Thunder and Lightning

Two things that associated with rain and often happen during rainy days are thunder and lightning. Before we talk about thunder, it's better to study what lightning is first.

Lightning is a form of visible electrical discharge that take place between rain clouds or between a rain cloud and the the earth. Lightning may appear as a jagged streak (forked lightning), as a vast flash in the sky (sheet lightning), or, rarely, as a brilliant ball (ball lightning). How rain clouds become charged is not fully understood, most of them are negatively charged at the base and positively charged at the top. There are numerous theories on why charges accumulate in the atmosphere. It is thought that temperature and water vapor pressure in rain clouds are associated with the positive and negative ions that cause lightning. Long-lasting lightning flashes with lower current are more damaging to nature and humans than shorter flashes with higher currents.

Lightning bolt heats the air around it so quickly (within a few millionth of a second) and to such a high temperature (about 10,000° C, or about 18,000° F) that the air molecules are pushed apart with great force, much like in an explosion. This results in an explosive sound which called thunder. Rolling thunder occurs either as a result of the time difference between sounds from the far and near end of a flash, or when mountains, layers of air, or other obstructions cause reverberations.

Because sound travels more slowly than light, thunder is heard after the lightning is seen. The distance between an observer and the lightning bolt can be estimated by counting the number of seconds between the lightning and the thunder. The light reaches the observer almost instantaneously, but the sound travels at about 1.6 km (about 1 mi) every 5 seconds. Thunder can seldom be heard from more than 24 km (15 mi) away.

Information is gathered from Encarta and Encyclopedia.com.

Graphics are from Graphic Garden, Graphics by Irene.