Definition: Thunder: A sharp or rumbling sound that accompanies lightning.
At a short distance away, the sound of thunder is brief, sharp and violent. When a lightning ground stroke is very close, a sound of short duration like that of tearing paper followed by a second sound like “vit” can often be distinguished before the sharp final crack. From a distant discharge, the thunder is heard as a dull rumbling or a prolonged roll that varies in strength. The duration of a roll of thunder, except in mountainous regions or cities, rarely exceeds 30−40 s.
Owing to the difference between the speed of light and the speed of sound, lightning is seen before the associated thunder is heard. The time interval increases with increasing distance between place of lightning discharge and observer. When the distance exceeds 20 km, thunder is usually not heard. Sometimes, thunder is not audible, even when the discharge occurs at appreciably shorter distances. This is due to the refraction of sound waves in the lower layers of the atmosphere.