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Tornadoes Are Possible – – – – – – – – – – – – – – – –– – – – – – – – (See “Up Close: Jarrell Tornado, Texas, 1997”)

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Tornadoes Are Possible – – – – – – – – – – – – – – – –– – – – – – – – (See “Up Close: Jarrell Tornado, Texas, 1997”) 4 FIGURE 15-1. –150 –120 –90 –60 –30 0 30 60 90 120 150 This worldwide map shows the Strokes 60 average density of per km2 annual lightning per year fl ashes per square kilometer. 30 50 40 30 20 0 10 8 6 4 –30 2 1 .8 .6 –60 .4 .2 Modifi ed from NASA. Modifi .1 wide, fl at. anvil-shaped tops 4( Figure 15-2). This is where lightning and thunder form. Cold air pushing under warm moist air along a cold front is a common triggering mechanism for these storm sys- tems, as the warm humid air is forced to rapidly rise over the advancing cold air. Isolated areas of rising humid air from localized heating during the day or warm moist air ris- ing against a mountain front or pushing over cold air at the surface can have similar effects. Individual thunderstorms average 24 kilometers across, but coherent lines of thun- derstorm systems can travel for more than 1,000 kilometers. Lines of thunderstorms commonly appear in a northeast- trending belt from Texas to the Ohio River valley. Cold fronts from the northern plains states interact with warm moist air R. L. Holle photo, NOAA, NSSL. from the Gulf of Mexico along that line so the front and its 4 FIGURE 15-2. A huge stratocumulus cloud spreads out at its line of storms moves slowly east. top to form an “anvil” that foretells a large thunderstorm. Thunderstorms produce several different hazards. Light- ning strikes kill an average of eighty-six people per year in + the United States and start numerous wildfi res. Strong winds + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + can down trees, power lines, and buildings. In severe thun- + + + + – derstorms, large damaging hail and tornadoes are possible – – – – – – – – – – – – – – – –– – – – – – – – (see “Up Close: Jarrell Tornado, Texas, 1997”). – – ––– – – – – ––– – – – – – – – – – – –– Lightning Lightning results from a strong separation of charge that builds up between the top and bottom of cumulonimbus clouds. Atmospheric scientists commonly believe that this charge separation increases as water droplets and ice particlesPROPERTY are carried in updrafts toward the OFtop of cumulo- CENGAGE LEARNING nimbus clouds and collide with the bottoms of downward- + + + + + + moving ice particlesNOT or hail. FOR The smaller upward-movingREPRODUCTION + + + + + + + + + + + + + + + + + + particles tend to acquire a positive charge, while the larger + + + + + + + + Modifi ed from NOAA, National Severe Storms Lab photo. Modifi downward-moving particles acquire a negative charge. Thus, the top of the cloud tends to carry a strong positive 4 FIGURE 15-3. In a thunderstorm, lighter positive-charged rain droplets and ice particles rise to the top of a cloud while the heavier charge, while the lower part of the cloud carries a strong neg- negative-charged particles sink to the cloud’s base. The ground has ative charge 4( Figure 15-3). This is a much larger but simi- a positive charge. In a lightning strike, the negative charge in the lar effect to static electricity that you build up by dragging cloud base jumps to join the positive charge on the ground. 1 © 2006 Thomson Brooks/Cole, a part of The Thomson Corporation. Thomson, the Star logo, and Brooks/Cole are trademarks used herein under license. WW3215.indb3215.indb 339898 33/1/05/1/05 111:15:061:15:06 AAMM Thunderstorms and Tornadoes UP CLOSE Jarrell Tornado, Texas, 1997 On May 27, 1997, around 1 P.M., a tornado watch was issued One woman had hidden under a blanket in her bathtub. for the area of Cedar Park and Jarrell, 65 kilometers north of Her house blew apart around her, and both she and the tub Austin, Texas. Many people heard the announcement on the were thrown more than 100 meters. She survived with only a radio or on television, but most went on with their daily work. gash in her leg. Some people watched the tornado approach Storms are common in the hill country. This case seemed fa- and decided to outrun it by car. They survived, but in other miliar: A cold front from the north had collided with warm, tornadoes people have died doing this when they would have water-saturated air from the Gulf Coast to generate a line of survived at home. Eyewitnesses reported that the Jarrell tor- thunderstorms. A tornado warning was issued at 3:25 P.M. nado lifted one car at least 100 meters before dropping it as Just before 4 P.M., a tight funnel cloud swirled down a crumpled, unrecognizable mass of metal. from the dark clouds 8 kilometers west of Jarrell, a com- This was the second tornado to strike Jarrell; the fi rst was NASA. munity of roughly 450 people. This tornado moved south– only eight years previously on May 17, 1989. southeast along Interstate 35 at 32 kilometers per hour One of several tornadoes during the same event moved rather than taking a more typical easterly track. A local warn- south through the town of Cedar Park, demolishing a large ing siren sounded ten to twelve minutes before the funnel Albertson’s supermarket, where twenty employees and shop- struck. pers huddled in the store’s cooler. One of us happened to be When trained spotters saw a tornado on the ground, the a few kilometers south of Cedar Park playing golf that hot and alarm was sounded and everyone who could took shelter. humid Texas morning. Thunderstorms began to build on the Some sought protection in interior rooms or closets; few horizon, and the sky took on a greenish gray cast. Early in the homes have basements because limestone bedrock is usually afternoon, golf course attendants quickly drove around the close to the surface. People in this area are advised to take course warning players that there were two spotted tornadoes shelter in closets and bathtubs with a mattress for cover, but in the area. Because thunderstorms and tornadoes are fairly in this case it did not matter. Within minutes, the F5 tornado common in the area, many people become complacent; sev- wiped fi fty homes in Jarrell completely off their foundation eral people thought about fi nishing their golf rounds. Reach- slabs. Hail the size of golf balls and torrential rain pounded ing the car in a drenching downpour, we realized that there the area. Wind speeds were 400 to 435 kilometers per hour was no safe place to go. Our cell phones were useless because for the twenty to twenty-fi ve minutes the twister was on the all circuits were busy. Fortunately, the tornadoes were north ground. At least thirty people died. of us, so we drove south into Austin to wait out the storm. your feet on carpet during dry weather, a charge that is dis- charged as a spark when you get near a conductive object. The strong negative charges near the bottom of the clouds attract positive charges toward the ground surface under the charged clouds, especially to tall objects such as buildings, trees, and radio towers. Thus, there is an enor- mous electrical separation or potential between different parts of the cloud and between the cloud and ground. This can amount to millions of volts; eventually, the electrical resistance in the air cannot keep these opposite charges apart,PROPERTY and the positive and negative regionsOF join CENGAGE with an LEARNING electrical lightning stroke 4( Figures 15-3 and 15-4). Because negative and positive charges attract one an- other, a negativeNOT electrical FOR charge may REPRODUCTION jump to the positive- charged cloud top or to the positive-charged ground. Air is a poor conductor of electricity, but if the opposite charges are strong enough they will eventually connect. Cloud-to- C. Clark photo, NOAA. ground lightning is generated when charged ions in a thun- 4 FIGURE 15-4. The return stroke on the left side of this photo dercloud discharge to the best conducting location on the is much brighter than both the small leader coming up from the ground. ground and the cloud-to-cloud stroke on the right. 2 Reprinted from Natural Hazards and Disasters, Hyndman & Hyndman, ISBN 0-534-99760-0 WW3215.indb3215.indb 339999 33/1/05/1/05 111:15:111:15:11 AAMM Negatively charged step leaders angle their way to- Danger from lightning strikes can be minimized by ob- ward the ground as the charge separation becomes large serving the following: enough to pull electrons from atoms. When this occurs, a ■ Take cover in an enclosed building. Do not touch any- conductive path is created that in turn creates a chain reac- thing that is plugged in. Do not use a phone with a cord; tion of downward-moving electrons. These leaders fork as cordless phones and cell phones are okay. One of us they fi nd different paths toward the ground; as they move was struck by lightning through a corded phone—not closer, positive leaders reach upward toward them from ele- something you want to experience. vated objects on the ground (see the lower right side of Fig- ■ ure 15-5). If you ever feel your hairs pulled upward by what Do not take a shower or bath or wash dishes. feels like a static charge during a thunderstorm, you are at ■ Stay away from high places or open fi elds or open wa- high risk of being struck by lighting. When one of the pairs ter. Water conducts electricity. of leaders connects, a massive negative charge follows the ■ Stay away from tall trees. If there are tall trees nearby, conductive path of the leader stroke from the cloud to the stay under low bushes or areas of small trees. ground. This is followed by a bright return stroke moving ■ If trapped in the open, crouch on the balls of your feet, back upward to the cloud along the one established con- away from other people.
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