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Calculate the distance to a lightning strike using the flash-to-bang method (counting seconds between lightning and thunder). Determine when you need to seek shelter, understand lightning safety rules, and learn about severe weather safety planning.
Lightning kills an average of 20 people per year in the US and injures hundreds more. Most lightning fatalities occur outdoors when people fail to seek shelter in time. The 30-30 rule: if the time between lightning flash and thunder is 30 seconds or less, seek shelter — and wait 30 minutes after the last lightning before going back outside.
Thunder is the sound produced by the rapid expansion of air heated by lightning to approximately 30,000 Kelvin (5× hotter than the sun surface). Sound travels about 1 mile per 5 seconds (343 m/s) — much slower than light which arrives essentially instantly. Counting the seconds between the lightning flash and the thunder rumble tells you the storm distance.
| Flash-to-Bang Gap | Safety Action |
|---|---|
| 30 sec / 6 miles (10 km) | Seek shelter immediately — 30-30 rule triggered |
| 15 sec / 3 miles (5 km) | Severe danger — take cover now |
| 10 sec / 2 miles (3 km) | Extreme danger — lightning imminent area |
| 5 sec / 1 mile (1.6 km) | Life-threatening — you are in the strike zone |
| 0-3 sec / <0.6 mi | Extreme emergency — lightning is on top of you |
The National Lightning Safety Council recommends the 30-30 rule: 1) If the flash-to-bang time is 30 seconds or less (storm within 6 miles), immediately seek substantial shelter. 2) Wait 30 minutes after the last thunder before resuming outdoor activities. Lightning can strike up to 10 miles from the visible storm — blue sky lightning is real. The 30-minute rule is critical because storms can redevelop and lightning can return.
SAFE shelters: substantial buildings with plumbing and electrical wiring (acts as Faraday cage directing lightning to ground), hard-top metal vehicles (not convertibles). UNSAFE: open structures (picnic shelters, dugouts), tall isolated trees, hilltops and ridgelines, open fields, bodies of water, near metal fences or poles, and caves. Crouch low if caught in the open — reduce your height and contact points with the ground.
Myth: Lightning never strikes twice in the same place — WRONG. The Empire State Building is struck 20-25 times per year. Myth: Rubber shoes and tires protect you — WRONG (lightning carries millions of volts). Myth: You are safe under a tree — WRONG (trees are a primary lightning target due to height and sap conductivity). Myth: If it is not raining, there is no lightning risk — WRONG (dry lightning strikes from distant storms are common).