The Flash-to-Bang Method
When a thunderstorm is in the area, one of the most important things you can do is estimate how far away the lightning is striking. This tells you whether you are in immediate danger and whether you need to seek shelter. The technique for doing this is called the flash-to-bang method, and it relies on a simple physical fact: light travels almost instantaneously, while sound takes a measurable amount of time to cover the same distance.
The method requires no equipment, no technology, and no training. Anyone can use it, anywhere, at any time. Here is how it works:
Step-by-Step Instructions
- Watch for a lightning flash. Keep your eyes on the sky. When you see a flash of lightning, note its location relative to you.
- Start counting immediately. The moment you see the flash, begin counting seconds. Use the "one-one-thousand, two-one-thousand" method, or count using a watch or phone timer for greater accuracy.
- Stop when you hear thunder. As soon as you hear the first rumble, crack, or boom of thunder, stop counting.
- Divide by five. Take the number of seconds you counted and divide by five. The result is the approximate distance to the lightning strike in miles. (For kilometers, divide the seconds by three instead.)
For example, if you count 15 seconds between a flash and the thunder, the lightning struck approximately 3 miles away. If you count only 5 seconds, the strike was about 1 mile away and you should already be inside a safe structure.
Distance Reference Table
The following table provides quick conversions from the flash-to-bang count to distance. Use this as a reference during thunderstorms to assess your level of risk.
| Seconds | Distance (Miles) | Distance (Kilometers) | Risk Level |
|---|---|---|---|
| 0–5 | 0–1 | 0–1.6 | Extreme: take cover immediately |
| 5–10 | 1–2 | 1.6–3.4 | Very high: you should already be sheltered |
| 10–15 | 2–3 | 3.4–5.1 | High: seek shelter now |
| 15–20 | 3–4 | 5.1–6.8 | Significant: prepare to take shelter |
| 20–25 | 4–5 | 6.8–8.5 | Moderate: monitor closely |
| 25–30 | 5–6 | 8.5–10.2 | Elevated: begin moving toward shelter |
| 30+ | 6+ | 10.2+ | Lower, but not zero: stay alert |
The Science: Why Five Seconds Equals One Mile
The flash-to-bang method works because of the enormous difference between the speed of light and the speed of sound. Light from a lightning bolt travels at approximately 300,000 kilometers per second (186,000 miles per second), reaching your eyes in essentially zero time regardless of distance. The flash is effectively instantaneous.
Sound, however, travels at approximately 343 meters per second at sea level and standard atmospheric conditions (20 degrees Celsius, or 68 degrees Fahrenheit). Converting to more familiar units, this is about 1,125 feet per second, or roughly one mile every 4.7 seconds. Rounding to 5 seconds per mile provides a practical and sufficiently accurate rule of thumb.
It is worth noting that the speed of sound is not constant. It varies with temperature, humidity, and altitude. In warmer air, sound travels slightly faster (about 0.6 meters per second faster for each degree Celsius increase). In colder air, it slows down. At high altitude, where the air is thinner and cooler, sound also travels more slowly. However, for the practical purpose of estimating storm distance and making safety decisions, the five-seconds-per-mile approximation is accurate enough to be reliable in virtually all real-world conditions.
When to Seek Shelter: The 30-Second Rule
The National Weather Service, the National Lightning Safety Council, and virtually every lightning safety organization in the world agree on this threshold: if the flash-to-bang count is 30 seconds or less, you should be inside a safe structure.
A count of 30 seconds corresponds to a distance of approximately six miles. While six miles might feel far away, lightning can and does strike the ground well ahead of an approaching storm. Positive lightning bolts, which originate from the upper portions of the storm cloud, can strike 10 to 15 miles from the storm center. Storms also move, and a thunderstorm traveling at 30 miles per hour can close a six-mile gap in just 12 minutes.
The 30-second threshold is not the point at which you should start thinking about shelter. It is the point at which you should already be inside. In practice, this means that if you are outdoors and you can hear thunder at all, you should be actively moving toward or already inside a safe building or hard-topped vehicle.
What Counts as "Safe Shelter"?
Not all structures provide adequate lightning protection. A safe shelter is either:
- A substantial building with plumbing and electrical wiring. The wiring and plumbing provide paths for lightning current to flow to the ground, reducing the risk to occupants. Homes, offices, stores, and schools all qualify.
- A hard-topped vehicle with the windows closed. The metal body of the car acts as a Faraday cage, directing lightning current around the exterior and into the ground. This is why convertibles, golf carts, ATVs, and motorcycles do not provide protection.
Structures that do not provide protection include: open pavilions, picnic shelters, tents, carports, dugouts, sheds without wiring, and bus stop shelters. If these are your only options, crouch low on the balls of your feet and minimize your contact with the ground until you can reach proper shelter.
Limitations of the Flash-to-Bang Method
While the flash-to-bang method is the best tool available to individuals in the field, it does have limitations you should be aware of:
- Multiple storms: When several thunderstorm cells are active simultaneously, it can be difficult to match a specific flash with its corresponding thunder, especially when multiple rumbles overlap.
- Continuous thunder: During intense storms, thunder from multiple strikes can blend into continuous noise, making it impossible to isolate individual flash-to-bang intervals.
- Intra-cloud lightning: In-cloud flashes are visible but may produce thunder that sounds different from or arrives at unexpected times compared to cloud-to-ground strikes.
- Terrain and buildings: Sound can be reflected or blocked by terrain, buildings, and vegetation, potentially making thunder seem closer or farther than it actually is.
- Wind: Strong winds can carry sound toward you (making the storm seem closer) or away from you (making it seem farther), though the effect on five-second-per-mile accuracy is generally small.
Despite these limitations, the flash-to-bang method remains the single most practical way for an individual to assess their lightning risk in real time. When combined with weather forecasts and real-time lightning detection data, it gives you a reliable basis for staying safe during storms.
Beyond Counting Seconds
Modern technology has made it possible to track lightning in real time without relying solely on your own senses. Satellite-based systems like the GOES-19 Geostationary Lightning Mapper detect every lightning flash across the hemisphere and can pinpoint its location with high accuracy. Learn more about this technology in our article on GOES-19 GLM lightning detection. For an exploration of the relationship between lightning and thunder, see our companion article on lightning vs thunder.