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What causes storms and why are there different kinds of storms, ranging from thunderstorms to hurricanes and tornadoes? Science journalist Jack Williams, weather editor of USATODAY.com, explains the origins of storms and offers safety advice, especially for lightning and storm-related floods, which kill more people than any other storm hazard.
By Jack Williams
No matter where you live or travel on Earth, you are likely to encounter a storm of some kind sooner or later. Some places, of course, are stormier than others. Some storms are deadly or cause great damage, while others bring little danger. Many storms bring the rain or snow needed to supply drinking water and keep crops growing. All storms, big or small, demonstrate nature’s power. Many scientists who study the atmosphere were first attracted to meteorology by the wonders of storms.
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All storms have two things in common: low atmospheric pressure at the storm’s center, and winds that are created by the flow of air from higher pressure outside the storm toward the low pressure at the center. Wind, in other words, is air that’s being pushed by high pressure toward low pressure.
Areas of low air pressure occur because the Sun heats the Earth unequally. The Sun shines down almost directly on Earth’s tropical regions near the equator, heating this region more than the polar regions around the North and South poles and the middle latitudes between the poles and the tropics. Storms, along with ocean currents, redistribute heat from the tropics to the middle latitudes and the poles. Without storms and ocean currents, the tropics would grow hotter and hotter until the oceans boiled. In other words, the Sun’s heat powers the weather.
Air pouring into an area of low pressure from all sides rises because it doesn’t have any place else to go. As air rises, it cools, and if it cools enough the water vapor in the air begins to condense. This condensation creates the tiny drops of water or tiny ice crystals that make up clouds. Under the right conditions, the tiny water drops or ice crystals merge to fall from the cloud as larger water drops of rain or ice crystals of snow. This is why storms bring clouds and usually bring rain, snow, or other kinds of ice. The University of Illinois Weather World 2010 Project’s Clouds and Precipitation Web page provides detailed information on how this works.
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Despite their dramatic behavior, thunderstorms are the smallest of the different kinds of storms. They average around 16 km (10 mi) across, but they often occur in lines that can be a few hundred miles long or clusters a couple hundred miles across. Thunderstorms are often part of larger storms such as hurricanes. While thunderstorms occur most commonly in warm weather, they can happen in the winter. Sometimes snow will fall from a thunderstorm instead of rain; this is called a “thunder snow.”
Every thunderstorm includes lightning because lightning causes thunder. Lightning creates a danger in any thunderstorm, no matter how large or small. Many thunderstorms drench the countryside with heavy rain, so you also have to worry about flash floods—that is, floods that rise very quickly, sometimes running over the banks of normally small streams. If a thunderstorm is strong enough, it can also create hail, or balls of ice that fall from the storm. Hailstones are usually less than 1.3 cm (0.5 in) across. Occasionally hailstones as big as softballs fall during storms, posing considerable danger because such large stones fall faster than 160 km/h (100 mph).
Some especially dangerous thunderstorms create winds called downbursts, which descend from the clouds and hit the ground going faster than 160 km/h (100 mph). Downbursts are sometimes concentrated in small areas less than 4 km (2.5 mi) across called microbursts. Thunderstorms also sometimes create tornadoes in the warm air that’s rising from the ground. Unlike microbursts, tornado winds spin in a circle as the air rises. Tornado winds can blow faster than 400 km/h (250 mph), but twisters this strong are rare.
Over the years, gigantic storms that originate in the tropics have killed more people around the world than any other single kind of storm. These storms are known by different terms in different regions. They are called hurricanes when they form over the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or the eastern Pacific Ocean north of the equator. The same storms are called typhoons when they form over the western Pacific and tropical cyclones when they occur over the southern Pacific or the Indian Ocean. They all produce sustained winds of 120 km/h (74 mph) and faster. In their earlier stages, when their winds range in speed from 63 to 119 km/h (39 to 73 mph), they are generally called tropical storms. Hurricanes, typhoons, and tropical cyclones receive individual names from the weather offices that forecast them and are the only kind of storms named as they occur.
Hurricanes occur over oceans with water that is 27°C (80°F) or warmer in areas where there is also a layer of humid air that’s a few thousand feet thick. These storms draw their energy from the warm water and humid air. During their lives, these storms flow along the paths of the Earth’s large-scale winds, much like twigs floating with the current of a river. When the storm moves over land or cold water it begins to die, but a storm can stay over a warm ocean for a week or two, sometimes even longer, growing stronger or at least not losing much strength. A hurricane consists of lines of thunderstorms that spiral into the storm’s calm center, which is called the eye. Winds grow stronger as you approach the eye, and the very strongest winds occur in the wall of thunderstorms around the eye, called the eye wall.
As a hurricane’s winds blow hour after hour, sometimes faster than 160 km/h (100 mph), water piles up on the ocean surface near the hurricane’s center. If the storm hits land it brings with it this mound of water, called a storm surge, which can be up to 6 m (20 ft) high. Until 1969 storm surges accounted for most of the hurricane-related deaths in the United States. Since then, better forecasts and well-organized evacuations have moved people out of the way of storm surges when hurricanes threaten, saving hundreds of lives. From 1970 through 2000 storm surges killed only half a dozen people in the United States. Since the 1970s the big killers in hurricanes have been floods on inland rivers and streams caused by the drenching rain that even a weak hurricane brings. Storm surges still present a real danger for those who don’t evacuate, of course. While water kills most hurricane victims, a hurricane’s wind, which can blow faster than 160 km/h, is also dangerous; it accounts for many deaths and causes a great deal of destruction.
Huge storms called extratropical cyclones or midlatitude cyclones form over land or cool oceans outside of the tropics. While hurricanes and the other kinds of tropical cyclones draw energy from warm water and humid air, extratropical cyclones draw their energy from contrasts between cold and warm air. In a tropical cyclone the air is warm and humid and only rain falls no matter what part of the storm you are in. But conditions vary dramatically in different parts of extratropical cyclones. The southeastern part of an extratropical cyclone might have warm, humid air while the northern or western parts might be cold. Rain will fall in the storm’s warm part, but snow, sleet, or freezing rain can fall in the cold portion. Because the temperature varies most widely in winter, extratropical storms generally generate more power then.
A large thunderstorm might affect a few counties. An average hurricane can smash maybe 80 km (50 mi) of coast with its full force. A single extratropical storm, however, can affect almost all of the United States. As such a storm moves onto shore it can drop heavy rain along the west coast and several feet of snow in the Cascade and Sierra ranges. As it moves inland the storm weakens, but it can still dump heavy snow on the Rockies. Just east of the Rockies, on the western Great Plains, the storm begins pulling in humid air from the Gulf of Mexico and frigid air from Canada, perhaps whipping up a blizzard for the Plains and Midwest. As the storm moves across the Appalachians, it can send a line of thunderstorms into the Southeast as it brings rain, freezing rain, sleet, and snow to the Middle Atlantic and Northeastern states before moving out over the Atlantic Ocean with dangerous winds.
The winds of an extratropical storm seldom blow faster than 110 km/h (70 mph), especially over land. While such storms don’t create the 3 to 6 m (10 to 20 ft) storm surge that a hurricane can bring, an extratropical storm that comes ashore from the Pacific Ocean or moves up the Atlantic Coast can cause widespread beach erosion and coastal damage for both the East and West coasts. The movie The Perfect Storm depicted an exceptionally powerful extratropical storm. Extratropical storms are generally most disruptive because of their ability to stop all travel—by road, rail, and air—across several states. In 1993, for instance, an extratropical storm closed every airport on the East Coast from Atlanta to Boston and stranded thousands of motorists.
While different kinds of storms bring different kinds of danger, some basic safety rules apply to all storms. To be safe you should learn what kinds of storms affect the area where you live or are visiting, and you should become familiar with the safety rules for those storms.
You can use the Federal Emergency Management Agency’s (FEMA) Project Impact Web page to find the hazards for any part of the United States. If you are in the United States, you should buy a special weather radio, which picks up broadcasts of regular weather reports and storm watches and warnings from the National Weather Service. When the Weather Service announces a watch, it means that a particular kind of storm is possible. A warning means that a storm is headed for your area. The best weather radios automatically turn on and sound an alarm when the nearest Weather Service office issues a warning. The online Federal Emergency Management Agency Library includes links to dozens of guides that can help you prepare for any kind of disaster.
Lightning and floods kill more people in the United States than any other storm hazard. To avoid being hit by lightning, stop all outdoor activities and get into a sturdy building when you see a flash of lightning or hear a clap of thunder. Don’t take shelter under trees or in an open picnic shelter. A vehicle with a metal roof is a safe place if you roll up the windows. When a hurricane threatens, you should heed any orders to evacuate areas that could be flooded. You should also find out if any nearby streams are subject to flash floods. If so, leave if you hear a flash-flood warning. Whatever you do, don’t drive your car onto a flooded road, even if the water looks shallow. Most flood accidents in the United States occur when vehicles drive into floodwater.
Tornadoes, severe thunderstorms with winds faster than 89 km/h (55 mph), and hurricanes all produce potentially deadly winds. When such a storm threatens, don’t waste time opening windows. The biggest danger from high winds comes from the things they blow around, including objects such as garbage cans and parts of buildings. Flying debris outside the house can hit a window and send glass flying all around a room. If possible, take shelter in a basement. If not, get into a small room with no outside windows, such as a bathroom or central closet. Anyone who lives in an area where tornadoes or hurricanes hit, and whose home isn’t likely to be flooded by a storm surge or flash flood, should build a safe room, which can serve double duty as a storage room and as a shelter from storm winds when needed.
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Meteorology; Weather; Storm
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