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Behind the Forecast: Why heat lightning does not exist

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Updated: Jun. 11, 2021 at 10:22 AM EDT
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LOUISVILLE, Ky. (WAVE) - During the summer months, heat lightning becomes a more frequent topic of discussion. However, heat lightning isn’t actually a thing.

“Heat lightning” is usually used to describe lightning from a thunderstorm that’s too far away for the thunder to be heard or the cloud-to-ground flash to be seen. It’s not a special type of lightning. It’s not created by hot or humid conditions.

The topography of an area can prevent someone from seeing the lightning flash so only a faint flash is seen as light is reflected off clouds higher in the atmosphere.

A mature thunderstorm can rise to 40,000 to 60,000 feet up in the atmosphere. Lightning from a thunderstorm can be seen from up to 100 miles away. However, according to the National Weather Service, thunder can only be heard within 10 to 15 miles of a thunderstorm.

Thunder is caused when a bolt of lightning rapidly heats the air around it, quickly expanding the air which produces what we know as thunder. Thunder is refracted through the lowest part of our atmosphere, called the troposphere. Sound travels at around 767 mph on average while light travels at 670,616,629 mph. How far sound travels is dependent on the density and temperature of the air; both of these change as you travel higher in the atmosphere. So they can change the speed of sound.

Air density and temperature can lead to what’s known as an acoustic shadow, where the lightning can be seen but the thunder not heard. The sound of thunder also bounces off of the planet’s surface and other objects, causing it to be not heard in certain locations. The curvature of our planet can cause thunder to be directed away from the surface before it’s heard far away.

If you are close enough to hear thunder then you are close enough to be struck by lightning.

Remember, “When Thunder Roars, Go Indoors” and if you “See a Flash, Dash Inside”!

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