How can I tell if it's too dangerous to be outdoors during a thunderstorm?

If you can hear thunder, you're close enough to be struck by lightning—making it immediately too dangerous to be outdoors. Lightning can strike more than 10 miles away from the location of rainfall, meaning blue sky overhead doesn't guarantee safety. The safest approach is to monitor weather forecasts before outdoor activities and seek substantial shelter (a building with plumbing and wiring, or a hard-topped metal vehicle) before storms arrive. If thunder rumbles while you're outside, move to shelter immediately without waiting to see how close the storm is.

Use the 30-30 rule to assess lightning danger and know when it's safe to resume outdoor activities. After seeing lightning, count the seconds until you hear thunder—if you count fewer than 30 seconds, the lightning is less than 6 miles away and you're in immediate danger. Seek shelter right away and wait at least 30 minutes after hearing the last thunder before going back outside. Many lightning deaths occur when people leave shelter too soon, not realizing that lightning can strike from dissipating storms. The 30-minute waiting period is critical because studies have shown that most people struck by lightning are struck not at the height of a thunderstorm but before and after the storm has peaked.

Certain situations dramatically increase your lightning risk. If you're on elevated terrain, in open fields, on water, or near isolated tall objects like lone trees, your danger is extreme once thunder is audible. In these high-risk locations, don't wait to assess—move to safety immediately at the first sound of thunder or sight of lightning. If you're caught outside with no shelter option and lightning is striking nearby, never lie flat on the ground, never shelter under an isolated tree, stay away from objects that conduct electricity, immediately get out and away from ponds and lakes, and spread out if you're in a group. However, understand that no outdoor position is truly safe—these are last-resort measures when trapped, not alternatives to proper shelter.