Quite a few people have heard of what is called “heat lightning”. This is the supposed phenomenon that happens when the air is so hot that it generates lightning without clouds and often without accompanying thunder. This is a myth, but it requires a little explanation.
Lightning is caused by a difference in electrical charge between one cloud and another, or between a cloud and the ground. The equalizing discharge is what we know as lightning. If you shuffle across a carpeted floor and then touch a grounded metal object, you might feel a shock. This is what lightning is, only on an enormously larger scale.
Hot air temperature, in and of itself, can increase the likelihood of a difference in charge that could lead to a lightening strike, but it doesn’t actually generate such a discharge since the air isn’t localized. What it can do, however, is cause evaporation and a strong updraft. This can in turn cause the formation of thunderclouds as the moist air shoots up to an altitude where the air temperature is cool enough for the moisture to condense. At that point, the charged particles are localized in the cloud and lightning can happen.
So what about when we see flashes of lightning, but hear no thunder? Lightning bolts cause thunder because the lightning super-heats the air it passes through, causing it to expand. This is rather like the bow wake of a ship. The expansion is rapidly followed by a contraction as the tremendous heat almost instantly dissipates and this causes the sound wave we know as thunder. Thunder can be thought of as the sound of the air slamming together after the vacuum caused by the passage of a bolt of lightning.
Transmission of sound depends on a lot of things; air density and distance being two of them. Most people are aware that the farther away someone else is, the louder they have to speak or yell in order to be heard. This is because of the distance factor. A sound wave in our atmosphere dissolves as it travels a greater distance.
However, you might note that on cool, crisp mornings, sound seems to travel farther. This isn’t an illusion and it is because cool air is denser than hot air.
With what people think as being heat lightning, both distance and air density come into play. The air isn’t very dense, so the sound waves don’t travel very far. Lightning is brilliant in the night sky, however, and if the strike is from a cloud that is 10,000 feet higher than we are, it can be seen for many miles. We probably just aren’t going to hear the thunder because the sound wave has dissolved by the time it reaches us.
Additionally, the air above a thunderstorm is generally far colder than the air below it, so it is much more dense above the cloud. A great deal of the sound wave will end up traveling through the denser air that is way above our heads than through the less dense and hot air that surrounds us. The sound wave dissolves even faster for us at ground level because most of it is reflected upward.
In fact, it is for this reason that most ‘heat lightning’ is seen on the horizon, where it can be difficult to see the clouds that spawned the lightning. The lightning can appear to come from a cloudless sky, though this isn’t what is really happening.
If you see a flash of lightning but never hear the thunder, it doesn’t mean that there isn’t any. It just means that you are too far away to hear it. Someone who is much closer to the lightning will almost certainly hear the thunder.
Heat lightning doesn’t exist. However, because of circumstances and conditions, we can be tricked into thinking that it does. This is an illusion that we can understand if we have some simple and basic understanding of lightning and thunder.