Experts say that if you can hear thunder, you're near enough to the storm to be struck by lightning. A more precise rule of thumb, suggested by the National Weather Service, is the "30-30 Rule."

30 SECONDS: When you see a flash of lightning, count off the seconds until you hear the accompanying thunderclap. If you count 30 seconds or less, you're close enough to be at risk of being struck by lightning. Get inside.

30 MINUTES: Stay indoors until 30 minutes after the last flash of lightning. After that it's safe to go back outside.

The 30-30 Rule applies even if it hasn't started raining, or if the storm clouds are far away. Lightning can travel horizontally as far as 10 miles before striking the ground, so even if the sky overhead is blue you can still be in danger.