a flash of light in the sky caused by an electrical current that may flow between parts of the same cloud, between different clouds, or between clouds and the earth

There are six different kinds of lightning, numbered according to frequency of occurrence: (1) from front of storm to back; (2) from upper to lower clouds; (3) "glow discharge" into the surrounding air; (4) from low raincloud to the earth; (5) from "squall cloud" to the earth; and (6) from upper cloud to earth. Arrows indicate the movement of air.

What Causes Lightning

Many things occur at the same time in a thunderstorm. All the updrafts and downdrafts make water droplets rub against each other, causing static charges. Positive and negative charges are separating, and electrical stress is set up by interactions with the wind as it blows water droplets and ice crystals around the cloud. Lightning is Mother Nature's way of relieving that stress. In the first picture below, a large positive charge is concentrated in the top of the cloud, a negative charge in the bottom, and the ground is positively charged. The positive charge on the ground can flow up tall objects like buildings and trees. Since air is not a good conductor of electricity, a lightning stroke does not happen right away. Instead, it waits until the difference between the positive and negative charges is great enough to overcome the resistance of the insulating air. A couple of leader strokes form a path through the atmosphere (second picture), then are followed by the ground to cloud bolt with positive charges (third picture).

A lightning flash is, therefore, a huge spark, similar to that produced in the sparkplug of an automobile, multiplied by several hundred thousand volts. Scientists have found that one stroke of lightning measures more than 125,000,000 volts of electricity. A spark between a cloud and the earth may measure as much as eight miles long, and may travel at a rate of 100,000,000 feet per second. Lightning that reaches between oppositely charged clouds may have a length of 20 miles. Photographs of lightning obtained by radar indicate that some cloud-to-cloud lightning strokes may measure 100 miles long.

How Lightning Produces Thunder

As lightning travels, it heats the air in its path -- up to about 50,000 degrees, heat matched only by the surface of the Sun. The sudden heating causes the air to expand violently. The hot air, bumping into the cooler air around it, makes the noise we call thunder.

Kinds of Lightning

All lightning strokes are basically about the same, but they appear to have different forms, depending on the position of the observer.

Forked, Zigzag, or Chain lightning is a chain of brilliant light that appears to zigzag (such as the picture at the beginning of this page). It actually follows a winding path, like that of a river. The single streak of lightning often breaks into several branches or forks.

Sheet lightning has no particular form. It is usually a bright flash that spreads all over the horizon and lights up the sky. Sheet lightning is actually forked lightning, but too far away for the observer to see the stroke. All that is seen is the flash, reflected from the clouds.

Heat lightning, often seen on summer evenings, is the same as sheet lightning, but the flashes are fainter. Thunder usually does not accompany them, as the lightning occurs too far away for thunder to be heard.

Ball lightning seems to consist of balls of fire, as small as walnuts or as large as balloons, that last about three to five seconds. They fall swiftly from the clouds until they strike the ground and explode. Sometimes they roll slowly along the ground and do not explode until they hit an obstacle. Ball lightning is the least understood of all forms of lightning. Scientists have produced ball lightning in the laboratory, but how it is produced in nature is still unknown.

"History of Lightning"

Benjamin Franklin was the first to show the connection between electricity and lightning. He made a silk kite and fastened a piece of wire near its top. Then he attached a long string to the kite and tied an iron key to the free end of the string. In 1752, Franklin sent the kite up in a heavy thunderstorm. As a thundercloud came near the kite, Franklin saw the loose ends of the string stiffen. He put his hand near the key and instantly felt a shock as a spark traveled from the key to his finger.

Lightning Myths and Facts

Myth No rain means no lightning.
Fact Lightning often strikes outside of heavy rain and may occur as far as 10 miles away from any rainfall.

Myth The rubber soles of shoes or rubber tires on a car provide protection from lightning.
Fact While rubber is an electric insulator, it provides no protection from a lightning strike. The average lightning bolt carries about 30,000 amps of charge, has 100 million volts of electric potential, and is about 50,000° F. What's more, the lightning bolt just traveled several miles through the atmosphere, which is an even better insulator than rubber. The 1/2 inch or so of rubber in the wall of your tire or on the bottom of your shoe isn't going to slow that lightning bolt down.

Myth If caught outside during a thunderstorm lie as flat on the ground as possible to avoid being struck by lightning.
Fact Although lying flat on the ground was once thought to be the best course of action, it is now known to be one of the worst. When lightning strikes the earth, it induces currents in the ground that can be felt up to 100 feet away. These currents fan out from the strike center in a tendril pattern, so in order to minimize your chance of being struck, you have to minimize both your height and your body's contact with the earth's surface. Your best bet is to crouch down on the balls of your feet and cover your ears. Although this position will minimize your chances of being struck, it will not guarantee your safety.

Gail Saunders-Smith Lightning Mankato, MN: Capstone Press, 1998

Jetstream--An Online School for Weather www.srh.noaa.gov/jetstream/lightning/lightning_intro.html

Benjamin Franklin

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