National Weather Service Taunton MA
1140 AM EDT Tue Jun 21 2005

National lightning safety preparedness week

Part two - the science of a lightning strike

At any given moment there are 1800 thunderstorms in progress somehwere on the earth, which amounts to 16 million thunderstorms each year! lightning researchers have a better understanding today of the process that produces lightning, but there is still much to learn about the role of solar flares in the upper atmosphere, the earth/S electromagnetic field, and ice in storms. We know the cloud conditions needed to produce lightning but cannot forecast the location or time of the next stroke of lightning.

Lightning occurs in volcanic eruptions, intense forest fires, surface nuclear detonations, heavy snowstorms, and large hurricanes, but it is most often seen in thunderstorms. thunderstorms form in air that is moist, unstable, and has a trigger that causes the air to rise, such as a cold front. Rising motions in the storm build the cloud to as high as 6 to 10 miles above sea level. Ice forms in the higher parts of the cloud.

The ice particles vary from small ice crystals to large hailstones. there are a lot of collisions between the particles which causes a separation of electrical charges. Positively charged ice crystals rise to the top of the storm and negatively charged particles and hailstones drop to the middle and lower parts of the storm. enormous charge differences develop.

A moving thunderstorm gathers another pool of positively charged particles along the ground that travel with the storm. Positively charged particles rise up taller objects such as trees, houses, and telephone poles. The particles can even move up you, have you ever been under a thunderstorm and had your hair stand up? if so, you May be the lightning target.

The negatively charged area in the storm sends out a charge toward the ground called a stepped leader. It is invisible to the human eye. When it gets close to the ground, it is attracted by all of these positively charged objects and a channel develops. You see the electrical transfer in this channel as lightning. There May be several return strokes of electricity within the established channel that you will see as flickering lightning. The lightning channel heats rapidly to 30,000 degrees or more and the rapid expansion of heated air causes the thunder. Since light travels faster than Sound in the atmosphere, the Sound is heard after the lightning. if you see lightning and hear thunder at almost the same time, the lightning is in your neighborhood!

Not all lightning forms in the negatively charged area low in the thunderstorm cloud. Some originates in the cirrus anvil at the top of the storm where there is a large positive charge. A strike originating in this area is called a positive flash. It is particularly dangerous for several reasons. It frequently strikes ahead of or behind a thunderstorm, away from the rain area, thus catching people by surprise, like a bolt from the Blue. Positive strikes typically last longer, so fires are more easily ignited. also, they usually carry a high peak electrical current which increases the lightning risk to an individual.

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