Researchers: hotter fires may heat underlying soil less than cooler fires

Researchers studying a 22-acre prescribed fire in Portugal have concluded that fires which burn hotter do not necessarily produce higher soil temperatures. Below is an excerpt from an American Geophysical Union press release.

When scientists torched an entire 22-acre watershed in Portugal in a recent experiment, their research yielded a counterintuitive result: Large, hot fires do not necessarily beget hot, scorched soil.

It’s well known that wildfires can leave surface soil burned and barren, which increases the risk of erosion and hinders a landscape’s ability to recover. But the scientists’ fiery test found that the hotter the fire—and the denser the vegetation feeding the flames—the less the underlying soil heated up, an inverse effect which runs contrary to previous studies and conventional wisdom.

Rather, the soil temperature was most affected by the fire’s speed, the direction of heat travel and the landscape’s initial moisture content. These new findings could help forest managers plan when and where to ignite small controlled burns to reduce dry vegetation and restore the ecosystem in at-risk areas, said Cathelijne Stoof, the soil and water scientist who led this study as part of her PhD research at Wageningen University in the Netherlands.

A report about the experiment by Stoof, who is now at Cornell University in Ithaca, New York, and her colleagueshas been accepted for publication by Geophysical Research Letters, a journal of the American Geophysical Union.

To some people this may be “counterintuitive”, but many firefighters know the residence time, or how much time high temperatures exist at a location, is very important in determining how much heat is transferred into the soil or the organic material that is not consumed by the fire.

On a related subject…

In two previous fire management jobs I had the opportunity to assist researchers who were measuring the temperature at which wildland fires burn. Obtaining this data is not the easiest thing in the world.

In southern California I helped place thermocouples, which can measure very high temperatures, on brushy hillsides prior to a prescribed fire. Then, trying not to disturb the vegetation which could influence fire behavior when the site was burned, I crawled under the brush running asbestos-covered wires from the thermocouples to data loggers which were about the size of a football. Each one recorded information from several scattered thermocouples. Then we had to bury the data loggers so they could survive the fire. This was in the 1970s. The researchers in the Portugal study mentioned above used thermocouples and data loggers, but the data loggers were probably about the size of a package of chewing gum — much easier to bury.

On another project the temperature was measured using heat sensitive paint, a much less accurate system. About eight different paints were used which discolored at specific temperatures. A narrow strip of each paint was brushed onto a very thin strip of aluminum which was stapled onto stakes, sticking out to the side at various heights above the ground. After the fire burned through you would examine the strips and you might see, for example, that the 1,200 degree paint discolored but the 1,250 degree paint did not, so you could conclude that the temperature at that location was between 1,200 and 1,250 degrees. Sometimes it was a judgement call about which paints discolored and which ones did not.

At what temperatures do forest fires burn?

We’ve been asked a few times, “what is the temperature of a forest fire”, so we placed an entry on our Frequently Asked Questions page:

An average surface fire on the forest floor might have flames reaching 1 meter in height and can reach temperatures of 800°C (1,472° F) or more. Under extreme conditions a fire can give off 10,000 kilowatts or more per meter of fire front. This would mean flame heights of 50 meters or more and flame temperatures exceeding 1200°C (2,192° F). (Information provided by Natural Resources Canada.)

How hot is the sun?

On our Facebook page someone once wrote that forest fires burn hotter than the sun. He, of course, was badly and sadly mistaken. According to

The temperature in the photosphere [near the surface] is about 10,000 degrees F (5,500 degrees C). It is here that the sun’s radiation is detected as sunlight.

The interior of the sun is much hotter and can reach more than 27 million degrees F (15 million degrees C).

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Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire.

One thought on “Researchers: hotter fires may heat underlying soil less than cooler fires”

  1. This is an interesting post about the different measurements of wildland and grassland fires and how they can be used to characterize fire behavior. Often, fire temperatures are considered to be important when in reality, it is the fire heat flux and flame behavior that is more interesting as a driver of fire spread, as flame temperatures are nearly constant for most fires involving common materials.

    Just as in the investigation of fire in a compartment, the more interesting part is the heat transfer to the boundaries via the intensity and duration of heat fluxes. A temperature measurement via a thermocouple in a fire scenario depends on many factors and is considered to be a secondary measurement of the underlying heat transfer processes that are occurring.


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