Matt Martinez, age 7, of Brownsville, Texas, for his question:

WHAT CAUSES THUNDER?

Thunder is sound we hear occasionally coming from the sky and clouds. It is caused by the violent expansion of air that has been heated by lightning.

Air is heated instantly when an electrical charge of lightning passes through it. The heat causes the molecules of air to expand, or fly out, in all directions. As the molecules seem to be looking for more room, they collide violently with layers of cool air. This sets up a great air wave which becomes the sound of thunder.

Deep thundering and rumbling roars are caused by the air wave set up by the part of the lightning trunks that is farthest away. The sharp crackle of thunder is set up when the large trunk of lightning forks out into many branches. The loud crash of thunder is caused by the main trunk of the lightning.

The sound of thunder reaches us after we have seen the lightning. This is because light travels 186,282 miles per second while sound travels at a rate of only about 1,100 feet per second.

By counting the time between the flash of lightning and the sound of thunder, you can tell about how far away the storm is. If it is five seconds between the flash and the thunder, the storm is about one mile away.