Thunderstorms And Tornadoes

Thunderstorms are so common in some locations that they can hardly be considered extreme. Each year, our planet hosts about 14

million of these wild tempests, which averages to 40,000 thunderstorms a day.

Although they can be extremely powerful and do a great deal of damage, thunderstorms are relatively small compared to other powerful storms such as cyclones. Thunderheads form individually or along the length of a cold front that can be as long as 600 miles (1,000 km). The most dangerous storms along such a squall line can grow to more than 50,000 feet (15 km) high. In the United States, squall lines form in the spring and early summer where the warm, humid air mass from the Gulf of Mexico meets the frigid continental air mass from Canada.

Thunderstorms are named for the earsplitting clap that accompanies the lightning strikes they create. Lightning is a powerful discharge of electricity. It may strike within a cloud, between clouds, between a

Lightning strikes in many spots during an intense storm. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

cloud and the surrounding air, or from a cloud to the ground. Luckily, only about 20% of lightning bolts strike the ground. Thunderstorms generate winds that damage crops and property, hail that pummels crops, heavy rains that bring flash floods, lightning that strikes people and ignites forest fires, and tornadoes that destroy local areas. Approximately 200 Americans die and 550 are injured by lightning strikes each year. One common misconception is that lightning does not strike the same place twice—in fact, it strikes New York City's Empire State Building about 23 times per year.

Thunderstorms are classified as either ordinary or severe. Ordinary thunderstorms are shorter-lived and milder than severe thunderstorms. These common storms build above warm ground, so they form most often in the late afternoon or early evening, in spring and summer. The storms are born when masses of warm, humid air rise into an unstable atmosphere. If these updrafts are extremely strong, cumulus clouds, followed by cumulonimbus clouds, swell upward. When the cloud top reaches the stratosphere, the winds shift it horizontally, forming the anvil shape of a thunderhead. Condensing water vapor in the rising air releases latent heat until the air inside the cloud is warmer than the air outside. The powerful updrafts keep water droplets and ice crystals suspended until they grow so large that they fall as rain or hail. The rain and hail cool the cloud as they descend, and the cooler air becomes dense and begins to sink, setting up a downdraft. This downdraft and the original updraft make up the two vertical limbs of a convection cell. Most thunderstorms have several convection cells; the side-by-side updrafts and downdrafts create wind shear. By now, the thunderstorm is mature, producing gusty winds, lightning, heavy rain, and hail. Although now extremely impressive, this thunderstorm is doomed. Cool temperatures at the base of the cloud weaken convection, cutting off the updraft of moist air and ending condensation. Without its source of latent heat, the storm dies, usually between 15 and 30 minutes after it began, although new thunderheads often build in the same region.

Severe thunderstorms are similar but more intense. In the United States, severe thunderstorms are most common in the Midwest but may occur in other locations. Severe thunderstorms have extremely strong updrafts, so raindrops and hailstones become huge before becoming heavy enough to fall. Severe thunderstorms have hail that is at least 0.75 inch (1.9 cm) in diameter and surface wind gusts of nearly 60 mph (97 km/hr). Once the hail falls, the intense precipitation of a severe thunderstorm cools the base of the cloud so much that severe downdrafts form. Rather than signaling the demise of the thunderstorm, these robust downdrafts strike the ground and wedge warm air upward, enhancing the updrafts. These convection cells are so powerful that they can keep a storm alive for many hours.

Sometimes the downdrafts hit the ground and spread horizontally, creating a microburst. In a microburst, winds may reach 170 mph (170 km/hr), damaging trees, buildings, and other structures. Microbursts were discovered in 1985 when a Delta Airlines jumbo jet attempted to

Cumulonimbus squall line associated with a cold front. (Historic NWS Collection)

land at Dallas-Fort Worth Airport, Texas, during a summer thunderstorm. As the plane made its final approach, the storm intensified and the aircraft was slammed down on the ground far short of the runway, smashing a car and killing 137 of the people on board the plane. Reviews of data from previous airplane crashes revealed that other planes were probably downed by microbursts. Pilots are now trained to detect and maneuver through these perilous winds.

The extreme wind shear in a severe thunderstorm causes the air mass to rotate, sometimes spawning tornadoes. These violently rotating, funnel-shaped clouds extend below the cumulonimbus cloud of the thunderstorm toward the ground surface. Tornadoes are intense—the winds may exceed 200 to 300 miles (320 to 480 km) per hour in speed—but they are relatively small, only a few feet to a mile in diameter. The updraft at the center may reach 200 miles (320 km) per hour. Tornadoes are found in great numbers along what is known as Tornado Alley, an area running through the states of Nebraska, Kansas, Oklahoma, and Texas.

Tornadoes grow along squall lines, with multiple storms occurring at the same time. In April 1974, a squall line produced 148 tornadoes—the largest number ever recorded for a 24-hour period— over 2,600 miles, through 13 states and Canada. During the peak of the storm, 15 tornadoes were on the ground simultaneously. The death toll was 315 people, with nearly 5,500 injured.

A tornado strikes south of Dimmitt, Texas, in June 1995. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

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