A heat flash is simply a lightning strike that occurs in a thunderstorm -- it is too far away to hear the thunder but close enough to see the flash from the lightning.
Most people see these flashes at night when it's hot and humid and the skies are relatively clear overhead.
Clear skies allow us to see these flashes from great distances.
While the heat and humidity can help create a thunderstorm, it is not the heat and humidity alone that cause the flash of lightning.
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