 Sound in dry air is approximately 343 m/s or 1,127 ft/s or at 20 °C (68 °F).This translates to approximately 3 seconds per kilometre (5 seconds per mile); saying "one thousand and one... one thousand and two..." is a useful method of counting the seconds from the perception of a given lightning flash to the perception of its thunder (which can be used to gauge the proximity of lightning for the sake of safety).A very bright flash of lightning and an almost simultaneous sharp "crack" of thunder, a "thundercrack", therefore indicates that the lightning strike was very near.