The speed of sound can vary, depending on air pressure and, more importantly, temperature. The conventionally accepted speed, which is useful for estimating the distance to a lightning stroke, is about one mile for every five seconds. More precisely, sound travels at 740 miles per hour (1,191 kilometers per hour) at one atmosphere pressure when the temperature is 32°F (0°C).

Sound travels faster through water and many other media that are denser than air. In general, because the temperature of the air cools as altitude increases, sound is refracted upwards and away from people on the ground. Thus, measurable sound decreases the farther away one is from the source. On the other hand, the stratospheric layer of the atmosphere increases in temperature as altitude increases, thus refracting sound downward.