Imagine sitting on your porch at dusk on a warm, summer evening and looking up to see a flash of lightning in the distance. But you don't hear the roar of thunder that usually accompanies a lightning strike. That's what people sometimes call heat lightning. But it's not actually a type of lightning, as the name suggests.
Heat lightning is simply a term used to describe the sight of lightning when it's too far away to hear thunder from a storm. The reason this is more common in the summer is because the higher humidity in the air tends to create a haze that can reflect light and make it appear farther up in the sky than it would otherwise.
However, even when the sky is clear, lightning can still be seen from quite a distance because the light emitted by a lightning bolt quickly heats up the air around it and creates a pressure wave. That pressure wave can be heard as thunder, which can travel as far as 10 miles from the lightning strike.
That's why it's important to remember that if you can hear thunder, you are close enough to be struck by lightning. When you're outside during a thunderstorm, always seek shelter in a substantial building with electricity and plumbing like a home, school or business, and avoid metal structures such as picnic pavilions or baseball dugouts. You can also use your car or an enclosed metal-topped vehicle as shelter, but only if it's not in motion.