Electricity in the sky.

Without any warning, the stormy sky flashes ghostly white. Jagged branches connect the earth and clouds in an eerie electic web, then disappear. Seconds later a resounding thunder-clap warns you that the storm's center is just a few miles away.

This scene, a lightning flash, is one of the most dramatic, dangerous, and yet common of all natural events. Lightning strikes somewhere on Earth a hundred times each second (see map, p. 12).

Lightning can happen whenever there is tremendous energy in the air: during sandstorms, snowstorms, volcanic eruptions and even rocket launches and nuclear blasts. But thunderstorms are the most common source.

Thunderstorms, also called electrical storms have always mystified people. The ancient Greeks thought lightning was hurled down at them by angry gods. Even back then, though, thinkers such as Socrates suggested that lightning was a natural event that could be explained by science.

But it was only some 250 years ago, in Philidelphia in 1752, that Benjamin Franklin "captured" lighting for study. He flew a silk kite into a thunderstorm and drew sparks from a key attached to the string to his bare knuckles to show that lightning acted just like electricity. (Don't try this at home, folks! Franklin's thundercloud probably wasn't highly charged. Otherwise, he wouldn't have lived to finish his experiment.) Franklin later invented the lightning rod, which protects building from damage by carrying lightning's electricity through a cable into the ground.

CHARGED UP

Lightning is difficult to study because it is one of the fastest of all natural events--and also one of the most unpredictable.

Scientists know that lightning is an electrical discharge--a release of electrical energy--from a thundercloud. But we still don't fully understand how lightning works.

Surprisingly, Earth plays an active role in the creation of lightning (see diagram, left). Our planet normally has a negative electrical charge, but as a thundercloud passes over, negatively charged electrons in the base of the cloud repel, or drive away, the negative charges on the ground. Result: A positively charged region forms on the ground beneath the cloud.

Like poles on a magnet, the negative charge at the bottom of the thundercloud and the positive charge of the ground are attracted to each other. But the air, highly resistant to the flow of electricity, insulates the charges from one another.

Still, the ground's charge moves beneath the cloud like a shadow, climbing any tall objects -- like a church steeple or a tree--that will bring it closer to the cloud. Tens to hundreds of millions of volts of electrical potential (difference in potential energy per unit of charge) develop between the cloud's base and the ground. (Compare that to the 110 volts that come out of your wall sockets.)

At a certain point, the attraction between charges becomes so strong that the air can no longer insulate the opposite charges from one another. Electrons begin to jump toward the ground and lightning strikes.

The streaming current heats the surrounding air to some 30,000[degrees]C --five times hotter than the surface of the sun--causing it to light up and expand as a shock wave. Second after the flash (can you explain why?), we hear the expanding shock wave as thunder.

A NECESSARY DANGER

For all its ferocity, lightning actually serves some useful purposes. Without it, Earth would lose its electrical charge in less than an hour. Lightning returns to the planet much, if not all, of the negative energy it loses to the atmosphere. Lightning also converts nitrogen to an oxide that falls with rain to fertilize soil. It starts wildfires that clear underbrush, allowing new plants to grow. Nearly half the fires in forests are started by lightning.

Yet electrical storms can be devastating. Except for floods, lightning is North America's most deadly weather-related event. Lightning can cause severe shocks and burns. In the U.S., an average of 120 people die and 350 are injured by lightning each year. If you spend time outdoors, learn to avoid being a target for lightning. Take a look at the illustration on the opposite page for some lightning safety tips.