Thunder is the sound produced by the rapid heating of air caused by a lightning strike. When lightning occurs, it heats the air around it to extremely high temperatures, up to about 30,000°C, almost instantly. This intense heating causes the air to expand explosively, creating a shockwave similar to a sonic boom. This shockwave travels through the air as sound, which we hear as thunder. Depending on how close you are to the lightning and the nature of the strike, thunder can sound like a sudden, loud crack or a longer, low rumble. The rumbling occurs because the sound waves from different parts of the lightning bolt reach the listener at different times due to the bolt's length and shape. Thunder always follows lightning because light travels faster than sound. You can estimate the distance of a thunderstorm by counting the seconds between seeing lightning and hearing thunder—every three seconds roughly equals one kilometer of distance. In summary, thunder is the acoustic effect of the rapid expansion of air heated by lightning.