Alison DeCourcy, age 9, of Rockford, Illinois, for her question:

Can we tell how far away lightning is?

Yes, we can do this, though we cannot measure the distance to the exact inch. The trick is to figure how long it takes the thunder and the lightning to get from there to here. In the storm cloud, the lightning and the thunder happen at the same instant. But they travel toward us at different speeds. The lightning is faster than fast. The flash reaches our eyes and we see it the moment it happens, but the sound of the thunder takes about five seconds to travel a mile.

The trick is to count off the seconds between the time we see the flash and the time we hear the roar. When the lightning flashes, start counting fairly slowly. If you reach up to ten before you hear the thunder, the storm cloud is two miles away  ¬more or less. If the flash and the roar are close together, they are almost overhead. Sometimes we see sheet lightning and hear no thunder at all. This is because the roar became worn out and lost along the way. The storm may be ten miles or more away and we see its lightning reflected in the cloud.