The old wives' tale that a hot, humid summer night can generate lightning without a thunderstorm, called "heat lighting," is exactly that--a meteorological myth.
Heat lightning is just normal lightning from a thunderstorm too far away for the sounds of thunder to be transmitted.
A meteorological myth?
Many people think oppressive heat causes lightning to develop, but according to AccuWeather.com meteorologists, heat lightning is actually a thunderstorm happening a significant distance from an observer, who is only able to see the flash and not hear the thunder.
While heat lightning is really just ordinary lightning from a far away thunderstorm, the typical sound of rumbling thunder is muffled either by long distances or by a blocking, mountainous terrain.
The cracking sound of lightning is thunder, which results from the rapid expansion of hot and cold air masses.
Another reason no sound accompanies heat lightning is the idea that thunder travels much more slowly than light, and it's unlikely to hear thunder from a distance greater than 10 miles.
The distribution of hot and cold temperatures around a thunderstorm causes sound waves to be bent, or refracted, upward into the sky rather than toward the ground where an observer may be listening.
Sometimes the distribution of temperature is so great that any sounds being generated from a thunderstorm pass much too high in the sky to be heard by the human ear.
However, because light travels much faster than sounds of thunder, it is possible to see lightning from thunderstorms over 100 miles away.
Light from a lightning bolt travels outward at over 100,000 miles per second. The flash from a lightning strike occurring within 10 miles will reach you almost instantaneously.
Heat lightning is known as such because thunderstorms spawning lightning often occur in the summertime when temperatures are at their warmest.
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