Thunder

Thunder is the sound produced by the rapid heating and expansion of air surrounding a lightning channel. When a bolt passes through the air, it heats it to temperatures on the order of tens of thousands of kelvin, causing the air to expand explosively. This rapid expansion generates a shock wave that propagates through the atmosphere and is heard as thunder.

The audible result ranges from a sharp crack to a long, rolling rumble. The crack is the initial shock wave from the lightning channel; the rumble often results from the extended channel and reflections off the ground, mountains, or clouds. The character and duration of thunder depend on atmospheric conditions, terrain, and the geometry of the lightning discharge.

Distance: Sound travels slower than light, so thunder is heard after seeing the flash. At sea level and typical temperatures, sound travels about 343 meters per second; a common rule is about three seconds per kilometer, or about one mile per five seconds. Thunder can be heard from several kilometers away, with the exact distance affected by temperature, humidity, wind, and obstacles.

Safety: If you hear thunder, you may be within reach of lightning; seek shelter indoors or in a sturdy building. In open areas, avoid tall objects and seek low ground. The 30-minute rule after the last thunderclap is commonly recommended before resuming outdoor activities.