Where does the moisture in a pyrocumulus cloud come from?

Pyrocumulus cloud fire

Above: CNN Meteorologist Chad Myers explains pyrocumulus clouds. This is a screenshot from Mr. Myers’ 52-second video. The Weather Channel has a similar explanatory video.

Cumulus clouds are puffy clouds, usually having a somewhat flat base but with some vertical development that gives them rounded towers on top. They can form when the sun heats the earth, which then heats the air above it causing the warmer air to rise. Rising air cools and the relative humidity increases. If it reaches 100 percent, water vapor condenses forming a visible cloud.

Above: time-lapse video of pyrocumulus over the King Fire in California.

Clouds can also form over vegetation fires. In some cases a very intense fire can produce enough heat that the air rises very quickly. If it is not dispersed laterally by wind it can rise high enough that a cloud forms. This can look like a cumulus cloud, but when they form over a fire they are called pyrocumulus clouds.

Occasionally these clouds will produce rain or even lightning. Water requires a non-gaseous surface to make the transition from a vapor to a liquid. Smoke helps out by contributing very small particles that are used as condensation nuclei on which water droplets form, to create clouds or rain.

If pyrocumulus clouds grow large they resemble cumulonimbus, thunderstorm clouds. What goes up must come down, and if not disturbed by a strong wind during the dissipating stage the updrafts can reverse and become downdrafts. This is sometimes called a “collapsing column”. When that descending air hits the ground it spreads out, sometimes in all directions, and can quickly and drastically change the wind direction at a given point on a fire. This can be fatal if firefighters find themselves in the wrong location at the wrong time.

I had always assumed that much of the moisture that formed a pyrocumulus came from a byproduct of combustion — water vapor — something that many burning fuels create. (Some TV meteorologists also make the assumption about the sources of the moisture.) A great deal of water vapor is produced when vegetation burns, and the higher the fuel moisture the more water vapor is created.

But I wanted to confirm that assumption before I wrote this article, and it turns out I was wrong. I found two research papers that were devoted to the subject and they were mostly in agreement. As the byproducts of combustion rise above a fire the water vapor is rapidly diluted before it reaches the condensation level, or what becomes the base of the pyrocumulus. One group of researchers in Germany calculated that 10% of the moisture in a pyrocumulus comes from the fire.

Others with the Bushfire & Natural Hazards CRC in Melbourne, Victoria, Australia determined that the contribution of water vapor from the fire is negligible since it is diluted before it reaches the height of the cloud:

Fire plumes entrain large amounts of environmental air as they ascend, which greatly dilutes the plume gases, including the fire moisture. Figure 3 shows the fire moisture dilution for the moist fire simulation(right panels of Fig. 2). The lightening shades of blue with height demonstrate the fire moisture dilution. When the plume reaches the condensation level (4.5 km) there is barely any fire moisture evident to contribute to cloud development. The dilution rate may be sensitive to fire size and intensity.

pyrocumulus moisture
From research by the Bushfire & Natural Hazards CRC, Melbourne, Victoria.

The development of pyrocumulus clouds is very dependant on the amount of moisture in the air, aside from the fire. Again, from the Australian research:

For a given temperature profile, PyroCu development is very sensitive to environmental moisture. Moister environments produce deeper plumes and lower condensation heights.

Sundance pile burn pyrocumulus Dave Porter
Pyrocumulus cloud over a pile burning operation on the Black Hills National Forest, January 6, 2016. Photo by Dave Porter.

This is no doubt why Dave Porter of the Black Hills National Forest was able to photograph a pyrocumulus cloud over a pile burning project on a winter day. There was far less heat than over a summer wildfire, but the relative humidity was probably high.

Above: time-lapse video of the Station Fire in southern California.

Thanks and a tip of the hat go out to Walt.

Author: Bill Gabbert

After working full time in wildland fire for 33 years, Bill Gabbert now writes about it from the Black Hills. Google+

4 thoughts on “Where does the moisture in a pyrocumulus cloud come from?”

  1. WE DID A FIRING JOB, AIR TEMPERATURE 106 DEGREES, Rh 15%. A corn field was being irrigated about 200 yards away. We had a good down pour, large drops for about 15 minutes. The capping cumulus from the firing operation was at least 20,000 feet.
    Scott Franklin Retired LA.County Fire

  2. A very worthwhile article. This will help people who are just starting to learn about fire behavior (such as homeowners who have encountered their first major wildfire), as well as fire personnel who are discussing and researching the greater complexities of wildfire behavior. I certainly have made a few assumptions on this topic that turned out to be wrong so thanks for making this aspect of fire behavior more understandable. You know it’s a good article when stimulating discussions start to occur right away!

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