Warm, wet air surges upwards into the sky and cools dramatically forming thunderstorms. Some of the water inside the clouds freezes and strong air currents make the ice and water droplets bump together. This knocks tiny charged particles called electrons from the ice and so there is a build-up of electrical charge. This charge is released by a stroke of lightning. The lightning heats the air around it to an incredible 30,
C (54,F), five times hotter than the surface of the Sun. this heat causes the air to expand very fast; i.e., faster than the speed of sound. It is this which causes the crash of thunder.
If the thunderstorm is overheard, we can hear thunder and see lightning simultaneously. If it is not overheard, we can see lightning first, as light travels much faster than sound.
By counting the seconds between the lightning and thunder and dividing by three we get the distance to the storm in kilometers and dividing by five we get the distance in Miles. Dark thunderclouds are formed on hot and humid days. A thundercloud is usually about 5 kms (3 Miles) across and 8 kms (5 Miles) high.