Two types of violent weather affect the United States each year: tornados and hurricanes. The U.S. has more tornadoes each year than any other country in the world. There are two conditions necessary for tornadoes to form. First, fast winds in the upper troposphere are slowed at the ground because of friction. This condition, of fast winds in the upper troposphere, and slower winds in the lower troposphere, causes a tube of horizontally rotating air to form. For a tornado to form, something must make that column of rotating air turn vertical. A strong updraft of rising hot air can be powerful enough to twist the horizontally rotating column of air into a vertical column of low pressure air.
Thunderstorms that precede tornadoes usually form in the late afternoon after the sun has warmed the ground. The air heated over the ground is buoyant and it rises and rapidly reaches the dew point causing the formation of massive cumulonimbus clouds. The updraft of the rising air is often enough to turn the horizontally rotating air in the troposphere into a vertical funnel cloud. In the late 1960’s, Dr. T. Theodore Fujita, an authority on tornadoes at the University of Chicago, proposed a scale (called the Fujita scale) for classifying tornadoes according to their rotational wind speed and the damage done by the storm.
Browse this web page for more information:NOAA FAQ about Tornadoes
Tropical cyclones are another awesome expression of nature’s power. Tropical cyclones go by several different names, depending on their location. Hurricane is the term used in the Atlantic and East Pacific, Typoon is the term used in the Western Pacific, and Cyclone is the term used in the Indian Ocean or near Australia. Cyclones are zones of intense low pressure. The Atlantic basin cyclones that you investigated in Lab 3 generally started on the western coast of Africa as zones of low pressure resulting from superheated land. These low pressure zones then moved out over the central Atlantic and gathered the abundant energy stored as latent heat in the warm water. By the time that they reached the eastern coast of the Americas they were fully developed, powerful cyclones.
Browse this NOAA page on Hurricanes for more information.
Note that tornadoes can be a bi-product of hurricanes!