Tornadoes are Earth's most violent storms

Tornadoes are the most violent storms on Earth. Winds spiraling into them usually exceed 100 mph and can reach speeds of 300 mph. In the USA, an average of 1,000 tornadoes spin up beneath thunderstorms each year, and these typically kill about 60 people.

Tornadoes and the threat of tornadoes are a key part of the USA's spring weather because spring brings favorable tornado conditions. But tornadoes can occur any time of the year, during the day and at night.

The National Weather Service defines a tornado as "a violently rotating column of air in contact with the ground and pendant from a thunderstorm." In other words, a thunderstorm is the first step in the creation of a tornado. Then, if other conditions are right, the thunderstorm might spin out one or more tornadoes.

The three key conditions required for thunderstorms to form are:

• Moisture in the lower to mid levels of the atmosphere.

• Unstable air. That is, air that will continue rising once it begins rising from near the ground.

• A lifting force. Something is needed to cause the air to begin rising. The most common lifting force is heating of air near the ground. As the air warms it becomes lighter and begins rising. Advancing masses of cool air, which force warm air upward, also trigger thunderstorms.

When all the conditions are present, humid air will rise high into the sky and cool and condense into towering clouds, forming thunderstorms. This air rising into a thunderstorm is called an updraft. Tornadoes form in within a thunderstorm's updraft.

The strongest tornadoes are often near the edge of the updraft, not far from where air is descending in a downdraft caused by the thunderstorms with falling rain or hail. This is why a burst of heavy rain or hail sometimes announces a tornado's arrival.

Tornadoes are commonly associated with the nation's heartland – in a 10-state area stretching from Texas to Nebraska that also includes Colorado, Iowa, Illinois, Indiana, Missouri and Arkansas, known as Tornado Alley.

But, they are not limited to this region. Tornadoes have occurred in all 50 U.S. states and are, in fact, more common in Florida than they are in Oklahoma.

Florida tornadoes are generally weak — for tornadoes — with winds around 100 mph.

Tornadoes that have hit Oklahoma, on the other hand, are some of the most violent on record. A tornado that struck Oklahoma City and its southern suburbs in 1999 had winds of nearly 320 just above the ground.

Tornadoes are ranked by the damage they do using the six-tiered Fujita Scale. F0 and F1 tornadoes on the scale are considered "weak" and cause minimal to moderate damage with winds from 40-112 mph. F2 and F3 tornadoes are considered strong, packing winds of 113-206 mph that can cause major to severe damage. Violent tornadoes are those classified F4 and F5 with winds exceeding 206 mph. Damage is extreme to catastrophic.

Most weak tornadoes last 10 minutes or less, traveling short distances. Violent tornadoes have been known to last for hours and a few have traveled more than 100 miles.

http://www.usatoday.com/weather/resources/2006-04-03-tornado-basics_x.htm