THUNDERSTORMS AND TORNADOES The material that is contained on the following pages was reprinted from the text entitled Natural Hazards and Disasters by Donald Hyndman and David Hyndman. In their book the focus is on Earth and atmospheric hazards that appear rapidly, often without signifi- cant warning. With each topic they emphasize the interrelationships between hazards, such as the fact that building dams on rivers often leads to greater coastal erosion and wildfires generally make slopes more susceptible to floods, landslides, and mudflows. By learning about the dynamic Earth processes that affect our lives, the reader should be able to make educated choices about where to live, build houses, business offices, or engineering projects. People do not often make poor choices willfully but through their lack of awareness of natural processes. 4 FIGURE 15-1. This worldwide map shows the average density of annual lightning flashes per square kilometer. Modified from NASA. Thunderstorms Thunderstorms, as measured by the density of lightning strikes, are most common in latitudes near the equator, such as central Africa and the rain forests of Brazil 4( Figure 15-1). The United States has an unusually large number of light- ning strikes and severe thunderstorms for its latitude. These storms are most common from Florida and the southeastern United States through the Midwest because of the abundant moisture in the atmosphere that flows north from the Gulf of Mexico 4( Figure 15-1). Thunderstorms form as unstable, warm, and moist air rapidly rises into colder air and condenses. As water vapor R. L. Holle photo, NOAA, NSSL. condenses, it releases heat. Because warm air is less dense 4 FIGURE 15-2. A huge stratocumulus cloud spreads out at its than cold air, this added heat will cause the rising air to top to form an “anvil” that foretells a large thunderstorm. continue to rise in an updraft. This eventually causes an area of falling rain in an outflow area of the storm when wa- ter droplets get large enough through collisions. If updrafts push air high enough into the atmosphere, the water drop- lets freeze in the tops of cumulonimbus clouds; these are the tall clouds that rise to high altitudes and spread to form wide, flat. anvil-shaped tops4 ( 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. Modified from NOAA, National Severe Storms Lab photo. Lines of thunderstorms commonly appear in a northeast- trending belt from Texas to the Ohio River valley. Cold fronts 4 FIGURE 15-3. In a thunderstorm, lighter positive-charged rain from the northern plains states interact with warm moist air droplets and ice particles rise to the top of a cloud while the heavier negative-charged particles sink to the cloud’s base. The ground has from the Gulf of Mexico along that line so the front and its a positive charge. In a lightning strike, the negative charge in the line of storms moves slowly east. cloud base jumps to join the positive charge on the ground. 2 © 2006 Thomson Brooks/Cole, a part of The Thomson Corporation. Thomson, the Star logo, and Brooks/Cole are trademarks used herein under license. 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 first 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 fifty homes in Jarrell completely off their foundation eral people thought about finishing 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-five 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. Thunderstorms produce several different hazards. Light- ning strikes kill an average of eighty-six people per year in the United States and start numerous wildfires. 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 particles are carried in updrafts toward the top of cumulo- nimbus clouds and collide with the bottoms of downward- moving ice particles or hail. The smaller upward-moving particles tend to acquire a positive charge, while the larger downward-moving particles acquire a negative charge. Thus, the top of the cloud tends to carry a strong positive C. Clark photo, NOAA. charge, while the lower part of the cloud carries a strong neg- 4 FIGURE 15-4. The return stroke on the left side of this photo ative charge 4( Figure 15-3). This is a much larger but simi- is much brighter than both the small leader coming up from the lar effect to static electricity that you build up by dragging ground and the cloud-to-cloud stroke on the right. Reprinted from Natural Hazards and Disasters, Hyndman & Hyndman, ISBN 0-534-99760-0 3 your feet on carpet during dry weather, a charge that is dis- In fewer cases, lightning will strike from the ground to the charged as a spark when you get near a conductive object. base of the cloud; this can be recognized as an upwardly fork- The strong negative charges near the bottom of the ing lighting stroke 4( Figure 15-5) rather than the more com- clouds attract positive charges toward the ground surface mon downward forks observed in cloud-to-ground strokes. under the charged clouds, especially to tall objects such as Lightning also strikes from cloud to cloud to equalize its buildings, trees, and radio towers. Thus, there is an enor- charges, although there is little hazard associated with such mous electrical separation or potential between different cloud-to-cloud strokes (visible in Figures 15-3 and 15-4). parts of the cloud and between the cloud and ground. This Lightning is visible before the clap of thunder because of can amount to millions of volts; eventually, the electrical the difference between the speed of light and the speed of resistance in the air cannot keep these opposite charges sound. Sound travels a kilometer in roughly three seconds, apart, and the positive and negative regions join with an while light will travel this distance almost instantaneously.