Why Does Thunder Come After Lightning? How to Tell How Far the Storm Is

Oct 8, 2026 · 4 min read · Sourced and fact-checked

The short answer: Lightning and thunder happen together, but you see the flash almost instantly while the sound takes about 5 seconds to cover each mile. Counting the gap gives a rough distance.

You see a flash, wait a moment, then hear a rumble. It feels like two separate events, but the flash and the thunder start at the same time. The gap is just the time sound needs to reach you.

Where thunder comes from

According to the US National Weather Service, thunder is created when lightning passes through the air. The discharge heats the air in its channel very quickly, and the air expands. The temperature in the channel may reach as high as 50,000 degrees Fahrenheit, which the Weather Service describes as 5 times hotter than the surface of the sun. Right after the flash, the air cools and contracts quickly. This rapid expansion and contraction makes the sound wave we hear as thunder.

Why you hear it later

The Weather Service puts it plainly: you see lightning immediately, and it takes the sound of thunder about 5 seconds to travel a mile. NOAA's education pages repeat the same figure. So the delay is not the storm waiting. It is sound moving much more slowly than the light that carries the flash to your eyes.

The counting trick

Count the seconds between the flash and the thunder, then divide by 5. The result is the distance in miles, as the Weather Service explains:

5 seconds is about 1 mile
15 seconds is about 3 miles
0 seconds means the lightning is very close

For kilometres, 1 mile is about 1.6 km, so a gap of 5 seconds is roughly 1.6 km. This is a rough guide, not a precise measurement.

Why thunder rumbles

A lightning bolt usually strikes one spot on the ground, but the channel travels many miles through the air. The Weather Service says you first hear the thunder from the part of the channel nearest you, and then from parts farther and farther away. A sharp crack or click usually means the channel passed nearby. A rumble means the lightning was at least several miles away. The loud boom sometimes heard comes from the main channel as it reaches the ground.

A safety note

The Weather Service says thunder can be heard only about 10 miles from the strike, and that if you can hear thunder you are within striking distance of the storm. It advises going to a safe place immediately, and to stay safe while counting rather than standing outside to measure.

Sources

US National Weather Service, Understanding Lightning: Thunder: https://www.weather.gov/safety/lightning-science-thunder
NOAA NESDIS, How Far Away Is Lightning?: https://www.nesdis.noaa.gov/about/k-12-education/severe-weather/how-far-away-lightning

Facts checked: October 8, 2026.