Why does thunder sometimes crack and sometimes rumble?
Updated · 6 min read
A sharp crack means the strike was close, so sound from the whole channel reached you almost at once. A long rumble means distance: sound from different parts of a kilometers-long channel arrives spread over seconds, and high frequencies are absorbed along the way.
Why does close thunder crack?[1][2]
Because the whole lightning channel is roughly the same distance from you. Sound from the top and bottom of the channel arrives within a fraction of a second of each other, so it stacks into a single sharp report rather than spreading out.
Very close strikes also preserve the initial shock front. Within a few meters of the channel the pressure pulse travels faster than sound, and even at a few hundred meters the leading edge is still abrupt, which is why a nearby strike is felt in the chest as well as heard.
The high-frequency content is what makes it sound sharp. Those frequencies are still present because they have not had far enough to travel to be absorbed by the air.
Why does distant thunder rumble?[1]
Because a lightning channel is several kilometers long, and sound from its far end arrives seconds after sound from its near end. That spread turns one event into a rolling sound, which terrain echoes then extend further.
Work the arithmetic and the effect is obvious. A channel 5 km long, oriented so one end is significantly further from you than the other, produces arrivals separated by many seconds at 343 m/s. The rumble is one flash smeared across time by geometry.
Air absorbs high frequencies far more strongly than low ones, so distance also acts as a filter. What arrives from 15 km away is the low-frequency remnant, which is why distant thunder is a deep roll while nearby thunder has a sharp edge on it.
What does the sound tell you about distance?[2][3]
Quality of sound is a rough guide, and counting is the accurate one. A sharp crack with no countable delay means the strike was very close. A low rumble arriving many seconds after the flash means several kilometers.
The 15 km limit is why heat lightning exists as a category. Beyond roughly that range the sound has been absorbed and refracted away from the ground, so the flash is visible with no thunder at all.
| Sound | Typical distance | What it means |
|---|---|---|
| Sharp crack, no delay | Under ~300 m | Effectively on top of you |
| Loud bang, short delay | 1-3 km | Close. High frequencies still present |
| Sharp start, then rolling | 3-8 km | Channel geometry spreading the arrival |
| Low continuous rumble | 8-15 km | High frequencies absorbed, echoes added |
| Nothing audible | Beyond ~15 km | Seen as heat lightning |
Why does thunder echo in some places?[1]
Terrain and atmospheric layering both reflect sound. Valleys and mountains bounce thunder off slopes, adding delayed copies, while temperature inversions can bend sound back toward the ground far from the strike.
Mountain thunder is famously prolonged for this reason. The channel geometry spreads the sound, and then each slope returns its own delayed reflection, stretching a single flash into a roll that can last well over half a minute.
Inversions do something less obvious: they refract sound downward that would otherwise escape upward, which can make a distant storm audible in one location and silent in another at the same range.
References
- Severe Weather 101: Lightning BasicsNOAA National Severe Storms Laboratory, 2026
- The variation of the specific heat ratio and the speed of sound in air with temperature, pressure, humidity, and CO2 concentrationJournal of the Acoustical Society of America 93(5), 2510-2516, 1993 · doi:10.1121/1.405827
- Lightning SafetyUS National Weather Service, 2026
Related guides
- How far away is lightning? Count the seconds between flash and thunderCount the seconds between the flash and the thunder, then divide by 3 for kilometers or 5 for miles. See why the rule works and where it goes wrong.
- The speed of sound and thunder: what actually determines the delayThe speed of sound changes with temperature, humidity, pressure and wind. See the exact Cramer equation and how much error the simple rule of thumb introduces.
- How hot is lightning, and what does that heat do?About 30,000 kelvin, roughly five times the surface of the sun. What that heat does to air, sand, trees and anything it passes through.
- The math and sources behind thunder timingEvery equation this map uses to time thunder: Cramer's speed-of-sound formula, slant range, wind advection, and the research each constant comes from.