Pyrocumulus cloud

By Phil Plait | June 24, 2012 7:00 am

As I write this, the High Park fire is the second largest wildfire in Colorado history, currently at 75,000 acres (over 300 square kilometers, or 115 square miles). It’s been burning more than a week, and fighting it has been difficult due to dry conditions, wind, and oppressive heat in the area.

I can see the fire from Boulder, but yesterday I got a really good, if terrifying, view of it driving home from the airport. There was nothing but farmland and one low range of hills between me and it. I stopped and took some pictures with my phone:

I was about 55 km (35 miles) south of the fire when I took this. Note how the plume is whitish and looks like a storm cloud. I discovered there’s a word for this: pyrocumulus; "pyro" means fire, and "cumulus" is Latin for heap or piled up. Cumulus clouds are the ones that are the big puffy cauliflower-shaped ones. The puffiness is from convection, which is when hot air rises and cold air sinks. Usually, warmer moisture-laden air punches upward into the cooler air above it. The water condenses, and all the little convection cells give the cloud that lumpy appearance.

In this case, the fire is hot, and the air is thick with smoke as well as water from the efforts to put it out [UPDATE: As Dan D points out in the comments below, water from the vegetation contributes to this as well]. It rises rapidly, forming the pyrocumulus cloud. They’re usually grey, but I suspect the towering cloud I saw is white due to the water vapor. The smoke plume from the fire is blowing to the east (right), and stretches for a long, long way:

It actually extends well off to the right, outside the frame of this picture. The smoke plume is noticeably reddish to the eye. I was cycling up that way last week, just the day after the fire started, and the smoke plume was reddish-brown with a blue tinge to it around the upper edges. I suspect this is due to scattering. Incoming light of all colors from the Sun hits the cloud. Longer wavelength red light penetrates deeply into the cloud, but blue light only gets a short way in before scattering off the smoke and ash particles. Think of them like blue photon ricochets, hitting the cloud and bouncing off in all directions.

The upshot is that we see blue light coming from the edges of the cloud where it gets scattered, but the lower part of the cloud looks redder because of the intrinsic color of the smoke, and also because only the red light form the Sun gets through it (similar to why sunsets look red). The overall effect is eerie, and unpleasant.

Which fits. This fire is pretty bad, and it’s joined by many other fires in Utah and New Mexico, not to mention in other countries like Russia. I’ll note that it’s difficult to pin this down to global warming, but as the planet does warm, different weather patterns will make some places wetter, others drier. One commonly predicted outcome is more and bigger fires, and it does seem we’re approaching or breaking a lot of records lately.

Related Posts:

Boulder fire from space
Sunset on an alien world
… I meant, Stardust "@" Home (more about this blue/red light effect from scattering)
The twice reflected Moon light

CATEGORIZED UNDER: Miscellaneous, Pretty pictures

Comments (40)

  1. Wzrd1

    It’s instinctive to see something like global warming in seasonal events, but Phil, YOU YOURSELF warned us of that tendency.
    Global warming will be proved by long trended data, not one or two harsh summers.

    That said, regarding this fire…
    Phil, has it reached firestorm intensity and size yet? Perhaps an article on firestorms might be in order? Quite a few of the phenomena of the firestorm are also common in astronomy.

  2. Damon

    ^^^^^ we already have long trended data

  3. Jay29

    “Global warming will be proved by long trended data, not one or two harsh summers.”

    @Wzrd1, I thought GW was already proven…

    But I agree that it’s dangerous to hint that events like these are triggered by GW until the science backs it up. Just because we don’t want to get in the habit of gut speculation. Leave that to the denialists.

  4. Michelle

    We have the data. These events were predicted a long time ago, and now the predictions are unfolding. Every year forest fires are getting more frequent, bigger, more destructive. It’s not dangerous to hint that these events are caused by global warming, it’s dangerous to say they are individual climate events. Folks, you’re not seeing the forest for the trees! And as each tree goes up in flame, you keep saying, it’s not GW, it’s not GW, it’s just one tree!

  5. Dan D.

    Nice article and picture! My first time commenting on here. A good pyrocumulus can even develop into a full-fledged pyrocumulonimbus, and even in some cases put out the very fire that started it!

    A few things:

    1) I know that you didn’t make this explicit claim, but the reason water condenses in a developing cumulus cloud is not because the warm air below is punching into the cooler air above — that’s true, but the warm air itself is cooling as it rises due to adiabatic expansion. Eventually, the rising air reaches it’s dewpoint, and a cloud forms. This will occur independent of how cool the environmental air it’s rising into is, although the latter does affect how *rapidly* the warm air rises, or even whether it rises at all. The colder the air aloft, the faster the warm air from below rises, due to the greater temperature difference between the rising air parcel and the environment it’s moving into — that is, greater buoyancy. Conversely, if the air aloft is too warm, it can be warmer than the air rising to replace it, and the rising air will decelerate and even sink back down. This is why thunderstorms eventually flatten out on top and produce a long horizontal anvil; the tropopause is very warm compared to the air below it!

    2) Actually, a good chunk of the water vapor that feeds a pyrocumulus comes from the water in the burning vegetation itself. I would hazard a guess that this source of water vapor is far greater than that derived from the water used to try to put the fire out, but I would need to research this a bit more to be sure. I believe that this has to be taken into account when modeling wildfires and coupling them with atmospheric models. (I’m a research meteorologist who deals with computer models of thunderstorms, but haven’t done any fire-related stuff myself).

    3) I tend to agree with the other commenters regarding GW. One can simply not place even a seasonal trend like the one that is leading to these fires into the context of GW without having many more years of data to confirm a longer-term trend and a physical connection. I’m not sure this has been done for, say, fires on the Great Plains. Perhaps it has and I’m simply ignorant of it (I’m not a climate scientist!).

  6. There for a minute I thought the BA had moved to southern california.

    ‘course y’all know that all that ash will reflect more sunlight into space, thus reducing global warming, right? Burn baby burn!

    I keed.

    Welcome to the future!

  7. Chris

    On the bright side, once all forests burn down we won’t have to worry about forest fires anymore.

  8. Dan D. (5): Ah, of course. I updated the post to add in the point about water. I didn’t talk about adiabatic expansion because I always worry that details like that may take away from the main point. I didn’t mean to imply the warm air cools because of conduction with cooler air, of course.

  9. Jenn D.

    Love the comments on this blog. Even the snarky ones are fun. Great addition Dan. Do you have a blog of your own? I’m not a scientist of any kind – just an interested bystander.

  10. 1) High atmospheric particulates cause Global Cooling by reflecting sunlight back into space. 2) Burning foliage and wood is carbon-neutral. 3) Removing surface flora ends root depletion of soil deep water. 4) Killing forest removes environment for the intensely destructive bark beetles. 4) Dousing wildfires with water pumps water gas (carbon monoxide and hydrogen) into the atmosphere, exacerbating the Greenhouse Effect. If you love the Earth, burn the entire US west to save it. Smokey Bear is a stateless international eco-terrorist.

  11. VinceRN

    Saw clouds like this many times living my first 33 years in Southern California where we occasionaly had fairly large fires. They can be especially impressive at dusk lit from below by fire while the tops are still catching sunlight. I never heard the term pyrocummulus, kind of cool that there’s a word for it.

    As for the GW aspect, there were massive fires, far bigger than anything we’ve experienced recently, before humans came on the scene. Wildfires are part of nature and I don’t think having one, or even a cluster of them, needs to be attributed to anything other than nature doing what nature does.

    Still, they really suck for humans.

  12. Wzrd1

    VinceRN, I was just reading on the Peshtigo fire, which killed at least 1500 in 1871, literally two days before the Chicago Fire. It tended to be forgotten, as Chicago was and is a major city and the Peshtigo fire burned more rural regions.

    I suspect some think I am a GW denier. I’m not. I simply mentioned that a few years do not a major trend make. The effects many promulgate (which BA does not promulgate) are effects of local events that may or may not be caused by GW. It’s a matter of a small area in scope and short trending, whereas global climate is measured in at the minimum, centuries, with shorter trends observed (El Nino, La Nina, etc).
    In short, a drought that contributes to forest fires may or may not be caused by long term global warming. If one were to point at an event that IS more alarming and indicates trending, one should look at the northwest passage opening totally last summer. THAT can only happen after many years of warming in the region. When one adds in arctic melting in Russian waters, one finds a more worthy trend than a regional forest fire that drought contributed to.
    Science is exacting. It requires great precision and evidence based theories. Not short term trends.
    Though, I DO tend to joke about local weather being GW, those who know me know it is intended in jest. Again, local phenomena are not global trends.

    @Uncle Al, burning forest is NOT carbon neutral, as wood burns, releasing carbon dioxide and carbon monoxide. Water gas is called steam and isn’t a gas, it’s vapor.
    However, fires ARE part and parcel of the ecosystem of forests. The difference is, excessive conservation prevented normal, lower intensity fires from clearing small brush. That proliferated, creating both a thicker than natural mat of organic material over the soil AND massive overgrowth. That is why the Forestry Service is experimenting with burn areas, to ascertain scientifically what levels of growth should be permitted before a fire can be “permitted” to clear overgrowth naturally. Rather than the massive, out of control firestorms we now have been getting.
    And I HAVE oversimplified that by a LOT.

  13. TeeDawg

    Hey Phil, I saw this too as I drove that way a few hours before you. I told my wife I’d love to see it a dusk as the farmland was in the shadow of the mountain and the clouds were lit up by the setting sun.

    Unfortunately, we were heading back to Denver in the late afternoon.

    Perhaps you’ll let me know!

    Cheers, Todd.

  14. SkyGazer

    Totally off topic, but very funny AND very important too!
    So here goes.

    A World Without Math

  15. kat wagner

    Here in south central Idaho we have been kicked out of our house for two fires – one was started on a horribly windy day by some guy burning his garbage in a barrel, way far away. I had gone for a run and saw the first wisp of smoke, and I thought, you idiot!

    RE global warming though, does Colorado have lots of dead trees due to the pine bark beetle? We do, because the winters haven’t been -30 for weeks and that’s one thing that kills the beetle. And when there’s enough water in the trees, they can spew out the beetles that do burrow in. Remember all the dead trees in Canada during the last Olympics? Same thing, dead trees because of the pine bark beetle and warmer winters.

  16. VinceRN

    Was doing a little googleing (googling?) on wildfires and found this. Interesting. Also found info on wikipedia about a three million acre fire in 1825. Three million acres? There are nine states in the US smaller than that, there are dozens of countries in the world smaller than that. Crazy.

  17. MadScientist

    The water contribution from the firefighting effort is insignificant compared to the water from combustion + moisture in the fuel. You can safely bet the clouds are virtually 100% water from the burning material. Even ‘dry’ fuel has quite a bit of moisture in it. Each load dropped by aircraft is only a few trees worth and the large fire engines only carry a few dozen trees worth of water.

  18. FMCH

    Phil, here in Florida the only thing that saved us was La Nina dissipating or else, we were on the way for another big fire season.

    Speaking of thunderstorms and fires, check out this gif of satellite images from Florida Forest Service showing a brush fire causing a huge thunderstorm to develop. As was typical for the summer of 1998, the storm didn’t provide any relief.

  19. Steve

    I just saw ABC news and their report from Colorado Springs. I hope you and yours are all safe and sound, and you are in our prayers.

  20. Jorge P

    You mean all the warming that didn’t happen over the last decade?

  21. David W

    Several years ago there was a huge fire in the White Mountains in Arizona. It consumed several hundred thousand acres, and created a breathtakingly huge cloud. We were driving to Prescott on I-17, and could see that huge cloud a hundred miles to the east. It was stunningly huge.

    Less huge, but locally devastating, was the Aspen fire in the Santa Catalina mountains on the north edge of Tucson. Much of Summerhaven, a small town in the highest part of the range, was destroyed. From down in the valley, we can still see skeletons of trees standing on the ridges. When the fire was burning, we could see the flames from all over Tucson at night. In the morning we would awaken to the smell of smoke in the air and the sight of ash in the pool–sometimes even whole aspen leaves.

    Both fires were human-caused.

  22. Karina

    Of course, if people would just stop setting fires (either accidentally or maliciously), that would be a big help too. Officials believe this was man-caused.

  23. Dan D.

    @Phil #8. Indeed, sorry for going off on too big of a tangent about convection!

    @Jenn #9. Thanks! I don’t have a blog devoted purely to my scientific work or field, but I’ve thought quite a bit about starting up one. I do however, have a more philosophical/theological blog that deals with the interplay between science and faith (at least how I see it!) at, which I don’t post to nearly enough.

  24. Daniel

    @Michelle, global warming has nothing to do with the increase in fires. We are reaping the consequences of 100+ years of ignorance about the role of fire in the ecosystem, and the subsequent and extremely misguided efforts to put out every single fire, no matter where or what it was burning. Probably every forest in the nation is overgrown and overfueled. This means that when fire does break out, it burns longer and hotter than it should–sometimes so hot that it destroys the natural regeneration and reseeding mechanisms. Forests today are distinctly unnatural systems, caused by ignorant fire-suppression.

  25. noen


    TEDxTheEvergreenStateCollege – David Roberts – Climate Change is Simple

    “Our present course leads to certain catastrophe”

    By 2100:
    Highest temps in 30 million years.
    Sea level rise of 3-6 feet.
    Drought over 40% of inhabited land
    Hundreds of millions of refugees.
    Half of all known species extinct.

    A 4 degree C rise is incompatible with an organized global community.

    We are currently on course, if we do nothing, for a 6 degree C rise. 6 degrees is an existential threat and climate change becomes irreversible. To ever have a hope of having a stable climate at all global climate emissions must peak in 5-10 years and decline rapidly after that.

    Every year we wait adds 500 billion to the cost of avoiding irreversible climate change.

    Because at 12 degrees C the surface of the Earth is uninhabitable.

  26. noen

    Daniel said: “global warming has nothing to do with the increase in fires.”

    Sure it does. Global Warming results in an increase in droughts. Droughts result in an increase in the number and severity of fires.

    In 30 years we will no longer be able to grow wheat in the continental US.

  27. noen

    VinRN said: “As for the GW aspect, there were massive fires, far bigger than anything we’ve experienced recently, before humans came on the scene. Wildfires are part of nature and I don’t think having one, or even a cluster of them, needs to be attributed to anything other than nature doing what nature does. ”

    Your argument is fallacious. Here is why.

    1. We are experiencing many large wildfires today
    2. Massive wildfires happened in the past
    Therefore we cannot attribute wildfires today to global warming.

    The argument is false because from the fact that wildfires happened in the past for reasons unrelated to human activity it does not follow that the fires we see today are not related to warming. It is a prediction of climate theory that global warming will result in an increase in the frequency and severity of wild fires. There is no reason to assume they are not related.

    It is irrelevant whether or not any wildfires today are caused by global warming. We already know that the present course is catastrophic. We already know that unless we take drastic steps now climate change could become irreversible and pose an existential threat to human civilization.

  28. Mick

    @Dan D #6, regarding your water content queries;

    I am a member of the rural fire brigade in Qld, Australia. Our medium attack trucks come equipped with a 2000L tank and the light attack vehicles each have a 600L tank. That’s good for about a half-hour to 45 mins of water if pumped at full pressure from a 64mm hose.

    If we’re fighting a large fire we simply do not have enough water to extinguish a blaze so our main method of attack is to create fire breaks by either bulldozing or back-burning the vegetation. If the blaze is well contained away from properties and bordered by natural terrain or roads/rivers then it is best to let the fire burn itself out and monitor it rather than risk putting people in danger. If we are doing aerial water bombing operations the main idea is to reduce the fire’s local intensity, it is just impossible to cover every square inch of terrain with water.

    What you should get from this is that it it very rare to see large amounts of water put on a wildfire. It would be a very safe assumption to say that there is more water contained in the vegetation than there is put on a fire by firefighters.

  29. Teacher Al

    The major products of the combustion of organic materials is carbon dioxide and WATER. That’s what comes out of your exhaust pipe and your chimney. Plenty of water coming from the burning of the driest forest materials to form pyrocumulus clouds.

  30. That probably was the fire in Estes Park that you “saw.” I was able to see this from Boulder and Longmont, and it coincided with reports of fire destroying (at that point) 21 homes near the Beaver Meadows entrance to Rocky Mountain National Park.

  31. mike burkhart

    Every year we seem to have these fires maybe we should look at ways to prevent forest fires, bring back Smokey the Bear to do those public service messages.

  32. Will

    The current intensification of fires in the western US has a lot more to do with forestry management practices than it does with climatic changes. Our blanket policy of stopping fires for the last six decades has, ironically, led to larger, less stoppable fires, even as more and more people have placed themselves in danger by building into firelands. The Forest Service has come around on this, but now that people have invested billions of dollars in homes in dangerous places, they are forced to pursue what they now know to be unsound fire suppression policies. Here is a nice little summary available from the Forest Service website

  33. VinceRN

    @noen – neither does it follow that they are related to human activity. The fact that human activity has some effect on the environment does not me that everything is caused by human activity and natural processes are no longer functioning as you seem to think.

    As for the rest of you ranting, you are taking an extreme worst case scenario and presenting it as irrefutable fact. Your doomsaying is no more likely to be true that any other sky is falling world is ending belief ever has been.

    There is certainly a problem, but the world is not ending, nor is there any reason to believe it is.

    Rant away in response, I’ll enjoy reading it.

  34. noen

    “The current intensification of fires in the western US has a lot more to do with forestry management practices than it does with climatic changes.”

    And they are made worse by the droughts, higher temps, more intense storms that are related to climate changes. Increased fires is a prediction of global warming and globally there has been such an increase. The fires also only make a bad situation worse. They put CO2 into the air, expose the ground and cause erosion, remove trees which draw CO2 down from the environment and one would expect dust storms to follow. Global warming isn’t going away and stripping vegetation from the land doesn’t help.

    Go outside and look around you. All of that can go away. It IS going away. People need to see these fires as a look at the future because whether or not they are directly caused by climate change now, they will be in the future.

    The mother of all fold catastrophes is headed our way. We can no longer stop it. That opportunity is gone. Our 6° future is coming and if we want just to survive we’d better get ready.

  35. Just to clarify a little, and forgive me if I missed it previously in the comments, but water is a product of the combustion itself. Most organic matter is carbon, hydrogen and oxygen (with nitrogen, phosphorus, sulphur). When the hydrogen is oxidized, it becomes water vapour. So wood that is dry as a bone will produce water vapour when burned, along with CO2 – and a zillion other things (like ash, soot, CO etc) from incomplete combustion and other constituents.

  36. skydaddy

    Whoever said that water vapor isn’t a gas… um, no. Water vapor IS a gas, just like mercury vapor or sodium vapor or gasoline vapors. Vapor = gas. When the water vapor condenses into cloud droplets, it’s little tiny droplets of liquid water.

    I *really* enjoyed the comment about “let’s wait for hard science to base decisions on and not jump to conclusions like the denialists.” Oh, the irony!

    Yes, fires today are more destructive, because we’re building expensive homes in fuel dumps.

    BTW you know what humans have historically done when the climate changes? Adapt.

  37. David C.

    No science here, just a memory from 1971, when I went west to make some money fighting fires; driving up the Columbia in a truck, from Revelstoke, BC, we were stopped at a ford, and could see the fire scorched land around us, and further up, was this massive cloud, the air had been still, and it rose so high the upper winds got hold of it, just a pillar of smoke, with a mushroom top being swept by the air currents; something you don’t forget, especially as that was where we were headed;

  38. Mark

    to even mention global warming when talking about nominal seasonal wildfires is just silly and hurts the bloggers cause. be that as it may, stay safe…..hope you all get out of that mess without too much harm

  39. Valerie

    After watching the weather channel report on pyrocumulus clouds I was wondering if they are only formed by fires from nature or natural things, such as, a wildfire or a volcano? Could they be formed from something like an oil fire? If that is possible, would this kind of cloud be potentially hazardous and dangerous? I am just an average in I.Q. so I figured there had to be someone much smarter than I who can answer this for me. Thank you for any input.

  40. Daryl

    We had pyrocumulus here in southern California on Sunday, Sept. 2nd, from the “Williams” fire. At first, only a smoke plume was visible, with a smoke layer spreading out that extended to the east. At that time (3pm), media reports were that 30 acres had been burned. By 4pm, a large pyrocumulus cloud extended well above the plume itself, with the base of the cloud brownish from the smoke, but the top glistening white. Media reports stated that the fire grew to 700 acres in that hour! Considering that the fire departments were just getting their assets into place (evacuations were the top priority), it would seem that most of the water vapor came from burning plant material, rather than water poured onto the fire (the fire was said to have broken out around 2:15-2:30 pm, and was in rugged, mountainous terrain above Los Angeles).


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