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Tornado Formation
Tornadoes are most common from spring to early summer when severe thunderstorms are more likely. During severe storms, the falling rain causes downdrafts, or sinking air, toward the rear of the cloud. At the same time, the strong updrafts, or rising air, along the advancing edge of the storm pull humid air into the storm. As the humid air cools, moisture trapped in it condenses into water droplets and heat is released into the air. The heat causes the mass of air to quickly rise which strengthens the storm. As the updrafts get stronger, they pull more air into the cloud. The thunderstorm's natural rotation causes some of this air to spin. As the spinning air mass gets smaller, it starts rotating faster just as a spinning ice skater does when she pulls in her arms.
Characteristics
Tornadoes can occur in any part of the world. However, they are most frequent in the United States, particularly Kansas, Texas and Oklahoma. The United States has an average of 800 tornadoes a year. Tornadoes move with the thunderstorm where they formed. They typically travel at an average speed of about 35 miles per hour for an average distance of about 5 miles. The size of the tornado can vary between 300 feet and 1 mile in diameter. The dark color associated with a tornado's funnel is caused by the dirt and debris it picks up as roars across the landscape.
Dust Devils
Also called whirlwinds and willy-willys, dust devils are miniature tornadoes that pick up sand, leaves and dust as they move. They are typically 10 feet or less in diameter and 300 feet or less in height. They form on dry, hot days in arid regions such as deserts when a layer of air just above the ground gets hot and rises in an updraft. Local winds cause the rising air to spin in either a clockwise or counterclockwise direction. Dust devils sometimes have winds faster than 50 miles per hour, though they are usually too weak to cause much damage.
Measurement
The Fujita scale, also called the F-scale, measures the tornado's severity. Meteorologist Tetsuya "Ted" Fujita, created the scale in 1951. It ranks tornadoes by their wind speed and the type of damage they cause. Category F0, or "light," tornadoes are the mildest, with wind speeds between 40 and 72 miles per hour. They can cause some damage to roof shingles, chimneys, TV antennas and trees. Category F5, or "incredible," tornadoes are the most severe, with winds between 261 and 318 miles per hour. These tornadoes can level homes and severely damage large structures such as motels and schools. Fortunately, fewer than one percent of all tornadoes are category F5.