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Question: I have heard a lot about heat lightning. What is it?
Folk weather mythology suggests heat lightning is caused solely by hot air expanding until it sparks on sultry summer nights. You will also find many credible sources telling you that heat lightning does not exist. But, this is mostly a matter of semantics.
Heat lightning is not a unique form of lightning, but normal thunderstorm lightning that flashes too far away from the observer for its thunder to be heard. It is most commonly seen as sheet lightning.
During hot, humid weather, scattered, rather short-lived thunderstorms may pop up across a region driven by the heat and humidity. Some, during their lifetimes, may travel overhead, bringing heat-relieving rain and a brief, cooling wind. Then there are those which pass a long distance from us whose presence is only noted by its towering cumulonimbus and flashes of lightning.
Because scattered thunderstorms do not produce a sky-filling cloud deck, we have a long line of sight through the mostly clear air, often extending to the horizon. Therefore, we can see the high levels of thunderstorms even at distances below the horizon.
Within those remote thunderstorms, when the lightning bolts flash, their light can be seen as far as 160 kilometres (100 miles) distant, depending on the height of the bolt within the cloud, the clarity of the air between us and the bolt, and our elevation above the ground. 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.
Therefore, we see the lightning, but do not hear the thunder — a situation we have taken to call heat lightning.
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