CHAPTER 17 Weather and Climate - Mr. Barrow's Science Center

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Severe WeatherThe continental United States is prone to severe weatherbecause of the extreme temperatures of warm and cold airmasses and the availability of moisture from tropical oceans.Such conditions lead to severe thunderstorms, hurricanes, tornadoes,and violent wind storms called downbursts.Thunderstorms Recall that cumulonimbus clouds formedfrom unstable air produce thunderstorms. A typical cumulonimbuscloud has ice crystals near its top, as shown in Figure 9.Sometimes these ice crystals act as nuclei to trigger furthergrowth of cloud droplets, and turbulence adds layers of ice duringmany cycles of sinking and rising. This forms hail. Hailstonescan grow to the size of softballs and can cause extensive damageto crops and structures.Downdrafts and Squalls The force of the falling precipitationin a thunderstorm may pull with it cold air bursts fromhigher in the cloud. This is why the air often feels cool after athunderstorm. This sinking current of cold air is called adowndraft. When a downdraft hits the surface with particularlystrong force, it spreads out in aseries of windy gusts called squalls.In arid regions, squalls producedust storms.Figure 9 When hailstones fall,they are repeatedly caught inupdrafts, coated with moisture,and frozen. This gives them alayered, onionlike appearance.Infer Why does this cumulonimbuscloud have an anvil-shaped top?Upper air flowDownbursts Cold air downdraftscan produce even more severeforms of weather. One example ofan extreme form of wind shear is adownburst. Here, cold air descendsfrom a thunderstorm and hits theground. When it hits the ground, itbursts outward like the spokes on awheel. The rapid change in windspeed and/or direction that a downburstcauses can be dangerous foraircraft during both take-off andlanding. The winds that result fromdownbursts can be as high as260 km/h. Fortunately, automatedwarning systems now alert pilots tolook for signs of downbursts whenapproaching an airport.12 kmDowndraftUpdraftAnvil topStorm travelSECTION 2 Weather 527
Topic: Hurricane TrackingVisit gpescience.com for Weblinks to information abouthurricanes and hurricane tracking.Activity Research six hurricanes.Make a table that lists how eachwas tracked as a tropical depressionand then as a hurricane.Figure 10 Texas, Oklahoma,and Kansas frequently experiencetornadoes.Fewerthanten101010Scale0 500 mi 100 1000 km10 10501002003005010010050Tornadoes and Hurricanes Two types of violent windstorms that differ greatly in their origins and effects aretornadoes and hurricanes. Tornadoes are intense, short-lived,localized storms in the mid-latitudes. They originate in cumulonimbusclouds under special conditions. Typically, tornadoesthat occur in the United States form when dry air from thedeserts of Mexico and the southwest overrides warm, moist airfrom the Gulf of Mexico. This happens frequently in the GreatPlains, the lower midwest, and parts of the south, as shown inFigure 10. In the south, they often accompany hurricanes.A twisting, funnel-shaped tornado cloud can move acrossland at a speed of around 50 km/h creating a path 150 m wideand 10 km long. Intense, circular winds in the funnel can reachspeeds up to 400 km/h. The extreme low pressure at the centercan result in more damage than that from the wind.Hurricanes are tropical storms that cover vastFewer thanten1010050505010areas and last for days. Those affecting the UnitedStates often form as tropical depressions over thewarm waters of the southern Atlantic off thecoast of Africa. When winds exceed 118 km/h, thestorms are called hurricanes. A typical hurricaneconsists of vast cloud bands that spiral inwardtoward the clear center, called the eye. Scientistsoften fly into the eye to study the storm. WesternPacific hurricanes are called typhoons.SummaryGlobal•Wind and Pressure SystemsFour major pressure systems produce threemajor wind systems in each hemisphere.•High and Low Pressure SystemsAir flows counterclockwise around lowsand clockwise around highs in the northern•hemisphere.Air rises in the center of lows and sinks in thecenter of highs.Weather Fronts and Severe Weather•Air masses interact at weather fronts.Warm fronts are associated with stratus-typeclouds and cold fronts with cumulus-type•clouds.Severe weather includes hurricanes, tornadoes,and downbursts.Self Check1. Explain how Earth’s rotation affects winds.2. Compare and contrast tornadoes and hurricanes.3. Infer the wind directions around a high in the southernhemisphere.4. Describe common differences between continental airmasses and maritime air masses.5. Compare and contrast warm fronts and cold fronts.6. Think Critically If the polar front jet stream were tomove southward over the U.S., what other weathersystems are likely to be affected?7. Use Percentage A tornado watch was issued on25 days during one year in a midwest city. Whatpercent of the year does this represent?528 CHAPTER 17 Weather and ClimateMore Section Review gpescience.com
Page 2 and 3: Start-Up ActivitiesAtmospheric Pres
Page 7 and 8: Figure 5 Earth’s continualcycling
Page 9 and 10: WeatherReading Guide■ Explain wha
Page 11: Figure 8 The symbols shownbelow eac
Page 15 and 16: Topic: World ClimatesVisit gpescien
Page 17 and 18: Ocean and Land Influence Oceans and
Page 19 and 20: ColdAridColdDry winterWarmAridWarmS
Page 21 and 22: Dinosaur ExtinctionThe impact of a
Page 23 and 24: Trace Gases Today’s atmosphere co
Page 25 and 26: Design Your OwnInvestigating Microc
Page 27 and 28: The Grapes of Wrathby John Steinbec
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CHAPTER 17 Weather and Climate - Mr. Barrow's Science Center

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