Lightning comes from thunderclouds, known as cumulonimbus, which are created when hot moist air rises into the atmosphere and condenses. Hot air rises when heated by the sun, carrying water vapor into the sky. As it rises, the hot air mingles with colder air, and the moisture condenses into water droplets. Clouds are created when these water droplets become visible. The droplets increase in size as the cloud grows and eventually become so heavy that they fall as rain. Thunderclouds are large, anvil-shaped masses that can stretch miles across at the base, and reach 40,000 feet or more into the atmosphere.

The genesis of lightning is a subject of great theoretical debate, says Dr. Vladimir Rakov of the Lightning Research Center at the University of Florida. We know that electrical charges build up within thunderclouds, but there is no single theory that fully describes why. One commonly discussed thesis suggests that small cloud particles acquire a positive charge, while other larger particles become negatively charged. These particles eventually separate, and the upper part of the cloud becomes positively charged, while the lower part becomes negatively charged.

The attraction, or electrical potential, between the positive and negative charges eventually grows strong enough to overcome the air's resistance to electrical flow. Racing toward each other, the charges connect, completing an electrical circuit, and discharging the accumulated electricity as lightning. Cloud-to-cloud lightning is the most common form of electrical discharge. Only about one-third of all discharges are cloud-to-ground. Bolts that shoot from cloud-to-air, known as "bolts from the blue," are even less common, but can strike up to 10 miles away.

When the current is discharged, it is accompanied by a flash containing millions of volts of electricity. This is a huge amount of energy, and the surrounding air is heated up to 54,000° Fahrenheit, five times hotter than the surface temperature of the Sun. The rapidly expanding heated air also produces tremendous shock waves, which become audible as the sound of thunder.

Although the flash and resulting thunder occur at essentially the same time, light travels at 186,000 miles per second, almost a million times faster than the speed of sound. The flash, if not obscured by clouds, is seen long before the thunder is heard.