How far away is lightning? Counting the seconds to thunder

Count the seconds between seeing a lightning flash and hearing its thunder, then divide by 3 for the distance in kilometres or by 5 for the distance in miles. A 6-second gap means the strike was about 2 km, or just over a mile, away. It works because the light arrives almost instantly while the sound travels at only about 343 m/s. The lightning distance calculator has a tap timer and adjusts for air temperature, but the most important point in this guide is simpler: if you can hear thunder at all, you are close enough to be struck.

Why counting works

Lightning heats the air in its channel extremely quickly, to as much as 50,000 °F (roughly 28,000 °C) according to the US National Weather Service, about five times hotter than the surface of the Sun. The air expands explosively and then contracts, and that sudden disturbance travels outwards as the sound we call thunder. The flash and the sound start at the same moment, but they travel at very different speeds. Light covers 10 km in about 33 millionths of a second, effectively instantly, whereas sound in air at 20 °C travels at about 343 m/s. Almost the whole delay is therefore the sound's travel time, and the distance is simply the speed of sound multiplied by the seconds you counted.

The 3-second and 5-second rules

At 343 m/s, sound takes about 2.9 seconds to travel a kilometre and about 4.7 seconds to travel a mile, so the rounded rules of 3 seconds per kilometre and 5 seconds per mile are close enough for a storm. The US National Weather Service and the UK Met Office both give these rules. Some examples:

• 3 seconds: about 1 km (0.6 miles).
• 5 seconds: about 1.7 km, or 1 mile.
• 15 seconds: about 5 km, or 3 miles.
• 30 seconds: about 10 km, or 6 miles.

Temperature changes the speed of sound slightly, from about 331 m/s at 0 °C to about 349 m/s at 30 °C, a difference of around 5%, which the calculator corrects for; the speed of sound calculator shows the full relationship, and the unit converter switches between kilometres and miles. Metric vs imperial: how we got two systems explains why there are two rules in the first place.

How accurate it is

The count measures the distance to the nearest part of the lightning channel. A channel can be kilometres long and run sideways through the cloud, so the first thunder you hear comes from the closest point and the long rumble that follows comes from parts further away. Start counting at the flash and stop at the very first sound, not the loudest crack. Your reaction time adds a few tenths of a second at each end, which barely matters at storm distances.

Thunder does not carry very far. The National Weather Service says it can be heard only about 10 miles from the strike, and the Met Office says it is rarely heard more than 20 km away, so distant flashes on a summer evening with no thunder at all usually come from storms too far off to hear. Temperature layers in the air can also bend sound upwards, away from the ground, and shorten that range.

Lightning safety

The distance is interesting, but it is not a safety margin. Lightning can strike well away from the rain, and the National Weather Service is blunt: there is no safe place outside when thunderstorms are in the area, and if you hear thunder you are likely within striking distance. Its slogan is "When thunder roars, go indoors."

• Get inside a substantial building or a hard-topped vehicle as soon as you hear thunder or the sky looks threatening. Rain shelters, small sheds and open vehicles are not safe.
• Indoors, avoid corded phones, anything plugged into an outlet, plumbing, and outside doors and windows. Mobile and cordless phones are safe to use.
• Wait 30 minutes after the last lightning or thunder before going back outside.

An older guideline, the 30-30 rule, said to take shelter when the flash-to-bang count fell to 30 seconds or less. The current advice is simpler and safer: go indoors as soon as you hear any thunder, because storms move fast and the first strike near you can come before the count ever drops that low. Always follow your local weather service's warnings, and if gusty winds come with the storm, the Beaufort scale converter helps describe how strong they are.

If someone is struck

People struck by lightning often survive, but they may need immediate medical help. Call the emergency services straight away. Lightning victims do not carry an electrical charge and are safe to touch, so you can check their breathing and start CPR, or use a defibrillator if one is available, while help is on the way. Make sure you are not putting yourself in danger from the same storm while you help. The Doppler effect: why a passing siren drops in pitch covers another everyday effect of the speed of sound.