Heat lightning

Heat lightning, sometimes known as silent lightning, is the name used for the faint flashes of lightning on the horizon or other clouds from distant thunderstorms that do not have accompanying sounds of thunder. This often occurs because of inter-cloud and intra-cloud lightning, where lightning charges travel from cloud-to-cloud or merely areas of differing electric potential within a single cloud, respectively. Because of this nature, heat lightning can often be seen from great distances, and thus can be an early warning sign that thunderstorms are approaching. The term could be considered a misnomer because it has nothing to do with the heat of the lightning itself.

Heat lightning can also occur if cloud-to-ground lightning occurs very far away and the sound dissipates before it reaches the observer. At night, it is possible to see the flashes of lightning from very far distances, up to 100 miles, but the sound does not carry that far. In Florida, heat lightning is often seen over the water at night, the remnants of storms that formed during the day along a sea breeze front coming in from the opposite coast.

Heat lightning also occurs where airborne matter muffles the thunder, such as heavy snow in winter storms (thundersnow) and dust and sand storms. In some instances, heavy falling snow has silenced thunder from cloud to ground lightning strikes as close as one to two miles (1.6 to 3.2 km) from the observer and severe dust storms are even more effective in many cases.

Heat lightning is not to be confused with electrically-induced luminosity actually generated at mesospheric altitudes above thunderstorm systems (and likewise visible at exceedingly great ranges), a phenomenon known as "sprites".

The movement of sound in the atmosphere depends on the properties of the air, such as temperature and density. Because temperature and density change with height, the sound of thunder is refracted through the troposphere. This refraction results in spaces through which the thunder does not propagate. The sound of thunder often reflects off the Earth's surface. The rumbling sound is partly due to these reflections. This reflection and refraction leaves voids where thunder cannot be heard.

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Wikipedia