Lightning is triggered by cosmic rays from outer space, according to a physicist at John Hopkins University, which would explain why lightning zigs and zags and why one lightning rod is never enough to protect a building.

"The electrical field generated by static forces in the air doesn't steer the lightning strike, which is why the bolt often hits the church roof instead of the steeple," said Dr. James W. Follin Jr. of Johns Hopkins' Applied Physics Laboratory. "The steering of the strike is done by the decay particles of cosmic ray showers, which cascade through the sky from all directions. That explains why lightning zigzags through the sky."

Insisting that his theory is not as new as it is overlooked, Follin said the first step in the generation of lightning is a strong shower of cosmic rays that penetrates a thundercloud and ionizes oxygen and nitrogen molecules in the air.

This strips electrons from the air molecules. The electrons are then accelerated by the existing electric field, Follin said, and as they accelerate they pick up energy and strip still more electrons from air molecules. In less than a second, an avalanche of electrons pours down from the sky to the base of the cloud.

The bolt itself strikes, Follin goes on when heavy electrons (the so-called muons) that form 90 per cent of the cosmic rays at sea level concentrate themselves at the bottom tip of the electron cascade. The tip of the cascade moves lower and lower as more heavy electrons weight it down until it strikes the ground and short-circuits the electric field in the cloud.

"You then get a very strong discharge that heats up the bolt and gives you the bright flash that you see," Follin said. "We're talking about enormous energies that force currents of 200,000 amperes and heat the air to more than 50,000 degrees Fahrenheit."

The bolt zigzags because the heavy electrons that decide what course the discharge will take are cascading down from all directions. The lighting strikes, Follin said, in whatever directions the heavy electrons are most concentrated.

"This explains why lightning rods are not very effective and why you have to put 20 lightning rods on top of a barn to protect it," Follin said. "The electric field in the cloud doesn't do the steering. The heavy electron does the steering, which means the lightning will hit the middle of the barn just as easily as it will hit each end where the lightning rods might be."

If the earth were not bombarded constantly by a stream of cosmic rays, Follin said, there would probably never be any lightning strikes on earth.

"You'd ned about 10 times the electrical fields in the clouds to generate any discharge on their own," the Johns Hopkins physicist told the fall meeting of the American Geophysical Union in San Francisco, "and clearly if you're going to grow the electrical fields by a factor of 10 you're going to get enormous electrical fields."

So big would these fields be, in fact, that the electrostatic forces would levitate the hail and raindrops that holding them in the sky: "The hail and rain would not fall from the sky."