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Lightning: Basics











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Spectacular, powerful, and sometimes deadly, lightning is one of the most common weather phenomena. Satellites detect more than 3 million lightning flashes each day around the world, or an average of more than 30 flashes per second. Lightning has also been observed on the planets Venus, Jupiter, and Saturn. Yet despite its frequent occurrence, lightning is still not completely understood.

How lightning forms

Lightning is generated in cumulonimbus clouds (thunderheads), which typically have a negative electrical charge near the middle of the cloud and a positive charge at the top. These charges are believed to build up mainly through collisions between small ice particles and very small hailstones or graupel. When they collide, the lighter ice particles tend to come away with a greater amount of positive charge and carry it high into the storm, while the small hail obtains mostly negative charge and stays lower in the cloud. The negative charge causes a "shadow" of positive charge on the earth below.

Conditions are then right to form an electrical circuit--this is what lightning is. An insulator--the air--keeps the connection from forming, but eventually the charges within the cloud grow too great for the air to restrain them. An electrical impulse, called a leader, reaches downward from the cloud in steps, each step covering about 50 meters (150 feet). When the leader nears the earth's surface, streamers arise to meet it, and the circuit is complete. A bright streak of electricity, the stroke of lightning, ascends along the same course the leader took. Several more strokes may follow this same path or a slightly different one.

The whole sequence is "lightning fast." The leader travels at 220,000 kilometers (136,000 miles) per hour, the pauses between steps take 50 millionths of a second, the return stroke moves at over 100 million kilometers (62 million miles) per hour, and all subsequent strokes are so fast the eye sees a single flickering lightning bolt.

There are many variations. More than half the time, lightning occurs within a single cloud or between clouds rather than reaching the earth. An uncommon but powerful type of lightning occurs when a flash originates in the positively charged region of a cloud, often when the wind has torn it away from its negatively charged parent. Lightning can appear in different forms as well: a ball, ribbon, sheet, or string of beads.

Studying lightning

Benjamin Franklin grossly underestimated the force of lightning when he did his kite-and-key experiment. An average stroke can easily release 250 kilowatt-hours of energy, enough to operate a 100-watt light bulb continuously for more than three months. And at 30,000 degrees Celsius (54,032 degrees Fahrenheit), lightning is five or six times hotter than the surface of the sun. All of this energy is contained in a channel about the width of a human thumb. (Why can't we harness his energy for human uses? As powerful as they are, lightning flashes are so brief that their energy pales next to the amount we use in our industrial society, and learning how to collect and direct the lightning energy would be an enormous task.)

Modern-day Franklins at NCAR use up-to-date techniques to investigate lightning. A specially equipped sailplane has gathered data from inside electrical storms over Florida, New Mexico, Montana and Colorado. Recent studies have used specialized lightning detectors to count all of the flashes in a storm, both those that remain wholly within the cloud as well as those that make it to earth. Scientists have also mapped entire paths of lightning flashes inside clouds to track where they originate and how they create chemical changes in storms. And scientists are getting a much clearer picture of lightning distribution over the U.S. with a nationwide network of detectors that tracks cloud-to-ground strikes. Data from this network often appear on television weathercasts. New instruments can monitor the total amount of lightning activity, including intracloud and cloud-to-ground flashes. These sensors will help scientists gain much more information on overall storm intensity.

Lightning safety

Lightning kills more people than tornadoes, hurricanes, or any other kind of bad weather except floods. But because lightning usually kills people one at a time, it tends to be underrated as a hazard. The best protection against lightning is to stay indoors during a thunderstorm. But stay away from the telephone; about 1% of people killed by lightning were talking on the phone at the time. If you cannot reach a building, a car offers excellent protection.

The highest point within the positively charged shadow under a thundercloud--a hilltop, a tree in a meadow, a golfer--is the easiest place for the leader to reach. So avoid high places during storms.

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Written by:
Carol Rasmussen
UCAR Communications

UCAR Communications UCARWeb

Created by Bob Henson, UCAR Communications
Last modified: 5 April 2000