Science File / An exploration of issues and trends affecting science, medicine and the environment.

Q: Does lightning strike down or up?

A: Although most lightning actually goes from cloud to cloud, lightning that touches the ground strikes upward. When static electricity builds up in a cloud, a thin, invisible discharge called a pilot stroke passes from the cloud to the ground. As soon as the pilot stroke creates a passageway of electrically charged air, the lightning stroke flashes up from the ground to the cloud. The stroke is so fast, however, that its direction is not apparent. The electrical discharge, usually more than 100 million volts, superheats the air to 60,000 degrees Fahrenheit. The rapid expansion of this hot air causes pressure waves that are called thunder. Damage to objects on the ground results not from their being struck by a bolt, but from the passage of the high voltage through them.