Thunder is the sound generated due to lightning. The flash of lightning and the accompanying thunder occur around the same time. Nonetheless, we all are aware that lightning is seen first, followed by the sound of thunder after few seconds. This phenomenon occurs as light wave travels much faster than the sound waves.
When the high temperature bolt hits the surrounding air, a sudden rise in temperature takes place. It brings along an instant expansion of heated air in the outward direction. The elevated pressure is obvious due to compression of heated air. The rapid expansion of compressed air then sends out shock wave or vibration, which we hear as a sound of explosion.
Thunder can be perceived clearly, if lightning occurs within a distance of 10 miles or more than that. You can estimate the distance from the strike; first count the interval between the flash of lightning and the sound of thunder in seconds, and then divide the interval by five. What you get is the rough distance in miles.
In case, you are near to the flash of lightning, you will hear thunder as a sharp crack; whereas, if you are far from the lightning stroke, then the sound will seem like a low rumble. The rumbling sound is because of the echo, occurred due to reflection of sound waves from the buildings, trees and hillsides.
In case, you are outside during a thunderstorm, it is advisable to avoid open fields, beaches and lakes, and also stay away from tall trees. The safest way is to get inside your home or stay in the car. In the latter situation, you should not touch metal, as it is a good conductor of electricity.