Heat lightning is probably one of the biggest misconceptions in the weather world. Growing up, you were likely told the frequent flashes were the clouds reacting to an extremely hot day. There is a little truth to that.
The "heat" term probably came about because these flashes are often observed on a warm and humid night, after daytime heating has triggered thunderstorms.
In reality, heat lightning is simply the lightning from a distant storm. The sound of thunder rarely travels more than 10 miles. However, lightning can travel for miles and miles, illuminating clouds in the night sky.
Think back to the last time you saw heat lightning. You never hear anything, or saw the actual lightning bolt, just the bright light in the clouds.
Sometimes, even on a clear night, you can see the stars above your head, but in the distance, a flicker in the clouds. That storm may be 25 to 50 miles away, depending on the amount of haze and humidity in the night sky.