Tom's Blog

Wednesday, February 28, 2018

An interesting phenomenon of grassland fire behavior

Yesterday (February 27, 2018) we did the annual burn of the tallgrass prairie on the south slope of Pleasant Valley Conservancy State Natural Area.

Because of the extensive bur oak savanna on the upper ridge, most of the burn was conducted as a backfire. However, there are no trees on most of the lower slope so to speed up the process, a headfire was used.

The principle fuel is little bluestem and Indian grass, with a few patches of big bluestem.

Yesterday the wind was gusty and variable, so that the grass burned either quietly or in quick bursts as the wind intensity varied. The photo below is typical of the headfire.

Headfire behavior. The flame has just been caught by a sudden gust of east wind (0648.jpg)

After the burn was over, Kathie noticed that on the lower slope the fire residue was in narrow linear strips, with white ash alternating with black ash (black char).

According to the fire literature, white ash is a product of complete combustion and black ash is the result of incomplete combustion. I assume that when a strong gust of wind passed over the burning grass, the burn temperature rose and complete combustion occurred, leading to white ash. When the wind was less strong, the temperature was less, and incomplete combustion produced black ash. The interesting thing is that there were several alternating strips of white and black.

The smoke in the photo below shows the linearity of the wind.

Fire behavior showing possible origin of the linear strips. The flames have just been caught by the wind. (0640.jpg)

Unfortunately, I did not get a photo of the long linear strips, but the photo below shows short strips of white and black ash from the same burn.
Bank of grass that has just been burned showing short strips of white and black ash (0620.jpg)
White ash is primarily inorganic substances such as potassium hydroxide, potassium carbonate, and non-alkali substances such as chloride and sulfate salts. The fragile leaf structure may be maintained temporarily but quickly disintegrates. Black ash (char) is primarily amorphous carbon.

Per NOAA, Feb 27 noon was 55 F and 55% RH. Wind 8-10 mph from south. However, at PVC, the wind often shifted to the east.


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