Thunder

Thunder is a powerful sound and one that we are all familiar with. It may come as a distant, heavy muttering sound on a warm summer night, perhaps accompanied by a light patter of rain that scarcely interrupts the crickets. It may be an ominous drumroll growing closer and closer, first rolling, then booming as the storm front sweeps closer. Or it may be a deafening crack of sound, exploding all around us at nearly the same instant as the blinding flash that made it. Whether it is near or far, soft or loud, its deep tones are unmistakable and are one of the most dramatic reminders of the elemental strength of Nature.

Thunder is made by lightning, which violently heats the air along its length, making the air expand explosively. The air around the discharge channel is heated to around 55,000 degrees Fahrenheit, which is roughly six times the temperature of the sun's surface. This causes the air to burst outwards, and since sound is transmitted as air waves, the expansion becomes an enormous crash of thunder. It is possible that this heating alone is not enough to cause the thunder we hear, and various research suggests that the lightning may actually cause a chemical explosion like a bomb, using the air as its explosive.

Thunder at a distance rolls because we are hearing the sound from different parts of the lightning discharge channel arriving at different times. When the lightning is close, the sound from the nearest part of the channel strikes us as a single overwhelming crash, while the rest of the sound waves from the remainder of the channel bypass us, meaning that we usually don't hear the 'roll' with a nearby lightning stroke.

Thunder is a dramatic phenomenon. Some people enjoy it like a kind of elemental music, while others are frightened or made nervous by it. However, it also has the practical effect of letting you estimate how far away the lightning strike was. Thunder travels a mile in roughly 5 seconds (or a kilometer in 3 seconds), so counting the time between the flash and the boom can give you a good notion how distant the thunder is. Under normal conditions, the sound of thunder can't be heard for more than 12 miles, so if you are hearing thunder at all, the storm is probably within a 12 mile radius of your location.

Determining distance by sound has its limits, too. One limit is that if there are frequent lightning strikes in a storm, it will probably be impossible to tell which flash belongs to which boom. Anyone who has experienced a Midwestern "thunder-boomer" will know that the rolling of thunder in a violent storm can be constant, and the flashing of lightning can be like a rapid artillery barrage. But if the thunderstorm is a more typical one, you can often determine how far away it is by listening carefully to its thunder -- the deep voice of its lightning.