Now that September is here, summer is winding down -- but it doesn't feel that way quite yet. Although the official first day of fall is less than three weeks away, there's probably still a summer thunderstorm or two left in the season.

A thunderstorm can be a sensational show. Huge dark clouds race across the sky. Lightning flashes and thunder booms. Just make sure you watch the storm from the safety of a sturdy building!

Why do storms produce lightning? The violent air currents inside thunder clouds toss ice crystals, water droplets and dust particles up and down. Winds inside the cloud carry the droplets at speeds up to 100 miles an hour. They rub up against each other again and again. This contact builds up charges of static electricity. (That's the same kind of electricity that builds up when you shuffle your shoes across a rug or run a plastic comb through your clean hair.)

A lightning bolt is basically a giant electrical spark. When the electrical charges inside a cloud reaches several million volts, the air can no longer hold it. Here's what happens: The top part of a cloud builds up a positive electrical charge. The bottom part develops a negative charge. The ground below the cloud gets a positive charge. Opposite electrical charges are attracted to each other. Eventually, one charge gets strong enough to overcome the resistance of the air between it and the other charge. Then it rushes to meet the other charge, creating lightning.

The electricity may jump from cloud to cloud, cloud to earth or even earth to cloud. Then lightning flashes. A single bolt can contain enough electricity to provide an average house with power for two weeks! As a lightning bolt moves, it heats the air around it to an astonishing 50,000 degrees Fahrenheit -- about five times as hot as the sun. The heat makes the air around the lightning bolt expand. This shock wave makes a loud noise: a thunderclap. You can use the sound of thunder to estimate how far away a thunderstorm is. Loud cracks mean the lightning is nearby; low, rumbly thunder is the signal of a faraway storm. When you see a lightning flash, start counting seconds until you hear thunder boom. Five seconds equals one mile. So if you counted to five, the storm is one mile away. If the boom followed the flash right away, the storm is right around you -- but you already knew that!

Lightning can ignite forest fires. On average, some 9,000 forest fires are started by lightning each year, according to Louis J. Battan, a meteorologist at the University of Arizona. Scientists there and at the University of Florida developed a method for mapping lightning strikes. A network of instruments has been set up over our national forests. The data they gather can show foresters where fires are likely to have started.

Some of the data the network has gathered is pretty amazing. During one storm along the Arizona-New Mexico border, for example, lightning flashed 2,229 time during one hour and 49 minutes!

Electrical storms are dangerous for people, too. Each year, between 150 and 300 people are killed by lightning in the United States -- more than by any other natural disaster, according to the American College of Emergency Physicians. And about 550 people are injured by lightning each year.

The electrical bolts can knock birds out of the air and kill wildlife on the ground, too. Your chances of getting hit by lightning are very small. Even so, it makes sense to be careful around something as powerful and unpredictable as an electrical storm.

If you hear a weather report about an approaching thunderstorm, plan to go inside and stay there until the storm has passed. During the storm, stay off the telephone and unplug your Nintendo and the TV. Anything that can carry current from outside your house to the inside could be a hazard if lightning strikes.

Tips for Parents

The Virginia chapter of the American College of Emergency Physicians offers these precautions to help you avoid injury from lightning:

Avoid seeking shelter in metal sheds, tents or convertible cars. Buildings or enclosed automobiles are your best bets.

If you're caught outside, put down metal objects such as golf clubs or umbrellas. Stay well away from metal fences and other metal objects.

Stay out of the water during an electrical storm; water is an excellent conductor.

If you are caught outside, stay away from hilltops and tall, isolated trees. Find a low area with thick growth. If you're caught in an open field, crouch as low as possible.

If you're with a group of people in the open during a storm, don't huddle together. Stay a few yards apart so in case of a strike fewer people will be injured.

Catherine O'Neill is a freelance children's writer.