Severe Thunderstorms

Florida has more thunderstorms than any other area of the United States.

Thunderstorms affect relatively small areas when compared to hurricanes, however they occur much more frequently. The typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. Nearly 1,800 thunderstorms are occurring at any moment around the world. That's 16 million a year!

Despite their small size, all thunderstorms are dangerous. Every thunderstorm produces lightning, which kills more people each year than tornadoes. Heavy rain from thunderstorms can lead to flash flooding. Strong winds (straight-line winds or downbursts), hail, and tornadoes are also dangers associated with some thunderstorms.

Straight-line winds are responsible for most thunderstorm wind damage. Some of these winds can exceed 100 mph. A downburst is a small area of rapidly descending air beneath a thunderstorm. When these occur, many times clouds are pulled toward the ground with the wind and give a tornado-like appearance.

Of the estimated 100,000 thunderstorms that occur each year in the U.S., only about 10 percent are classified as severe. The National Weather Service considers a thunderstorm severe if it produces hail at least 3/4-../inch diameter, wind 58 mph or higher, or tornadoes.

What makes a thunderstorm?

Every thunderstorm needs moisture to form clouds and rain, relatively warm unstable air that can rise rapidly, and lift in the form of fronts, sea breezes or mountains capable of lifting air to help them form.

The life cycle of a thunderstorm consists of three stages. The developing stage, the mature stage, and the dissipating stage.

Lightning - The Underrated Killer

Lightning kills more people every year than do tornadoes and hurricanes combined. Between 1940 and 1989, more than 8,000 people were killed by lightning.

The first strike of lightning from a thunderstorm may be the most dangerous, not in terms of impact, but because of the element of surprise and not knowing it is in the area. Your chances of being struck by lightning are about 1 in 600,000. That can be reduced by following the common sense safety rules listed below.

What is lightning?

As thunderstorms develop, interactions of charged particles produce an intense electrical field within a cloud. A large positive charge is usually concentrated in the frozen upper layers of the cloud, and a large negative charge, along with a smaller positive area, is found in the lower portions.

As the thunderstorm passes over the ground, the negative charge in the base of the cloud induces a positive charge on the ground below and for several miles around the storm. The ground charge follows the storm like an electrical shadow, growing stronger as the negative cloud charge ../increases.

The attraction between positive and negative charges make the positive ground current flow up buildings, trees, and other elevated objects in an effort to establish a flow of current, but air, which is a poor conductor of electricity, insulates the cloud and ground charges, preventing a flow of current until a huge electrical charge builds up.

Lightning flashes when the attraction between positive and negative charges become strong enough to overcome the air's high resistance to electrical flow.

What causes lightning?

There are different kinds of lightning: lightning within the clouds, lightning between the clouds, lightning from clouds to clear air, and lightning from cloud to ground.

The lightning that concerns people the most is cloud to ground. These flashes are the ones that start fires, splinter trees, knock out electrical power and even kill people. It is estimated that cloud to ground lightning strikes represent only about 20 percent of all lightning strikes.

Here is what happens. Negatively charged electrons zigzag downward in a forked pattern (known as a stepped leader). As the stepped leader nears the ground, it draws a stream of positive charge upward, usually through something high such as a tree or building. As the leader and the streamer come together, a powerful electrical current begins flowing and contact begins the return stroke with an intense wave of positive charges traveling upward about 60,000 miles per second. This is the light that we see and the process can repeat several times along the same path in less than half a second, making lightning flicker.

What causes thunder?

An average lightning strike produces currents of 30,000 amperes, some could approach 30 million volts at 100,000 amperes. The rapid rise in current produces temperatures in the lightning channel of more than 50,000 degrees Fahrenheit which causes the air in the channel to expand. The expanding air cools, then contracts. Rapid expansion and contraction of the air around lightning starts molecules moving back and forth, making sound waves we hear as thunder.

Lightning safety tips

Watch for signs of an approaching storm. If outside, get inside a building or a vehicle (not a convertible). Never seek shelter under trees and try to avoid using small sheds. If caught outdoors and no shelter is near, your best protection is to crouch in the open, keeping twice as far away from trees as they are high. Stay indoors and do not venture outside unless absolutely necessary. Stay away from open doors and windows, fireplaces, stoves, metal pipes, sinks and electrical devices. Do not take a bath or shower during a storm. Turn off and unplug (if possible) TV's, computers and air conditioners. Power surges from lightning may damage them. Avoid using the telephone unless it is an emergency. Get out of boats and away from water.