Tom's Blog

Tuesday, March 20, 2018

Prescribed burning in cold weather

This season we have been doing prescribed burns in fairly cold weather. The dew points have been quite low, so that there was no dew on the grass even when the temperature dropped to the mid 20s (F).

Leading up to each burn day we had several days of clear, sunny weather with temperatures reaching the upper 40s. But for the two successful prairie burns we did on Friday March 16, 2018 the temperature never got above 32 F! We burned the East Basin (5 acres, with complicated topography) and the Valley Prairie (4 acres adjacent to the PVC wetland).

I had no success finding any information in the fire literature about burns in cold weather. Perhaps most people think you can’t burn when it is cold.

One of the most important parameters for a prescribed burn is dead fuel moisture content. This time of year prairie grass is fully cured, which means that its moisture content is completely controlled by external conditions, most importantly rainfall (or lack of it) and relative humidity (R.H.). Although temperature does have an effect on the rate at which moisture of the grass is taken up or lost, the most important influence of temperature on dead fuel moisture is the effect on R.H. When we burned Friday, the R.H. was below 50%.

Although the fire literature has tables for estimating dead fuel moisture, these are only approximations and don’t take local conditions into consideration. But there is a simple way of estimating dead fuel moisture of oak leaves, which are a fairly good surrogate for grass.

This is McCarthy’s test for assessing the moisture content of hardwood leaves by bending.

Moisture content
Behavior during bending
Leaves crack if creased but do not break entirely
Leaves crack if folded more than a right angle
Leaves crack if bent at a right angle but do not break freely, especially at the veins
Leaves break entirely apart if bent at right angles
below 10%
Leaf crumbles when you pick it up

Source: McCarthy, E.F. 1927. Weather and forest inflammability in the southern Appalachians.
Monthly Weather Review, March 1927.

 Here is a guide to using the McCarthy test: When using the bend test, take several samples in both sunlight and shade and if on a slope, at the top, middle, and near the bottom of the unit. If the leaves have been wet from precipitation, wait at least one day of full sunshine with medium winds and RH below 50%.

Just before we started the East Basin burn, I used the oak leaf bend test. It gave a reading of 14%, which is a good dead fuel moisture content for a prairie burn

The photos show the fire behavior in the East Basin Prairie, a restored prairie. The fuel here, mainly Indian grass and little bluestem, was in its 8th growing season and obviously well established.

According to NOAA weather radio, the early afternoon temperature was 32 or 33 F over all of southern Wisconsin.

Successful prairie burn under cold-weather conditions

Keeping a straight burn line on  the east side of the unit

Doing some interior lighting on a later stage of the burn
This prairie has a complicated shape, with level areas surrounding
a steep south-facing basin

Two fire lines meeting on the final stage of the burn

Almost complete burn coverage with just a few unburned patches.
The white ash indicates complete combustion.

The south fire line (facing north) still had piles of snow.
Note also the snow on the distant hill.

Final check of the burn.
Most of the small smokers are well inside the unit and will be left to burn up

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