help, refer to the SkillHandbook.

1.anemometer

4.barometer

7.weather

instruments

2.hygrometer

5.thermometer

8.air pressure

3.humidity

6.temperature

9.wind speed

Figure 12-14 This infraredimage shows a huge stormsystem over the easternUnited States.



12.412.4

12.4 Weather Analysis 317

After weather observations are gathered, meteorologists plot thedata on a map, using station models for individual cities or towns. Astation model is a record of weather data for a particular site at aparticular time. Meteorological symbols, such as the ones shown inFigure 12-15, are used to represent weather data in a station model.(For a more complete list of the symbols that meteorologists use torepresent weather data, see Appendix E.) A station model allowsmeteorologists to fit a large amount of data into a small space. It alsogives meteorologists a uniform way of communicating weather data.You’ll use station models and weather maps to forecast the weatherin the Mapping GeoLab at the end of this chapter.

SURFACE ANALYSISStation models provide information for individual sites. To plot datanationwide or globally, meteorologists use isopleths, which are linesthat connect points of equal or constant values. The values representdifferent weather variables, such as pressure or temperature. Lines ofequal pressure, for example, are called isobars; lines of equal temper-ature are called isotherms. The lines themselves are similar to thecontour lines—lines of equal elevation—that you studied in Chap-ter 2. Just as you can make inferences about elevation by studyingcontour intervals on a map, you can also make inferences aboutweather by studying isobars or isotherms on a map. For instance, youcan tell how fast wind is blowing in an area by noting how closelyisobars are spaced. Isobars that are close together indicate a large

Weather Analysis

OBJECTIVES

• Analyze a basic surfaceweather chart.

• Distinguish betweenanalog and digital forecasting.

• Describe problems withlong-term forecasts.

VOCABULARY

station modelisoplethdigital forecast analog forecast

Environmental ConnectionEnvironmental Connection

Type of high cloudsType of middle clouds

Temperature (̊ C)

Type of precipitation

Dew pointtemperature

Change in barometricpressure in last 3 hours(in tenths of millibars)

Barometric pressure inmillibars with initial9 or 10 omitted

Wind speedand directionType of low clouds

20 188

19–12

Figure 12-15 A stationmodel shows weather datafor a particular area at aparticular time.


Create and analyze isobarson a weather map Areas ofhigh and low pressure can beindicated on a weather map bylines of approximately equalpressure called isobars.

Analysis1. On a blank piece of paper,

trace the diagram shownhere, along with the pressurevalues at various locations,which are given in millibars(mb).

2. A 1004-mb isobar that encir-cles one location on this maphas been drawn and labeled.Complete and label the 1000-mb isobar that has been started. Finally, draw a 996-mb isobar and a 992-mbisobar. The isobars may not

completely encirclea location in a mapof this scale.

Thinking Critically3. What is the contour

interval of the iso-bars on this map?

4. A blue letter H anda red letter L in thecenters of closedisobars mark the areas ofhighest and lowest pressure,respectively. On your map,place a blue H or a red L—whichever is appropriate—inside the closed 1004-mbisobar.

5. What type of weather is com-monly associated with thispressure system?

991

992

992

996

996

999

1000

994

997

1004

1006

1001 10001000

Interpreting Scientific Illustrations

pressure difference over a small area. A large pressure differencecauses strong winds. Conversely, isobars that are spread far apartindicate a small difference in pressure. Winds in these areas would belight. As shown in Figure 12-16, isobars also indicate the locations ofhigh- and low-pressure systems. This information is especially usefulwhen combined with isotherms, which identify temperature gradi-ents and, consequently, frontal systems. Using isobars, isotherms,and station-model data, meteorologists can analyze current weatherconditions for a particular time and place. This is crucial informa-tion—meteorologists must understand current weather conditionsbefore they can move on to forecasting the weather. You’ll learn moreabout isobars in the Problem-Solving Lab on this page.

SHORT-TERM FORECASTSIn the early days of weather forecasting, meteorologists simplyobserved current weather conditions, compared these conditions tothose that had occurred a day or two before, and then extrapolatedthe changes a day or two into the future. The resulting positions of

the weather systems served as the basis for their forecasts. Weatherforecasting, however, is too complicated to rely on extrapolating thepast movements of weather systems. Weather systems change direc-tions, speed, and intensity with time. These changes take place inresponse to changes in the upper atmosphere, so a reliable forecastmust analyze data from different levels in the atmosphere.

Digital Forecasts The key to unlocking the forecast puzzle lies inthe fact that the atmosphere behaves much like a fluid. Thus, we canapply many of the same principles to the atmosphere and its variables,such as temperature, pressure, density, and so on, that we can apply toa fluid. Furthermore, these principles can be expressed in mathemat-ical equations to determine how atmospheric variables change withtime. For meteorologists to solve these equations on a global ornational level would take an impossibly large amount of time.Fortunately, high-speed computers can do the job. A forecast such asthis that relies on numerical data is known as a digital forecast.Digital forecasting is the main method used by modern meteorolo-gists, such as the one shown in Figure 12-17. It is highly dependent onthe density of the data available—basically, the more data, the moreaccurate the forecast.


Figure 12-17 This meteo-rologist is preparing aweather forecast.

TX

NMAZ

NV

IDOR

MT

WY

COKS

NE

UT

NDMN

WI MIIA

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WA

CA

FL

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LA MSAL

GA

SC

NC

VA

PA

NY

NJ

ME

NHVT

MACT

DE

MD

RI

WVKY

TN

IN

Air-Pressure Data

Figure 12-16 This map shows air-pressure data for the UnitedStates. Where would you expect light winds?

Analog Forecasts Another type of forecast, an analog forecast,involves comparing current weather patterns to patterns that tookplace in the past. The assumption is that weather systems behave ina similar fashion. Analog forecasting is so called because meteorolo-gists look for a pattern from the past that is analogous, or similar to,a current pattern. To ensure an accurate analog forecast, meteorolo-gists must find a past event that is similar to a current event throughall levels of the atmosphere, and also over a large area. This is themain drawback of analog forecasting. Still, analog forecasting is use-ful for conducting monthly or seasonal forecasts, which are basedmainly on the past behavior of cyclic weather patterns. Let’s explorethe strengths and weaknesses of long-term forecasts.

LONG-TERM FORECASTSRegardless of the forecasting method used, all forecasts become lessreliable when they attempt to predict long-term changes in the weather.Why? Even high-tech computers cannot model every factor that affectsthe weather. Recall that mountains, valleys, rivers, lakes, cities, andcountless other features on Earth’s surface affect the amount of heatabsorbed in a particular location. This, in turn, affects the pressure andtherefore the wind of that area, which in turn affects cloud formationand virtually all other aspects of the weather. Over time, all these fac-tors interact to create progressively more complicated scenarios.

The most accurate and detailed forecasts are short-term innature, as shown in Figure 12-18. For hourly forecasts, extrapola-tion is a reliable forecasting method because the current weatheris dominated by small-scale weather features that are readily observ-able by radar and satellite. Forecasts in the one- to three-day range,


Long-Term vs. Short-Term Forecasts

Fore

cast

lea

d t

ime

Forecast uncertainty

Minutes

Hours

Days

6–10 days

8–14 daysMonths

Seasons Years

Figure 12-18 Thisgraph shows thatforecast uncertaintyincreases with time.

however, are no longer based on the movement ofobserved clouds and precipitation, which changeby the hour. These forecasts are dependent on thebehavior of larger surface and upper-level fea-tures, such as low-pressure systems. A one- tothree-day forecast can somewhat accurately pre-dict whether the day will be rainy or dry, and, ifrainy, when the precipitation will occur. At thisrange, however, the forecast will not be able topinpoint an exact temperature or sky condition ata specific time.

Accuracy Declines with Time At the four-to seven-day range, forecasts must attempt to pre-dict changes in surface weather systems based oncirculation patterns throughout the troposphere and lower stratos-phere. Meteorologists can estimate each day’s weather but can offerlittle detail as to when or what exact weather conditions will occur.At the one- to two-week range, forecasts are based on changes inlarge-scale circulation patterns. Thus, these forecasts are vague andbased mainly on analogous conditions.

Long-term forecasts involving months and seasons are basedlargely on patterns or cycles. Several of these cycles, such as the oneshown in Figure 12-19, involve changes in the atmosphere, oceancurrents, and solar activity, all of which might be occurring at thesame time. The key to future improvement in weather forecasts liesin identifying the many influences involved, understanding theseinfluences and how they interact, and finally, determining their ulti-mate effect on weather over progressively longer periods of time.


1. Find an example of a station model inyour local newspaper and describe thesymbols on the model.

2. Compare and contrast analog and digitalforecasting.

3. Explain why long-term forecasts aren’talways accurate.

4. Thinking Critically Based on what youhave learned about digital forecasting,what single improvement do you think

would be necessary to increase thereliability of this type of forecasting?

SKILL REVIEW

5. Forming a Hypothesis For a time periodof three days or less, hypothesize whichwould be more accurate: digital forecast-ing or analog forecasting. Explain yourhypothesis. For more help, refer to theSkill Handbook.

Figure 12-19 Changes inocean-surface temperaturescan trigger changes inweather patterns. This satel-lite image shows a cyclicevent known as El Niñowherein the Pacific Oceanwarms along the equatorand triggers short-term climatic changes.



ProblemHow can you use a surface weather mapto interpret information about currentweather and to forecast future weather?

Materialspencilruler


Interpreting aWeather Map

It’s time to put your knowledge of meteorology into action.The surface weather map on the following page shows

actual weather data for the United States. In this activity, youwill use the station models, isobars, and pressure systems onthe map to forecast the weather.

Preparation

Procedure

Analyze

Conclude & Apply

1. What is the contour interval of theisobars?

2. What are the highest and lowest iso-bars? Where are they located?

3. In which direction are the windsblowing across Texas and Louisiana?

4. What and where are the coldest andwarmest temperatures that you canfind in the continental United States?

1. The map scale is given in nauticalmiles. Refer to the scale when calcu-lating distances.

2. The unit for isobars is millibars (mb).In station models, pressure readings

are abbreviated. For example,1021.9 mb is plotted on a stationmodel as 219 but read as 1021.9.

3. Wind shafts point in the directionfrom which the wind is blowing.

1. Would you expect the weather inGeorgia and Florida to be clear orrainy? Why?

2. Both of the low-pressure systems ineastern Canada and off the Oregon

coast are moving toward the east atabout 15 mph. What kind of weatherwould you predict for Oregon andfor northern New York for the nextfew hours? Explain.

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Mapping GeoLab 323

Does your local airport use the ASOS? Go to the Earth Science Web Site at earthgeu.com to research which airportsuse the ASOS, or contact your local airport.Write to airport officials for statistics on aviation accidents both prior to and afterthe installation of the ASOS. Has therebeen a change in the number of aviationaccidents since the airport implementedthe ASOS? Use this information to write a short opinion piece on the continued use of the ASOS.

Activity

Some aviation professionals, however, disagree. The National Air Traffic ControllersAssociation believes that human observers werereplaced by machines primarily to save money,and that the loss of trained weather observers isdetrimental to aviation safety. On the evening thatKennedy’s plane went down, the ASOS indicatedthat visibility was 13 to 16 km. In actuality, visibil-ity was reported to be so poor by other pilots thatthe lights of Martha’s Vineyard could not be seenfrom the air. The National Weather Service admitsthat the ASOS needs to be refined and continuesto implement upgrades. Still, this group insiststhat widespread use of the ASOS will reduceweather-related aviation accidents.


Safety is a priority in the aviation industry.Pilots must be aware of weather conditions toavoid crashing when landing or taking off. Prior to 1990, National Weather Service personnelwere responsible for gathering and communicat-ing weather data to pilots. These professionalscollected data on air pressure, wind speed, temperature, cloud cover, and precipitation.

ASOSConcern about possible human errors

prompted scientists to develop a more efficientsystem for transmitting weather data to pilots.The resulting computerized system, called theAutomated Surface Observing System (ASOS), is now the cornerstone of weather forecastingand communication in this country.

The Pros and ConsToday, more than 1000 ASOS units are in

operation at major airports, continuously record-ing air pressure, temperature, wind speed anddirection, runway visibility, cloud ceiling, and pre-cipitation intensity. Data are automaticallyupdated every minute. While human observa-tions are based on what can be seen from agiven vantage point, computerized observationsare not affected by varying light and terrain con-ditions. For this reason, many aviation profes-sionals believe that ASOS data are moreconsistent than manually collected data.

Tracking Atmospheric ChangeOn July 16, 1999, John F. Kennedy Jr. piloted a small private planebound for Martha’s Vineyard. The plane never reached its destination.Searchers determined that the plane crashed into the Atlantic Ocean,killing all three people on board. While no one will know exactly whathappened, the dense fog may have been a factor in the crash.

http://earthgeu.com

Summary

Vocabularyair mass (p. 301)air mass modifica-

tion (p. 304)climate (p. 300)meteorology

(p. 299)weather (p. 300)

VocabularyCoriolis effect

(p. 305)front (p. 308)jet stream (p. 307)polar easterlies

(p. 307)prevailing westerlies

(p. 306)trade winds (p. 305)

Vocabularyanemometer

(p. 313)barometer (p. 312)ceilometer (p. 313)Doppler effect

(p. 315)hygrometer (p. 313)radiosonde (p. 314)thermometer

(p. 312)

Main Ideas• Meteorology is the study of the atmosphere. Weather is the

current state of the atmosphere, and climate is the averageweather over a long period of time.

• An air mass is a large body of air that takes on the characteris-tics of the area over which it forms.

Main Ideas• The Coriolis effect deflects air to the right in the northern hemi-

sphere and to the left in the southern hemisphere. The Corioliseffect combines with the heat imbalance found on Earth to formthe trade winds, prevailing westerlies, and polar easterlies.

• Weather in the middle latitudes is strongly influenced by fast-moving, high-altitude jet streams.

• A front is the boundary between two air masses of differentdensities. The four types of fronts are cold fronts, warm fronts,occluded fronts, and stationary fronts.

Main Ideas• Two of the most important factors in weather forecasting are

the accuracy and the density of the data. Surface data are easierto gather than upper-level data.

• The most common instrument for collecting upper-level data is a balloon-borne radiosonde. Radiosondes measure temperature,pressure, humidity, wind speed, and wind direction.

• Weather radar pinpoints exactly where precipitation occurs.Weather satellites use both visible-light imagery and infraredimagery to observe weather conditions on Earth.

SECTION 12.1

The Causes ofWeather

SECTION 12.2

Weather Systems

SECTION 12.3

GatheringWeather Data

Study Guide 325

Vocabularyanalog forecast

(p. 320)digital forecast

(p. 319) isopleth (p. 317)station model

(p. 317)

Main Ideas• A station model is a record of weather data for a particular site

at a particular time. On a weather map, lines of equal pressureare called isobars and lines of equal temperature are calledisotherms.

• Digital forecasting uses numerical data. Analog forecasting com-pares current weather patterns to patterns that took place in thepast. All forecasts become less reliable when they attempt topredict long-term changes in the weather.

SECTION 12.4

Weather Analysis

earthgeu.com/vocabulary_puzzlemaker

http://earthgeu.com/vocabulary_puzzlemaker


1. Which term best describes a snowflake?a. hydrosphere c. lithometeorb. hydrometeor d. electrometeor

2. What winds blow between 30° and 60° northand south latitude?a. trade winds c. polar easterliesb. prevailing westerlies d. jet streams

3. What would be the most likely classification foran air mass originating over Alaska and Canada?a. mT c. cTb. cP d. mP

4. What would be the most likely dominant air massover the eastern United States in summer?a. cT c. mTb. cP d. mP

5. What instrument is used to measure the heightsof the bases of clouds?a. radiosonde c. hygrometerb. ceilometer d. barometer

6. Which term describes changes in air motionresulting from Earth’s rotation?a. jet stream c. Coriolis effectb. convergence d. Hadley cell

7. What forecast method is best for researching pastweather events?a. digital c. extrapolationb. analog d. numerical

8. Which of the following is NOT characteristic of ahigh-pressure system?a. sinking air c. fair weatherb. dense air d. thunderstorms

Understanding Main Ideas 9. Which of the following would NOT be included ina station model?a. humidity c. pressureb. wind d. temperature

10. What does an anemometer measure?a. humidity c. wind speedb. air pressure d. wind direction

11. Describe the relationship among meteorology,weather, and climate.

Use the surface weather chart below to answerquestions 12 and 13.

12. What location will soon experience a warm front?

13. At what location would you forecast the greatestprobability of thunderstorms? At what locationwould you forecast the weather to turn colderover the next day?

FOCUS While you take a test, pay absolutely noattention to anyone other than the teacher orproctor. If someone tries to talk with you duringa test, don’t answer. You’ll become distracted.Also, the proctor may think that you were cheat-ing. Don’t take the chance. Focus on the test,and nothing else.

Test-Taking Tip

A

B D

LC

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Assessment 327

1. Which of the following types of air massesare most likely to form over land near theequator?a. mP c. cPb. mT d. cT

2. Which wind system flows between 30° and60° latitude north and south of the equatorin an easterly direction toward the poles?a. trade winds c. prevailing westerliesb. Coriolis effect d. polar easterlies

INTERPRETINGDATA Use the phototo answer question 3.

3. Meteorologists usemany differentinstruments togather atmos-pheric information.What is the instru-ment shown herecalled?a. a hygrometer c. a radiosondeb. a ceilometer d. radar

4. What does Doppler radar monitor?a. the motion of moving raindropsb. atmospheric pressurec. temperature, air pressure, and humidityd. the height of cloud layers

5. The data gathered by Doppler radar can beused to make a type of forecast that relies onnumerical data. What is this type of forecastcalled?a. an analog forecast c. an isoplethb. a digital forecast d. ASOS

14. Is meteorological data easier to gather on land oron water? Explain your answer.

15. What is Doppler radar?

16. What happens to an air mass as it moves awayfrom its source region?

17. Why are rain showers common near the ITCZ?

18. In your own words, describe the global wind systems.

19. Construct a station model using the followingdata: temperature: –5°F; dew point: –12°F;wind: north, 20 knots; barometric pressure:1038.5 mb; sky: clear.

20. Forecast the weather for the next 24 hours for anarea experiencing the same conditions as thoseincluded in the station model you constructed inquestion 19.

21. Like the north pole, the south pole receives littlesolar radiation during the winter. Unlike the northpole, however, the south pole does not send out-breaks of extremely frigid air as far as the sub-tropics. Why? (Hint: You may want to study aworld map to answer this question.)

22. You hear on a news report that an area hasreceived nearly twice its normal snowfall duringthe winter. What can you infer about the positionof the jet stream from this report?

23. Review Figure 12-4 on page 305, which showsglobal wind systems, then note the relative posi-tions of North America and Europe on a worldmap. Hypothesize why the winds that blowbetween 30° north and south latitude and theequator are called the trade winds.

Thinking Critically

Applying Main Ideas Standardized Test Practice

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Chapter 12: Meteorology · WEATHER AND CLIMATE Atmospheric phenomena, shown in Figure 12-1,interact...

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