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Heat Lightning: Why We Can See Lightning Without Hearing Thunder

© Keith C. Heidorn

Heat lightning are visible flashes of distant bolts that have no thunder sound accompanying them. It is not a unique form of lightning, but normal thunderstorm lightning that flashes too far away for its thunder to be heard.

It most commonly manifests as sheet lightning, which is a standard lightning bolt whose light is reflected off clouds or is diffused as it passes through the atmosphere.

Lightning Without Thunder

Scattered thunderstorms often occur in hot, humid weather. Some pass overhead, bringing rain. Others may pass a moderate distance away. We see lightning forked, sheet, ribbon) and hear thunder, but no rain falls. Finally, there are storms that pass a long distance from us whose presence is only noted by their clouds and flashes of yellow-tinted lightning.

Because scattered thunderstorms do not produce a dense cloud deck, we have a long line of sight through the air, and we can see the upper levels of faraway thunderstorms.

Within those distant thunderstorms, the lightning bolts can be seen as much as 160 kilometres (100 miles) from us, depending on the height of the bolt, the clarity of the air, and our elevation. Thunder, in comparison, has a much shorter range of detection - usually less than 25 km (15 miles) in a quiet rural setting and under 8 km (5 miles) in a noisy city environment.

Why Thunder Can't be Heard from Far Away

Air molecules scatter and attenuate sound, particularly diminishing the higher pitched frequencies. When thunder reaches a listener several kilometres away, the predominant sound will be a low-pitched rumble. Further away, the rumble ceases to be heard.

Changes in temperature and wind speed through the lower atmosphere also affect the propagation of thunder by refracting sound waves away from the listener. Because the air temperature generally decreases with height and sound travels faster in warmer air, thunder sound waves curve upward, away from the listener. High altitude-originating thunder may bend away from the ground and never be heard.

Wind can also affect thunder. Sound moves faster downwind that it does upwind. The variation of wind with height may refract sound waves too, similar to the effects of vertical temperature gradients. Wind usually increases with height near the earth's surface, and sound waves refract upward, away from the surface-based listener.

The combined effects of scattering, attenuation, refraction and, in some cases, reflection, limit the distances at which thunder may be heard by a ground-based observer. Although this distance varies with temperature, wind speed, and lightning height, thunder generally will not be heard further than 10 to 25 kilometres (6 to 15 miles) from the lightning bolt.

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