The Science of Lightning

How it strikes | Thunder | Estimating distance | Facts

During any given minute, there are more than a thousand thunderstorms around the Earth causing some 6,000 flashes of lightning. Every minute!

Thunderstorms are caused by rapidly rising and falling currents of air. The friction from this moving air creates electrical charges within a cloud. Water droplets and ice pellets fall, carrying charged electrons to the lower portion of the cloud, where a negative charge builds. A positive charge builds up near the top of a cloud.

Most of the electrical energy in a thunderstorm is dissipated within the clouds, as lightning hops between the positively and negatively charged areas. Lightning becomes dangerous, though, when it reaches for the Earth.

How lightning strikes

When the negative charge in the cloud becomes great enough, it seeks an easy path to the positively charged ground below. The current looks for a good conductor of electricity, or a tall structure anchored to the ground (such as a tree or a tall building). The negative charge sends out a feeler, called a stepped leader, which is a series of invisible steps of negative charges.

As the stepped leader nears the ground, a positive streamer reaches up for it. Only then, once this channel is made, does the visible lightning happen. A return stroke runs from the ground to the clouds in a spectacular flash.

Though the bolt appears continuous, it is actually a series of short bursts. Most lightning strikes occur in less than a half second and the bolt is usually less than 2 inches in diameter.

Thunder

The air around a lightning bolt is superheated to about 54,000 degrees Fahrenheit (five times hotter than the sun!). This sudden heating causes the air to expand faster than the speed of sound, which compresses the air and forms a shock wave; we hear it as thunder. Since the bolt is actually several short bursts strung together, multiple shock waves are created at different altitudes; this is why thunder seems to rumble -- each shock wave takes a different amount of time to reach your ear.

Estimating distance

Because light travels faster than sound (186,291 miles per second vs. 1,088 feet per second) you see lightning before you hear the thunder. When you see it, count the seconds before the thunder arrives. Divide this number by 5, and you'll know approximately how many miles away the lightning was (5 seconds = 1 mile).

Lightning facts

• A lightning charge contains 30 million volts at 100,000 amperes.
• The total energy in a large thunderstorm is more than that in an atomic bomb.
• About a hundred U.S. residents are killed by lightning every year.
• Benjamin Franklin's famous kite experiment, in 1752, showed that lightning was electricity.
• The Empire State Building in New York City is struck by lightning about 25 times every year.