The time-of-arrival technique refers to locating the source of an emitted signal from a precise recording of the time that a signal is observed. For example, the time interval between an observed lightning flash and the arrival of the thunder can be used to estimate the distance to the lightning flash. On the average, a time arrival difference of five seconds indicates that a lightning flash occurred one mile away from the observer, since the speed of sound in air is approximately 1000 ft s⁻¹.