Lightning is caused by the rapid movement of air inside the tall and frightening clouds we see in the sky during hot-weather storms. These clouds contain small pieces of ice which are constantly hitting one another because of the air movement. The millions of little collisions cause the air in the clouds to become electrified. Some of this electrified air has a positive charge; some has a negative charge. As the clouds—which are called “cumulo-nimbus” clouds—get bigger and bigger, the positively charged air moves toward the top, and the negatively charged air moves toward the bottom. The electrical difference continues to build up until, suddenly, with a huge flash, a very powerful electrical current jumps between the positive and negative parts of the cloud. Often, on a hot evening, we see this kind of lightning as a bright flash in a distant cloud. We call it “sheet lightning.”

That is what usually happens, but quite often the electrical current does not flow upward from the negatively charged part of the cloud to the postively charged part at the top. Instead, it flows downward to the positively charged earth. We see this kind of lightning as a long bright, quickly moving line. We call this a “bolt” of lightning. These bolts move very fast; they travel more than ten kilometers in less than a second. Each one contains millions of volts of electricity. When the bolt leaves the cloud, it is very wide, about ten meters across. By the time it reaches the earth, it is only about two centimeters thick, but it is still just as powerful. And it is extremely hot—around 30,000°C, which is about five times as hot as the sun’s surface.

The heat of the lightning bolt is the cause of thunder—the loud noise that comes with lightning. The air around the lighting heats up suddenly as the lightning passes through it. The heat causes the air to expand and that causes loud sound waves.

Lightning bolts have a negative charge. As they get close to the earth, positively charged electricity from any tall thing that happens to be in the area moves up to meet them. This tall thing could be a tree, or a building—or even a person standing in the middle of a field. The downward negative charge meets the upward positive one about a hundred meters above the ground. When the two meet, all the electricity from the cloud suddenly flows through the tall thing and into the earth.

In cities, if there is one building that is higher than all the others, that building will attract a lot of lightning. For example, the CN Tower, in Toronto, Canada is hit about two hundred times every year.

Lightning is common. At any moment there are about two thousand thunder storms world wide. The earth is hit by lightning approximately five million times a day. Considering how much lightning there is, it’s surprising that it doesn’t kill more people than it does. In fact, only one out of every 600,000 deaths is caused by lightning. Even so, protecting yourself is not easy. For one thing, you can be hit by lightning long before a storm arrives; it can strike as much as fifteen kilometers away from rain. The best way to tell whether you’re in danger is to measure the distance between you and the lightning by counting the seconds between the moment you see the flash and the moment you hear the thunder. It takes sound about one second to travel 300 meters. If you hear the thunder before you count to thirty, that means the lightning hit the ground less than ten kilometers away. And that means you’re in danger if you’re outside.

You should get into a building if you can and, if you can’t get into a building, you should get into a car. If you aren’t near a car or a building, you should stay away from anything tall—a telephone pole, for example, or a tall tree. You should also avoid standing in a empty field because, by doing that, you become a “tall thing” yourself. And if you’re cycling when you get caught in a thunderstorm, you should quickly move a away from your bike because, just as lightning is attracted to tall things, it’s also attracted to metal.

If you are hit by lightning, you probably won’t be killed but you might be very seriously injured. Your skin might be badly burnt by the heat. Your hearing or your eyesight might be damaged. You might lose memory. You might become permanently confused—or find that you are never able to sleep properly again.

- fl, 2002 (revised 2005, 2008)

-information from "Struck by Lightning," by Dane Lanken in "The Canadian Geographic," July/August, 2000