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| This, dear readers, is a pyrocumulus cloud. Photo courtesy Art Rangno. |
Some of you may recall a previous posting about the supposed pyrocumulus cloud over the Big Hump Fire on the Olympic Peninsula last month. It turns out that what we were seeing in Dale Ireland's time-lapse video was not a pyrocumulus cloud, just a puffy plume of smoke. The Accidental Naturalist is guilty of wishful thinking; I am studying clouds, not smoke, for my next book.
I got the full explanation of pyrocumulus from my meteorologist, cloud maven Art Rangno, who kindly sent me the two photographs of real pyrocumulus posted here.
The image above was taken at a controlled burn in what Rangno referred to as "the god-forsaken town called Hornepayne" in Northern Ontario, Canada in 1989. In this photo, you can clearly see the difference between the smoke from the forest fire and the pyrocumulus cloud--the smoke being gray (likely from the fluids used to ignite the fire) and the cloud being white (because water droplets in the clouds are large and scatter light that appears white to our eyes). The smoke has provided tons tiny particles that became condensation nucleii; water vapor condensed on these and eventually became large water droplets and cloud.
The image above was taken at a controlled burn in what Rangno referred to as "the god-forsaken town called Hornepayne" in Northern Ontario, Canada in 1989. In this photo, you can clearly see the difference between the smoke from the forest fire and the pyrocumulus cloud--the smoke being gray (likely from the fluids used to ignite the fire) and the cloud being white (because water droplets in the clouds are large and scatter light that appears white to our eyes). The smoke has provided tons tiny particles that became condensation nucleii; water vapor condensed on these and eventually became large water droplets and cloud.
The image below was taken during a research flight over the Puget Sound area conducted by the University of Washington Department of Atmospheric Sciences (Rangno's pre-retirement workplace). You can see the white pyrocumulus cloud in or above the stratiform layer of smoke. You can also see some natural clouds above the peaks of the Olympic Mountains in the background.
The flattening out of the rising smoke is something I did observe in the video of the Big Hump Fire plumes. Here is how that happens: The air heated by the fire rises in the surrounding cooler air. As it rises, it cools and then reaches a level where it is the same temperature as the surrounding air and is no longer buoyant. It then settles where it can remain buoyant and then spread out horizontally--as you can see clearly in the photograph above.
What is "fun" about this smoke (and,by the same token, smoke stacks or steam plants) is that you can actually see the warm air rising and eventually leveling out. The same kind of thing is happening to the air beneath clouds--but we can't see the air during this part of the process: the sun warms invisible parcels of air near the surface of the earth, the warmed air rises then reaches a level at which is is no longer buoyant. In the case of water vapor, this is called dew point or condensation level--the point at which the water vapor in the air condenses and forms a visible cloud. The base of that cloud rests at this level. Depending on the type of cloud, it may continue to lift (creating cumiliform clouds) or it may spread out horizontally (creating stratiform clouds).
Either way, the base of the clouds remains constant. When you read that altocumulus clouds are typically 6,500 to 26,000 feet high, know that those numbers refer to the height of the bases of the clouds above the earth. Altocumulus clouds rarely rise more than .62 miles above that. Cumulonimbus clouds may rest their bases at 2,000 feet (the condensation level or dew point) but grow vertically to 39,000 feet.
So, when you next go cloudwatching, imagine all the warm air rising beneath each of the clouds. Just beneath the cloud base, there is invisible water vapor; once it reaches condensation level, becomes visible cloud.
While I am not a fan of forest fires, smoke stacks, or steam plants, they do give us a visual impression of what the atmosphere is hiding...which is, as always, more than we can imagine.
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| Smoke--vertical and horizontal and a pyrocumulus cloud. Photo courtesy Art Rangno. |
What is "fun" about this smoke (and,by the same token, smoke stacks or steam plants) is that you can actually see the warm air rising and eventually leveling out. The same kind of thing is happening to the air beneath clouds--but we can't see the air during this part of the process: the sun warms invisible parcels of air near the surface of the earth, the warmed air rises then reaches a level at which is is no longer buoyant. In the case of water vapor, this is called dew point or condensation level--the point at which the water vapor in the air condenses and forms a visible cloud. The base of that cloud rests at this level. Depending on the type of cloud, it may continue to lift (creating cumiliform clouds) or it may spread out horizontally (creating stratiform clouds).
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| The dark bases of these cumiliform clouds are at the same altitude--the condensation level or dew point--but the height or thickness of the clouds themselves vary. The cloud height indicates the altitude of cloud's base above the ground. |
So, when you next go cloudwatching, imagine all the warm air rising beneath each of the clouds. Just beneath the cloud base, there is invisible water vapor; once it reaches condensation level, becomes visible cloud.
While I am not a fan of forest fires, smoke stacks, or steam plants, they do give us a visual impression of what the atmosphere is hiding...which is, as always, more than we can imagine.