Most of us have likely been in this scenario before. You walk outside or are driving somewhere during a sultry summer evening. All of the sudden, the sky is illuminated by a flash of lightning. You wait expecting to hear a clap of thunder but it never comes. What you have just experienced is widely known as heat lightning.
Does it Actually Exist?
Ok, but does heat lightning actually exist? The short answer is no, not technically at least. The American Meteorological Society defines heat lightning non technically as “the luminosity observed from ordinary lightning too far away for its thunder to be heard.” As the AMS stated, what you are actually experiencing is lightning from a normal thunderstorm that is too far away to hear the clap of thunder, see the rainfall, or even see the clouds.
Many people believe this instance of lightning is created simply by the hot and humid conditions without the presence of rain or thunder hence the term heat lightning.
You can view lightning up to 100 miles away from the thunderstorm producing it. Unless you are within around 10-15 miles of the strike, you most likely won’t hear the thunder produced.
Lightning Without Thunder?
Why can’t you actually hear the thunder? Well, there are actually a couple of reasons. One common reason is the curvature of the earth. The soundwaves from the lightning strike, aka thunder, travel outward from the epicenter of the strike. Since the earth is curved, the sound waves emanating outwards will eventually travel into the ground. The curvature is also responsible for sometimes hiding the cumulonimbus clouds completely from the observer’s sight if the storm is far enough away.
Another possible reason is that the sound waves could have been bent away from your location due to refraction in the atmosphere. Refraction occurs due to the change in density or temperature through different layers in the atmosphere. As the thunder travels through the different densities, the sound waves bend. This process can keep you from hearing the rumble.
The sound waves can also strike the ground and reflect upward which would also keep you from hearing the sound as well.
Our local terrain can also impede us from hearing the crack of thunder. The shockwaves from the lightning strike can be absorbed by the surrounding terrain. When this occurs the thunder either becomes very muffled or it is not heard at all.
So next time you see lightning in the summer night sky, know that it is from an actual thunderstorm that is simply located too far away to hear the thunder.