Jagged bolts of lightning rip the sky from cloud to ground, tripping off peals and claps of thunder, dazzling onlookers throughout the world an estimated 100 times a second.

Lightning, once thought to be punishment hurled by angry gods in the sky, remains one of nature's most harmful forces. Each year it kills more than a hundred Americans, causes millions of dollars of damage, starts forest fires and blacks out power supplies.

Despite its tremendous capacity for destruction, lightning creates, too. Its energy welds molecules of nitrogen gas with oxygen, yielding a form that falls to the ground as fertilizer for plants. Lightning maintains the electrical balance between the ground and the atmosphere. By igniting forest fires, lightning helps clear away underbrush that would strangle forest growth. And billions of years ago, lightning may even have helped form organic chemicals necessary for Earth's first forms of life.

Lightning straddles the fence between electrifying and electrocuting nature's other creations. A Barrage of Electrons

As Benjamin Franklin discovered in 1752, lightning is a giant spark of static electricity. It is a barrage of electrons, the negatively charged particles that, with positively charged protons, maintain electrical balance in an atom. If an atom either gains or loses an electron, this balance is tipped, and a charge is conferred.

As physicists generally explain

it, most types of clouds have little net charge -- positive and negative charges are generally balanced. In a thunderhead, however, rapid updrafts traveling as fast as 200

feet per second carry water droplets and other particles. As the droplets ascend, they freeze. Electrons are transferred from the rising bits of ice to larger chunks of ice being pulled downward by gravity. (Usually the ice melts into rain before hitting the ground.) Thus, the upper part of a thundercloud becomes positively charged while the lower part becomes negatively charged.

As the separated opposite charges grow, the attraction between them increases. In a sense, the excess electrons seek a path to flow back and restore equilibrium. Lightning occurs when they find that path. More than half of the lightning flashes in a typical storm occur within the cloud, causing it to light up like a pinball machine.

Most other lightning flashes, as many as several thousand during a single storm, strike the ground or an object reaching up from the ground. First, a so-called leader descends from a cloud, forking every couple of hundred feet and traveling several hundred times faster than sound.

The Earth doesn't just passively absorb a lightning strike. As the leader approaches the ground, positive charge from the Earth rises to meet it. What happens is that electrons drain away from the area, leaving it positively charged. Often, objects that rise above the surrounding features serve as conduits for the ground's charge. The rush of charge from the ground can make your hair stand on end, if you're unlucky enough to be in the vicinity of an oncoming bolt.

The two charges meet about 150 feet above the ground. When the cloud and the ground connect, a wave of positive charge, called the return stroke, flows from the ground up to the cloud through the channel created by the leader. The cycle of leader and return stroke may repeat several times in quick succession as other charged regions of the cloud find the newly created path. The human eye can just barely notice the interval between strokes, which is why lightning appears to flicker.

The return stroke carries a

typical peak current of 30,000

amperes, strong enough to briefly heat the lightning channel to approximately 60,000 degrees Fahrenheit.

Click, Crack and Rumble

The sudden heating of the air around the channel makes it expand at supersonic speed. The shock waves are heard as thunder.

Thunder is sometimes divided into three components: a click, a crack and a low, lengthy rumble. The click happens when the leader forges the channel, the crack

happens when the return stroke enters the channel, and the rumble is made by the return stroke going through the channel at higher altitudes. Although the bolt is over in a fraction of a second, the rumble continues because the highest part of the bolt is much farther away and the sound takes longer to reach a listener.

For every second that passes between the lightning and the thunder, the storm is one-fifth of a mile away.

Being Hit -- and Surviving

Many more people are struck by lightning and live than are killed by it. Many of those who are struck suffer burns. Some go into cardiac arrest. And a few emerge unscathed, even from direct strikes.

"The effects of lightning on people are complicated," said E. Philip Krider, chief of atmospheric sciences at the University of Arizona. He said that when a person is struck, the bulk of the electric current often won't penetrate the skin. "If someone's wet from the rain or from perspiration, the surface of their skin is a better conductor than their body," Krider said. "It's possible, then, that someone who's wet would be more likely to survive than someone who's dry."

Unusual Phenomena

Rare forms of lightning intrigue scientists but the quest

to document such phenomena, let alone explain them, has been difficult.

Top-of-the-cloud lightning. Jets of lightning exploding from the top of a thundercloud and into the upper atmosphere were caught on film by researchers at the University of Minnesota last year, during a storm associated with Hurricane Hugo. Such discharges may pose a threat to both aircraft and spacecraft, Krider said.

Clear-air lightning. Sometimes lightning seems to come from out of the blue, even though it really is from a storm several miles away. Apparently, however, under extraordinary conditions, clear air can generate enough charge to initiate lightning. French scientists have developed a way to trigger lightning in the clear air by launching a small rocket that trails a metal wire; when the rocket gets several hundred feet high, lightning will jump from the rocket.

Airplanes and rockets can trigger lightning as well. At the beginning of the Apollo 12 mission to the moon, the rocket was hit by two lightning strikes, even though the weak cold front on the day of the launch wasn't producing any other lightning. Heat lightning. These are nor- mal bolts from a thunderstorm out of earshot, more than 15 miles away.

Bead lightning. Rarely, after lightning strikes, a glow resembl- ing sausage links persists for about half a second. Some scientists believe that lightning occasionally

encounters electric obstacles that force it to take a twisted path,

producing a disjointed appearance. The flash also may appear as beads because different areas of the lightning bolt cool at different rates.

Ball lightning. Red spheres ranging in size from an orange to a basketball, and persisting between one and five seconds, have been reported both indoors and outdoors. People have reported ball lightning inside planes, coming through electric outlets, and floating horizontally from the strike point of a lightning bolt.

Not all scientists agree that ball lightning exists. Krider, however, believes it is real. "When I was 3 years old, I may have seen a lightning ball. At least I told my mother I saw one," Krider said. "Unfortunately, I don't remember the incident at all now."