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Understanding Fire Behavior

Pretty Cool and Pretty Hot

Is humidity important?

Is humidity unimportant?

Is fuel temperature important?

Is the difference in shaded and sunlit fuel temperatures more important than average fuel temperature?

Look at it this way: The tendency of fuel to dry out is a function of the difference in vapor pressure between the fuel and the air right around the fuel (= driving force, like a voltage). As the fuel heats up the air around the fuel also warms: the water molecules in the fuel bounce around more (= vapor pressure increase) while the warming air in the fuel microclimate can hold more water because it is hotter. (The amount of water that air can hold [called the saturation vapor pressure] is a curve the has an ever-increasing slope as a function of temperature. ) These changes in driving force are poorly indicated by the relative humidity in the ambient air.

Relative humidity = actual vapor pressure in air divided by the saturation vapor pressure at air temperature.

Increasing the temperature of fuel from 77° F to 93° F (? 16° F) at least doubles the tendency (100% greater) of fuel to dry out even without considering that the hotter air in the microclimate of sunlit fuels could hold much more water (increased difference between actual and saturation vapor pressure).

That difference is called the vapor pressure deficit.

In contrast, the difference between warming fuel from 77° F to 93° F only gets the fuel 2% closer to the flash point of cellulose 450° C.

That relationship continues to get steeper. Even warming the fuel 80° C only brings fuels 18% closer to flash point, whereas that warming would cause fuels to dry out many tens, if not hundreds of time faster.

So although ambient relative humidity has a tiny effect on fuel moisture, fuel temperature acting through moisture loss is a better candidate to explain increased fuel flammability than does an increase in air temperature or relative humidity alone.

This difference in shaded and sunlit fuel temperatures as compared to average fuel temperature or air temperature is pretty hard to swallow at first glance.

But now apply what you heard above as you answer the question, "Do 1000-hour fuel logs dry from the inside to out or does the outside dry out and the drying move progressively inward???

Does a wet piece of fire-wood on your campfire have to dry to the center before it burns?

Here is the answer...The sunlit side of a "1000-hr log" acts much closer to a "1 hour" fuel than it does to "1000 hour" fuel.

It is the surface that counts in a fast moving fire.

The shady side of a "10-hour stick" may effectively be a 100-hour fuel.

The fuel moisture of the surface of sunlit fuels, driven to dry out as explained above, is a primary factor in the driving force of a fire's progression, and explains observations presented by Doug Campbell in the Campbell Prediction System.

J. Timothy Ball, Ph.D.
President
Fireball Information Technologies, LLC
1240 Fairfield Ave
Reno, NV 89509
tim@fireballit.com
Reno, NV 89509
775-848-4462