Having dispelled the myth of heat lightning, there is another phenomenon that people may have heard of called dry lightning. They might wonder if there is such a thing and the simple answer is yes.
Dry lightning is also related to heated air, particularly at ground level but sometimes reaching several thousand feet above the ground. A dry lightning storm produces lightning, but no precipitation hits the ground. One of the things that is particularly interesting about this kind of lightning is that from a distance, a person can often clearly see that the clouds are dropping rain. So why isn’t the rain reaching the ground?
The key factor for this to happen is the humidity of the air. All air at the ground level contains moisture, even in the driest places on Earth. However, if a mass of hot air has very little moisture in it, the rain doesn’t have much chance to reach the ground. The heat of the air is enough to cause rapid evaporation and since the air is exceptionally dry, it has the capacity of holding a lot of the evaporating moisture.
The result is that rain falls, evaporates, becomes absorbed by the air and can even be forced right back up into the cloud. This can be a perpetual cycle and it will usually happen until the air has enough humidity that the rain can reach the ground without evaporating.
Dry lightning most often happens over places that are exceptionally dry, such as deserts, though it can occur in other places as well, if there isn’t much humidity. It also normally happens during the hottest and driest time of the year. Death Valley and the Sonora Desert in the United States, the Sahara Desert and several other deserts, along with plains and savannas in numerous places around the world all record dry lightning on almost a yearly basis. Quite often in this sort of electrical storm, a majority of the lightning that is produced is also cloud to cloud lightning, rather than cloud to ground lightning. Since this kind of discharge appears relatively broader than ‘regular’ lightning, this lightning is sometimes known as sheet lightning.
It might be mentioned that much of this is an optical illusion. A typical bolt of lightning is seldom more than four inches across, often far less. It is so brilliant and the light reflects off of particles in the air, making the shaft of lightning appear far broader than it really is. If there are objects in the foreground that are large, this can further exaggerate how broad the bolts of lightning appear to our eyes. With sheet lightning, the bolts can be quite a bit broader than four inches, occasionally even double that amount. However, the same optical illusion exists so they look substantially more than eight inches wide. Additionally, if the cloud cover is dense so that the strike is veiled by cloud cover, the lightning can easily look like they are sheets of electricity as the light reflects off of water vapor, though the bolts are still only eight inches wide or less.
Interestingly, if cloud to cloud sheet lightning is photographed, it normally doesn’t appear nearly as broad as it does to the naked eye. The picture of cloud to cloud lightning at the top of this article is an example. To an on the ground observer, this would appear as sheet lightning. Notice the broad reflection where part of the bolt is obscured by clouds, adding to the illusion.
It should also be remembered that a bolt of lightning produces exceptionally high temperatures for a brief time. According to the National Oceanographic and Atmospheric Administration (NOAA), a bolt of lightning can produce temperatures as high as 60,000 F (33,000 C). This can increase the illusion of broadness by creating sudden intense updrafts. (As we all probably know, hot air rises. Exceptionally hot air rises very fast.)
Dry lightning does occur. In fact, dry lightning is the most likely sort of lightning to cause forest fires, since the air conditions are dry and there is no moisture reaching the ground that could deluge the fire. Although the lightning can be primarily sheet lightning, some of it does usually strike the ground. Dry lightning shouldn’t be confused with heat lightning, though. The first exists and the second doesn’t. NOAA actively tracks dry lightning storms because of the number of major fires they are capable of producing.