Storms

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Printed: November 13, 2014

www.ck12.org Chapter 1. Storms

CHAPTER 1 Storms

Lesson Objectives

• Define storm.• Explain why thunderstorms occur.• Describe tornadoes.• Explain how hurricanes form.• Identify two types of winter storms.

Vocabulary

• blizzard• hurricane• lake-effect snow• lightning• storm• storm surge• thunder• thunderstorm• tornado• windchill

Introduction

In 2005, Hurricane Katrina caused a huge flood in the city of New Orleans. Figure 1.1 shows what the city lookedlike after the hurricane. Mile after mile of homes and businesses were covered with flood water. Billions of dollars ofdamage were done. More than 2,000 people died. Hurricanes are extremely strong storms and Katrina was strongerthan most. What are storms? What causes them? And what gives a storm its strength? Read on to find out.

What Are Storms?

A storm is an episode of severe weather caused by a major disturbance in the atmosphere. Storms can vary a lot inthe time they last and in how severe they are. A storm may last for less than an hour or for more than a week. It mayaffect just a few square kilometers or thousands. Some storms are harmless and some are disastrous. The size andstrength of a storm depends on the amount of energy in the atmosphere. Greater differences in temperature and airpressure produce stronger storms. Types of storms include thunderstorms, tornadoes, hurricanes, and winter stormssuch as blizzards.

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FIGURE 1.1A coast guard officer looks for survivors ofHurricane Katrina.

Thunderstorms

Thunderstorms are are known for their heavy rains and lightning. In strong thunderstorms, hail and high winds arealso likely. Thunderstorms are very common. Worldwide, there are about 14 million of them each year! In the U.S.,they are most common —and strongest —in the Midwest.

What Causes Thunderstorms?

Thunderstorms occur when the air is very warm and humid. The warm air rises rapidly to create strong updrafts.When the rising air cools, its water vapor condenses. The updrafts create tall cumulonimbus clouds called thunder-heads. You can see one in Figure 1.2.

FIGURE 1.2A thunderhead is a cumulonimbus cloud.

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Lightning and Thunder

During a thunderstorm, some parts of a thunderhead become negatively charged. Other parts become positivelycharged. The difference in charge creates lightning. Lightning is a huge release of electricity. Lightning can jumpbetween oppositely charged parts of the same cloud, between one cloud and another, or between a cloud and theground. You can see lightning in Figure 1.3. Lightning blasts the air with energy. The air heats and expands soquickly that it explodes. This creates the loud sound of thunder.

Do you know why you always hear the boom of thunder after you see the flash of lightning? It’s because light travelsfaster than sound. If you count the seconds between seeing lightning and hearing thunder, you can estimate how faraway the lightning was. A lapse of 5 seconds is equal to about a mile.

FIGURE 1.3Lightning flashes across an Arizona sun-set.

Tornadoes

Severe thunderstorms have a lot of energy and strong winds. This allows them to produce tornadoes. A tornado isa funnel-shaped cloud of whirling high winds. You can see a tornado in Figure 1.4. The funnel moves along theground, destroying everything in its path. As it moves it loses energy. Before this happens it may have gone up to25 kilometers (16 miles). Fortunately, tornadoes are narrow. They may be only 150 meters (500 feet) wide.

Classifying Tornadoes

The winds of a tornado can reach very high speeds. The faster the winds blow, the greater the damage they cause.Wind speed and damage are used to classify tornadoes. Table 1.1 shows how.

TABLE 1.1: Fujita Scale (F Scale) of Tornado Intensity

F Scale (km/hr) (mph) DamageF0 64-116 40-72 Light - tree branches fall

and chimneys may col-lapse

F1 117-180 73-112 Moderate - mobile homes,autos pushed aside

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TABLE 1.1: (continued)

F Scale (km/hr) (mph) DamageF2 181-253 113-157 Considerable - roofs torn

off houses, large trees up-rooted

F3 254-332 158-206 Severe - houses torn apart,trees uprooted, cars lifted

F4 333-419 207-260 Devastating - houses lev-eled, cars thrown

F5 420-512 261-318 Incredible - structures fly,cars become missiles

F6 >512 >318 Maximum tornado windspeed

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FIGURE 1.4Tornadoes are small but mighty storms.

Tornado Alley

Look at the map in Figure 1.5. It shows where the greatest number of tornadoes occur in the U.S. Tornadoes canhappen almost anywhere in the U.S. but only this area is called “tornado alley.” Why do so many tornadoes occurhere? This is where warm air masses from the south run into cold air masses from the north.

FIGURE 1.5Tornadoes are most common in the cen-tral part of the U.S.

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Hurricanes

Tornadoes may also come from hurricanes. A hurricane is an enormous storm with high winds and heavy rains.Hurricanes may be hundreds of kilometers wide. They may travel for thousands of kilometers. The storm’s windspeeds may be greater than 251 kilometers (156 miles) per hour. Hurricanes develop from tropical cyclones.

Hurricanes form over warm very ocean water. This water gives them their energy. As long as a hurricane stays overthe warm ocean, it keeps growing stronger. However, if it goes ashore or moves over cooler water, it is cut off fromthe hot water energy. The storm then loses strength and slowly fades away.

The Eye of a Hurricane

At the center of a hurricane is a small area where the air is calm and clear. This is the eye of the hurricane. The eyeforms at the low-pressure center of the hurricane. You can see the eye of a hurricane in Figure 1.6.

FIGURE 1.6The eye of this hurricane is easy to seefrom space.

Classifying Hurricanes

Like tornadoes, hurricanes are classified on the basis of wind speed and damage. Table 1.2 shows how.

TABLE 1.2: Saffir - Simpson Hurricane Scale

Category Kph Mph Damage1 (weak) 119-153 74-95 Above normal; no real

damage to structures2 (moderate) 154-177 96-110 Some roofing, door, and

window damage, consid-erable damage to vegeta-tion, mobile homes, andpiers

3 (strong) 178-209 111-130 Some buildings damaged;mobile homes destroyed

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TABLE 1.2: (continued)

Category Kph Mph Damage4 (very strong) 210-251 131-156 Complete roof failure on

small residences; majorerosion of beach areas;major damage to lowerfloors of structures nearshore

5 (devastating) >251 >156 Complete roof failure onmany residences and in-dustrial buildings; somecomplete building failures

Storm Surge

Some of the damage from a hurricane is caused by storm surge. Storm surge is very high water located in the lowpressure eye of the hurricane. The very low pressure of the eye allows the water level to rise above normal sea level.Storm surge can cause flooding when it reaches land. You can see this in Figure 1.7. High winds do a great deal ofdamage in hurricanes. High winds can also create very big waves. If the large waves are atop a storm surge, the highwater can flood the shore. If the storm happens to occur at high tide, the water will rise even higher.

FIGURE 1.7Storm surge can cause serious flooding.

Winter Storms

Like hurricanes, winter storms develop from cyclones. But in the case of winter storms, the cyclones form at higherlatitudes. In North America, cyclones often form when the jet stream dips south in the winter. This lets dry polar airpour south. At the same time, warm moist air from the Gulf of Mexico flows north. When the two air masses meet,the differences in temperature and pressure cause strong winds and heavy precipitation. Two types of winter stormsthat occur in the U.S. are blizzards and lake-effect snow storms.

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Blizzards

A blizzard is a snow storm that has high winds. To be called a blizzard, a storm must have winds greater than 56kilometers (35 miles) per hour and visibility of 1

4 mile or less because of wind-blown snow. You can see a blizzardin Figure 1.8.

FIGURE 1.8Blizzard in Washington, D.C. Blizzardsare unusual in Washington, D.C manyparts of the United States. Do they everoccur where you live?

Blizzards are dangerous storms. The wind may blow the snow into deep drifts. Along with the poor visibility, thesnow drifts make driving risky. The wind also makes cold temperatures more dangerous. The greater the windspeed, the higher the windchill. Windchill is what the temperature feels like when the wind is taken into account. Itdepends on air temperature and wind speed, as you can see in Figure 1.9. Higher windchill will cause a person tosuffer frostbite and other harmful effects of cold sooner than if the wind isn’t blowing.

Lake-Effect Snow

Some places receive very heavy snowfall just about every winter. If they are near a lake, they may be getting lake-effect snow. Figure 1.10 shows how lake-effect snow occurs. Winter winds pick up moisture as they pass over therelatively warm waters of a large lake. When the winds reach the cold land on the other side, the air cools. Sincethere was so much moisture in the air it can drop a lot of snow. More than 254 centimeters (100 inches) of snowmay fall in a single lake-effect storm!

Lesson Summary

• A storm is an episode of severe weather. It is caused by a major disturbance in the atmosphere. Types ofstorms include thunderstorms, tornadoes, and hurricanes.

• A thunderstorm is a storm with heavy rains and lightning. It may also have hail and high winds. Thunderstormsare very common. They occur when the air is very warm and humid.

• A tornado is a storm with a funnel-shaped cloud. It has very strong, whirling winds. Tornadoes are small butpowerful. They occur with thunderstorms and hurricanes.

• A hurricane is a large storm with high winds and heavy rains. Hurricanes develop from tropical cyclones.They form over warm ocean water. Much of the damage from hurricanes may be caused by storm surge.

• Winter storms develop from cyclones at higher latitudes. They include blizzards and lake-effect snow storms.

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FIGURE 1.9Windchill temperatures may be very low in blizzards because of the high wind speeds. How long does it take forfrostbite to occur when the air temperature is 0° F and the wind speed is 55 miles per hour?

FIGURE 1.10Lake-effect snow falls on the east side oflakes in North America. These snows areheaviest on the east sides of the GreatLakes.

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Lesson Review Questions

Recall

1. Define storm. List three types of storms.

2. Why do thunderstorms occur?

3. What is lightning? What causes it?

4. Where is tornado alley? Why do so many tornadoes occur there?

5. Where do hurricanes form? Where do they get their energy?

Apply Concepts

6. Figure 1.11 shows damage caused by a tornado. Explain how you could use the photo to classify the tornado.

FIGURE 1.11

7. Describe in words what this graph shows (see Figure 1.12). Explain the pattern in the graph. (Hint: How dohurricanes form?)

Think Critically

8. Predict which part of the U.S. is most likely to have blizzards. Explain your prediction.

9. Explain why lake-effect snow storms occur on the east side of lakes in the U.S.

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FIGURE 1.12

Points to Consider

Storms can be very dangerous. But with advance warning, people can take steps to stay safe. For example, if ahurricane is predicted, they can leave the coast and move inland.

• How can storms be predicted?• What data are needed? How are the data collected?

References

1. Courtesy of Petty Officer 2nd Class NyxoLyno Cangemi, U.S. Coast Guard. http://commons.wikimedia.org/wiki/File:New_Orleans_Survivor_Flyover.jpg . Public Domain

2. Flickr:Nicholas_T. http://commons.wikimedia.org/wiki/File:Thunderhead_%28nicholas_t%29.jpg . CC BY2.0

3. sandro visintin. http://www.flickr.com/photos/sandro87v/7346419324/ . CC BY 2.04. Courtesy of Jarrod Cook/NWS. http://commons.wikimedia.org/wiki/File:AbingdonTornado_2.JPG . Public

Domain5. Courtesy of National Oceanic and Atmospheric Administration. http://commons.wikimedia.org/wiki/File:T

or_alley.jpg . Public Domain6. Courtesy of Jeff Schmaltz, MODIS Rapid Response Team at NASA GSFC. http://earthobservatory.nasa.gov

/NaturalHazards/view.php?id=45770 . Public Domain7. NOAA. http://www.nhc.noaa.gov/HAW2/english/surge/surge_big.jpg . Public domain8. Dave Newman (Flickr:newmanchu). http://www.flickr.com/photos/groovysoup/4343759488/ . CC BY 2.09. Courtesy of NOAA. http://www.nws.noaa.gov/om/windchill/ . Public Domain

10. Hana Zavadska. CK-12 Foundation . CC BY-NC 3.011. User:Nubilt/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Windsor-co-tornado-2008-05-22

.jpg . Public Domain12. Courtesy of NOAA. http://www.tampagov.net/appl_tampa_announcements/UploadedFiles/Images/peakofseas

on.gif . Public Domain

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