On a day when the air temperature is 30 C, a lightning flash is seen and the thunder is heard 6.0 s later. Estimate the distance to the lightning strike. (Light reaches you almost instantly, so the delay is the sound travel time.) Solution: Speed of sound at 30 C: v = 331 + 0.6(30) = 331 + 18 = 349 m/s. Distance = v x t = 349 x 6.0 = 2094 m, about 2.1 km. The common rule of thumb - count 3 seconds per kilometre - gives 6/3 = 2 km, matching closely.