Acoustics/Generation and Propagation of Thunder
Thunder is the sound made by lightning. Depending on the nature of the lightning and distance of the listener, thunder can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave which produces the sound of thunder, often referred to as a clap, crack, or peal of thunder. The distance of the lightning can be calculated by the listener depending on when the sound is heard vs. the vision of the lightning strike.
A short sample of a crack of thunder
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The d in Modern English thunder (from earlier Old English þunor) is epenthetic, and is now found as well in Modern Dutch donder (cp Middle Dutch donre, and Old Norse þorr, Old Frisian þuner, Old High German donar descended from Proto-Germanic *þunraz). In Latin the term was tonare "to thunder". The name of the Germanic god Thor comes from the Old Norse word for thunder.
NOTE:The text above is taken from the wikipedia entry.