Fire clouds
What do a forest fire, a volcanic eruption, and a nuclear explosion have in common? What, fire? Oh, right, fire. Oh, okay. But not just fire!
How fire creates clouds
Clouds are normally caused by the vaporization of water. Particles of vapor rise into the sky with the warm air, cool down, «stick» to smaller particles of dust/pollen/pollution, and become liquid water droplets again, from which clouds form.
The key element here is warm air: f to become less dense and begin to rise, it must be heated by something. Usually, the sun does this, generously pouring heat onto the earth and ocean, which in turn heats the air.
But there are other ways. Sometimes it’s a fire that heats the earth. And not a barbecue or burning your ex’s stuff at 5 a.m., but something more serious. It is not difficult to guess that even a small fire will heat the air and evaporate water better than the gentle rays of the sun. This means that conditions for cloud formation will occur more rapidly.
Such clouds generated by fire—as a result of a fire, eruption, or powerful explosion—are called pyrocumulative clouds.
Pyrocumulative clouds are usually gray or slightly brown in color. Photo: Malachi Brooks / Unsplash
How water creates fire
Any fire spews huge amounts of soot particles into the sky. Any of these particles which water vapor can «stick» to (i.e. condense) help to form clouds.
Therefore it turns out that a fire is the perfect factory for thick clouds. That’s why the media sometimes say that wildfires «create their own weather.»
And this is how a pyrocumulative cloud looks from space. Argentina, 2018. Photo: NASA
But where there are thick clouds, there is also precipitation. After all, when a cloud gets too heavy from liquid water, the water spills back to the ground. And pyrocumulative clouds are no exception: a cloud generated by fire can also extinguish the fire that spawned it.
However, in addition to rain and hail, a pyrocumulative cloud is often struck by lightning. Because of the extremely powerful upward flows of countless ash flakes, the particles inside the cloud rub against each other and exchange electrons even more actively than in a normal cumulus-rain cloud.
Since rain and hail create downward air currents, the upward and downward currents under the cloud mix and lead to powerful winds and whirlwinds—which can increase in power to become a fire tornado.
It turns out that the combined effect of rain, thunderstorms, and strong winds may not extinguish the fire, but on the contrary, they may fan the fire and spread the embers beyond the boundaries of the original epicenter. Lightning, in particular, can ignite trees and buildings, and create additional fire centers.
A fire tornado is one of the possible consequences of a pyrocumulative cloud. Photo: Chris Tangey, Alice Springs Film & TV
Where and when pyrocumulative clouds have been observed
Such clouds are mostly recorded in places where forest fires often occur: in the United States, Canada, Siberia, and Australia.
Due to powerful upward flows, the upper boundary of pyrocumulative clouds can rise very high—even to the level of the stratosphere: 15 kilometers.
The mushroom cloud after a nuclear explosion is also considered to be pyrocumulative. But in the photo from Hiroshima, taken on the day of the nuclear explosion on August 6, 1945, the cloud is so huge that experts believe its source is not the explosion, but the city fires that followed the explosion. Photo: US military
If the cloud reaches the stratosphere, a huge amount of smoke and ash spreads and can persist in the sky, obscuring part of the sunlight for a very long time—for days and weeks, in rare cases, months or even years, as happens after powerful eruptions. The authors of the «nuclear winter» hypothesis rely on this, among other things.
While fires may seem to be a force of nature, and nuclear winter just a feature of post apocalyptic fiction, we need to be aware that at least some of the causes of these fires are man made and avoidable.
In order to avoid fires, which can lead to the appearance of a pyrocumulative cloud, it is extremely important to observe safety in the forest—at least put out campfires and don’t carelessly toss out smoldering for cigarette butts.
If we do all that we can to reduce unnecessary fire risks, we still won’t stop all naturally occuring pyrotechnic shows, but by being good stewards of our natural resources, perhaps we can minimize the impact when natural fires are sparked beyond our control.
Text: Eugenio Monti, a meteorologist and a climatologist
Cover photo: Manny Becerra / Unsplash
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