Rex Trulove
Have you ever heard the term, ‘Dry Lightning’? You might wonder what it is and if it even exists.
Dry lightning does exist and it is 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.
The lightning in this video is primarily sheet lightning, though it isn’t dry lightning. Rain can clearly be heard hitting the ground:
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 the clouds, 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.
That’s fascinating, Rex. Thanks for the info! On the question of sheet lightning, my impression is that we get more of it than forked lightning in the UK.
I remember an occasion – years ago – when I was cycling at night in Wiltshire, not far from Stonehenge. I was aware of flashes in the distance that seemed to be coming from the other side of a ridge of hills. I thought at first that they were car headlights – from cars climbing the other side of the ridge and pointing their light skywards. However, I soon realized that this was impossible and what I was seeing was distant sheet lightning.
That is quite possible, John. We get a lot of both kinds of lightning here, often in the same storm. The sheet lightning rarely causes a problem, since it goes from cloud to cloud. However, when we have sheet lightning, there is very often forked lightning, too. That is what causes the forest fires and since the conditions are so dry, they flare up rapidly.
I’m curious; I know that there are still forests in the UK. Do you ever have forest fires there?
Our biggest problem with the forest fires during a dry lightning storm is that they are usually extremely hard to get to. With dry conditions, by the time a firefighting team can get to the fire, it is already expanding at a tremendous rate. The local fire we had last year was spotted when it was only a few acres in size. It was in rugged terrain, though, and it took time to get people to it. Most of the fire teams were also fighting other fires. They were caught flat-footed. The end result was that small fire grew to destroy over 40,000 acres of forests.
Yes – we do get forest fires, but they are rare and nothing like the scale of the recent fires in the US. For one thing, the forested areas are smaller, and managed forests are provided with fire breaks – cleared areas – that usually prevent small fires from becoming large ones. For another, our generally wetter climate does not often allow tinder-dry conditions to develop.
Heath fires are more common, and these are sometimes stated intentionally with a view to burning off old cover and allowing new growth to break through.
I have never heard of the term dry lightning but I did learn something new about respect for the weather.
That is a really good thing. Dry lightning is extremely hazardous, especially in the western US.
I have never heard of dry lightning.
It occurs all over the place, in areas that at least occasionally have very dry conditions.
Interesting article. From a distance, I adore watching lightning storms… But once I hear thunder less than 5 seconds after the flash, I rather stay indoor.
This is the first time I heard about dry lightning though.
That is a wise precaution as that puts the lightning within a mile from you. You may have experienced dry lightning without even knowing it. If you’ve ever seen a lightning storm where there wasn’t any rain or hail, it was dry lightning.
Interesting and I know it is a challenge to photograph lightning.
I have never heard of dry lightning.
Have you ever been in a thunderstorm that didn’t rain on you or drop hail? If so, you’ve seen dry lightning storms. If not, you haven’t yet.
Yes as a matter of fact I have, interesting!
Great post, another very informative one!
Thank you. Last year when there were so many devastating forest fires in Montana (over 9 million acres burned), nearly all of those fires were caused by dry lightning.
There was once a Redgager who photographs lightning. He posted in one of his articles that during a thunderstorm, he would set up his camera in a tripod and set the time value to a few seconds and sometimes minutes. Of course he does this from a distance.
I forgot his name though.
That makes sense. In today’s world, though, what some people do is simply videotape the storm. They can then go frame by frame and extract the pictures with the lightning.
Well done! Nice post!