That's if you see a nuclear flash. Don't look at the flash - it could blind you! Drop flat to the ground or behind any shelter that can b e taken instantly. Cover any exposed areas with anything that can limit burns from the intense light, or injury from flying debris. Stay covered for two minutes. Count the number of seconds from the flash to the blast wave arrival. If the blast does not arrive within two minutes, it will be considerably weaker. The number of seconds between the flash and the blast wave arrival divided by 5 gives a rough idea of your distance in miles from ground zero.
Just saying... If you live in the dfw area or any large city, you should already know this! :)
Except the problem is that humans are curious, and would look towards a source of light.
Does that work with lightning too?