Estimate storm arrival from radar.
Arrival time is distance divided by closing speed, and closing speed is the storm's speed reduced by how directly it is heading toward you — a cell moving obliquely closes far more slowly than its ground speed suggests. Lightning becomes a hazard well before the rain arrives, so the sixteen-kilometre threshold used by outdoor safety guidance is subtracted separately to give the time before you should already be indoors.
Storm arrival time
Closing speed = storm speed x directness/100; minutes to arrival = distance / closing speed x 60; lightning range time subtracts the last 16 km
Lightning is lethal and storm tracks change rapidly. Do not use this as a basis for staying outdoors — follow official warnings and seek substantial shelter at the first sign of thunder.
Count the seconds between the flash and the thunder and divide by three for kilometres, or by five for miles. Thunder is rarely heard beyond about 16 km.
If thunder follows lightning within 30 seconds — roughly 10 km — go indoors, and wait 30 minutes after the last thunder before returning outside.
Because only the component of motion aimed at you closes the distance. A fast cell on a glancing track may never arrive, while a slow direct one certainly will.