STORMCHASERS: Weather

photo of tornado Severe storms and their consequences have fascinated people for centuries. This fact sheet provides additional information about monsoons, hurricanes, tornadoes and lightning, and some of the noteworthy events they produced.


Tornadoes

-The American Midwest suffers the world's most treacherous thunderstorms and tornadoes. Every spring, cool, dry air from the Rocky Mountains meets cold Canadian air in the North and warm moisture from the Gulf of Mexico in the South. When these extremes of temperature and humidity collide, conditions are ripe for the ignition of thunderstorms and their fiercer consequences, supercells -- rotating mixes of updrafts and downdrafts that have the potential to spawn tornadoes.

-Researchers are still exploring how a tornado develops from a thunderstorm, which is like a giant pump in which warm, moist air rises, cools, and condenses. When it has risen as high as possible, cloudy air spreads out into a characteristic anvil shape. As the air cools and condenses, rain forms and creates a downdraft. A difference in wind speed near the ground and higher up in the atmosphere can set the system rotating. But the transition of this rotation to the violent spinning of a tornado, and the fact that only a few rotating clouds produce them, remains a mystery.

-University of Chicago professor Theodore Fujita invented the tornado damage scale in the late 1960s, rating twisters from F-0 to F-5, correlating damage with wind speed.

-Tornadoes are the shortest lived and most unpredictable of violent storms. Nearly 800 tornadoes hit the U.S. yearly, most of those in a strip including parts of Texas, Oklahoma, Kansas and Nebraska known as "Tornado Alley," as well as other areas of the Midwest.

-The largest and most damaging rash of tornadoes in history, the "Super Outbreak," occurred April 3 and 4, 1974, with 148 twisters in the Midwest and even in the western part of five eastern seaboard states. Winds in at least six of the tornadoes whirled faster than 261 mph, making them F-5 storms on the Fujita Scale; and some were among the strongest ever recorded.

-Tornadoes began in Indiana the morning of April 3, and ended early in the morning on April 4, in West Virginia and Virginia. Xenia, Ohio, 16 miles east of Dayton, was devastated. A half-mile wide tornado lifted freight cars off a passing train and demolished the nearly-deserted Central High School. A teacher and students rehearsing a play fled the auditorium seconds before the twister tossed two school buses through the wall and onto the stage.

In all, 315 people in 11 states were killed; 6,142 were injured. Total damage exceeded $600 million.


Hurricanes

-A hurricane is a heat engine that derives its energy from ocean water. When water evaporates, it absorbs energy. When water evaporates, it absorbs heat and as that water vapor rises and cools within the hurricane, it condenses, releasing heat that sustains the system. Winds are at least 74 miles per hour and blow in a large counter-clockwise spiral about the eye -- the calm, almost cloud-free center of the hurricane. Bordering the eye is the eyewall, a ring of thunderstorms where winds are strongest and rains are torrential. Cloud bands accompanied by rain and hurricane-force winds spiral inward toward the eyewall.

-Storm systems are rated 1 to 5 on the Saffir-Simpson Hurricane Intensity Scale, developed in the early 1970s by Herbert Saffir, a consulting engineer in Coral Gables, Florida, and Robert Simpson, then-director of the National Hurricane Center. The scale correlates damage from central pressure, wind speeds (ranging from 74 to over 156 mph) and potential storm surges, with 5 assigned to the most intense storms.

-Of the 126 tropical storms or hurricanes that struck the United States Atlantic and Gulf coasts between 1949 and 1990, twenty-five (19.8%) were classified as "major," rating 3 or higher on the scale. The last category 5 hurricane on the Atlantic coast was Gilbert in September 1988, equaled only by Camille in 1969 and the unnamed 1935 Florida Keys hurricane.

-Hurricanes from the Atlantic Ocean, Gulf of Mexico and Caribbean Sea inflict some of the planet's worst damage. In fact, the Caribe Indians, of the West Indies, originally used the word "Hurricane" to mean "evil spirit." Storms from the Pacific, west of Mexico and Central America rarely hit land.

-Even more than powerful winds, which can give a coconut the force of a cannonball, a storm surge poses the greatest threat of destruction during a hurricane. About nine out of 10 hurricane fatalities are caused by the storm surge, which is a large dome of water perhaps 50 miles wide that sweeps across the coastline near where the hurricane eye makes landfall. A storm surge, superimposed on the normal tidal fluctuation and topped by waves, may exceed 20 feet with an intense hurricane.

-More than 6,000 people perished in Galveston, Texas on Sept. 8, 1900, when a hurricane sent a 20-foot wave surging through the low-lying barrier island. Winds estimated at over 120 mph hit the city, and more than 3,600 houses were destroyed. The National Weather Service rated the storm as Category 4 on its Saffir-Simpson Scale.

-Hurricane Andrew, the fourth most powerful hurricane of the century, pummeled the Bahamas, south Florida and coastal Louisiana for five days in August 1992 with winds gusting up to 175 mph. Insured losses were more than double those of the 1994 Los Angeles earthquake

Fortunately Andrew, rated a Category 4 storm, produced relatively few deaths -- 63, a fraction of the losses extolled by Camille or the Florida Keys Storm -- largely due to the National Hurricane Center, which tracked the storm and issued warnings in time for evacuation

-A storm is officially named when its winds reach tropical storm strength (39 mph/63 kilometers per hour). Countries affected by hurricanes suggest names to be approved by the world Meteorological Organization's Region 4 Hurricane Committee.

Long before meteorologists made it official in 1950, those in Spanish-speaking areas named storms for the corresponding saint's day. In 1941, author George R. Stewart published a novel, Storm, featuring a forecaster who gave women's names to extra-tropical storms. And during World War II, military forecasters informally attached women's names to storms. By 1979, men's names were added to the list, which now alternate with women's names in alphabetical order.


Monsoons

Monsoons characterize regions where seasonal reversals in prevailing winds cause wet summers and dry winters. Dry winter winds blow from land to sea and there is very little rainfall. By late spring, however, the land has warmed more than the ocean surface has and prevailing winds begin to shift. Heated air rises over the land and is replaced by cool, humid air that sweeps inland and, in turn, is heated and rises. With the ascent of humid air, water vapor condenses into clouds and rainfall develops. By autumn, the land cools more than the sea surface and prevailing winds reverse direction, bringing the rainy season to an end.

camelcart race -Derived from the Arabic word for season (mausim), monsoon rains come on the heels of a searing rohini, or dry period. "Monsoon" traditionally refers both to the seasonal winds and, by extension, to the life-sustaining heavy rains they bring. Monsoons have their greatest impact over portions of Africa and Asia, where 2 billion people depend on monsoon rains for their water supply.

-A world record for precipitation was set in Cherrapunji, India in the Himalayan foothills, when an incredible 1,042 inches -- almost 3 inches per day -- fell between Aug. 1, 1860 and July 31, 1861. That season's monsoon also set the record for rain during one month: 366 inches in July 1861.

-Nations depend on the monsoon for their very survival. When a monsoon bypassed parts of China in 1877, it resulted in crop failures and claimed between 10 million and 13 million lives.


Lightning

-Lightning occurs when electricity travels between areas of opposite electrical charge in clouds, or from a cloud to the ground. One theory postulates the collision of raindrops and hailstones in a cloud leaves them electrically charged, with polarity determined by particle size. The larger, and thus heavier, particles carry a negative charge as they sift to the cloud base, while the smaller, lighter pieces remain positively charged at the top. Another theory holds that updrafts carry positive charges near the ground upward, while downdrafts carry negative charges from the upper air downward. The U.S. annually experiences about 40 million lightning strikes, making it the nation's second biggest weather killer.

(back to Stormchasers Home)


Join Us/E-mail | TV Schedule | Schedule | Teachers
Archive | Search | Site at a Glance | Shop | To Print | NOVA Home

© 1997 WGBH