Where does the moisture in a pyrocumulus cloud come from?

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.

Continue reading “Where does the moisture in a pyrocumulus cloud come from?”

Pyrocumulus over a pile burn

Sundance pile burn pyrocumulus Dave Porter

Dave Porter of the Black Hills National Forest captured this image of a pyrocumulus cloud January 6 over a pile burning project southeast of Sundance, Wyoming.

A pyrocumulus over a pile burn is rare.

From Wikipedia:

A pyrocumulus cloud is produced by the intense heating of the air from the surface. The intense heat induces convection, which causes the air mass to rise to a point of stability, usually in the presence of moisture. Phenomena such as volcanic eruptions, forest fires, and occasionally industrial activities can induce formation of this cloud.

An article posted earlier today about the bushfire in Western Australia had a photo showing a pyrocumulus cloud over the fire.

Wildfire briefing, August 23, 2013

Firefighter dies in Portugal

A female firefighter was killed and nine were injured Thursday on a wildfire in Portugal near the small city of Tondela. Commander Antonio Ribeiro of the Serra de Caramulo firefighters said the crew ran from the fire but the firefighter who died fled in the wrong direction. Euronews reports that three firefighters have died this month. High temperatures and strong winds have contributed to the spread of 13 large fires in Portugal.

The national wildfire situation

Today there are 49 uncontained large fires listed on the national Situation Report in the United States, and that number does not include individual fires within complexes. There are currently 854,480 acres within the perimeters of those active fires. The national Preparedness Level has reached the highest category, PL 5, for the first time since 2008. And while it may seem like much of the west is on fire, the number of acres burned to date, 3.4 million, is much less than average, which is 5.6 million.

Competition for firefighting resources is occurring. There is only one California-based Type 1 or Type 2 incident management team available that is not assigned to a fire; 33 IMTeams are assigned nationwide. But surprisingly, there are no Area Command Teams committed.

We have 11 large and very large air tankers working right now on exclusive use contracts, and there are another 9 that the USFS has borrowed from the military, the state of Alaska, and the Canadian government. In 2002 there were 44 large air tankers on contract.

Forest Service runs out of money for firefighting

For the sixth time in the last ten years the U.S. Forest Service has run out of funds for suppressing wildfires. Even though the number of acres burned to date this year is below average, the USFS is having to divert funds from other non-fire accounts in order to cover the shortfall. This is due in part to reductions in the amount of money Congress allocates for the FLAME fund, which is supposed to fund firefighting while protecting other accounts. The Washington Post has more details.

Scott Olsen writes about a firefighter’s first day on the job

You may have seen the articles written last year by W. Scott Olsen, a professor of English at Concordia College in Moorhead, Minnesota about “the war on wildfires out west, meeting shot-callers and looking at the operation from the inside”. He has just published a new article at the Huffington Post about a wildland firefighter’s first day on the job.

Granite Mountain 19

The issues surrounding the deaths of the 19 members of the Granite Mountain Hotshots June 30 on the Yarnell Hill Fire continue to make the news. Firefighters with the New York City Fire Department have raised $30,000 so far for the families of the 19, and they are hoping to add to that total. The Prescott Daily Courier asked the candidates for Mayor and the City Council to express their positions on the discrepancy between the benefits for the seasonal and full time members of the crew. And there is a debate about whether the city’s hotshot crew should be rebuilt.

Investigative reporter John Dougherty has two recent articles about the Yarnell Hill Fire: “Yarnell Hill Fire: The Granite Mountain Hotshots Never Should’ve Been Deployed, Mounting Evidence Shows” and “A Granite Mountain Hotshot’s Father Says the Blaze That Incinerated His Son Could’ve Been Controlled“.

Montana residents contribute for free coffee for firefighters

Residents near Lolo, Montana are contributing to a fund to provide free, good quality coffee for firefighters working on the Lolo Creek Complex. According to an article at KZBK, Samantha Harris, a barista at Florence Coffee Company in Lolo, said customers have been donating money to give firefighters coffee.

“We have a huge tab here so all the firefighters’ coffee is paid for,” Harris said. “Which has been really fun to tell them their coffee is free.” The tab is at nearly $300, she said.

Florence Coffee Company is at 11880 HWY 93 in South Lolo, Montana.

Photos of pyrocumulus

The Alaska Dispatch has some very impressive photos of pyrocumulus smoke columns caused by wildfires.

Goat manure fire stinks up town

A burning pile of goat manure is affecting the quality of life for residents of Windsor, Vermont. The pile ignited from spontaneous combustion Wednesday at George Redick’s 800-goat dairy. Windsor Town Manager Tom Marsh said he could smell the fire at his home which is five miles from the dairy.

Studying pyrocumulus

Pyrocumulus on the Jasper Fire
Pyrocumulus on the Jasper Fire, August 25, 2000. Photo by Bill Gabbert

You may be familiar with pyrocumulus (pyroCb) clouds that form over intensely burning vegetation fires. They can be a combination of smoke and condensation. Some firefighters call this “ice capping”.

Chuck Bushey, a Fire Behavior Analyst and former President of the International Association of Wildland Fire, is a member of a small group studying pyroCb led by Mike Fromm of the US Naval Observatory.

Chuck sent me a link to an animation of pyroCb forming over the Silver Fire recently in southern New Mexico, and explained:


“…this is one recent example of the sort of products our small global pryoCb group watch. The group also examines lots of particulate and atmospheric information (some from ground based instrumentation such as Lidar, as well as from other satellite channels) to make sure it’s a real fire event or something else. There are other events that may look similar from orbit and some of the more remote incidents that this group sees we may be the first observers.

We can sometimes track these major upper atmospheric (stratospheric) events multiple times around the globe mixing with other weather systems. The most global round-trips I have observed has been four in the northern hemisphere.

We can only speculate on what the input of the volatile elemental and organic chemicals and other pyrolyized materials (such as soil and ash) are having on the cold, upper atmosphere and our climate. We also guess that these events may be more frequent now and maybe more intense than in the recent past but we really don’t know because no one was watching and our capabilities were limited.”