The short rule
If you see a flash but never hear thunder, you are usually watching a storm beyond earshot, not a unique “heat” species of bolt. The popular name stuck because those flashes are common on hot, hazy summer evenings when storms sit on the horizon.
Compare that with a flash that arrives with a sharp crack within a few seconds: that strike is close enough that you should move indoors and off ridgelines. For other look-alike weather columns, see dust devil vs tornado.
Side-by-side: what you see and hear
| Cue | Heat / distant lightning | Nearby lightning |
|---|---|---|
| Thunder | None, or a soft rumble much later | Clear bang or roll within ~30 seconds |
| Sky look | Horizon flicker; storm may be below the horizon | Bolt or in-cloud flash overhead / nearby |
| Safety | Low immediate risk if no local storm | Seek sturdy shelter; avoid open fields |
| Decision rule | Check radar for a storm 20-100+ km away | If thunder follows, treat as close enough to hurt |
National Weather Service explainers note that thunder is rarely heard beyond about 10-15 miles (roughly 15-25 km) under typical conditions, while the light from a strike can travel much farther, especially when reflected by haze or clouds.
What heat lightning is not
It is not lightning caused by heat alone with no storm. Surface heat helps build thunderstorms, but the flash still comes from a charged cloud system somewhere in the distance.
It is also not the same as rare upper-atmosphere events (sprites, elves) or the debated glow called ball lightning. Those have different shapes, heights, and evidence standards.
If radar shows no storm for a hundred miles and you still see brief sky flashes, consider human lights, meteors, or power-line arcs before inventing a new weather type.
Practical checklist
Count the seconds from flash to thunder; every five seconds is about a mile. No thunder after half a minute usually means the cell is well outside that range.
Open a radar app: a line of storms on the far side of the county matches classic “heat lightning” nights.
If gust fronts, mammatus, or a falling barometer show up locally, stop watching the horizon show and prepare for the storm to arrive.