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J. Timothy Ball, Ph.D.
President
Fireball Information Technologies, LLC

I think I know why Doug Campbell is "more right than he ought to be."

Two things have bothered me about CPS.

(1) Why it appears to Doug that humidity is unimportant and fuel temperature is vitally important.

(2) Why the difference in shaded and sunlit fuel temperatures is more important than (sunlit or average) fuel temperature.

Even warming the fuel 80 degrees 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 bulk air humidity has a tiny effect on fuel moisture, fuel temperature acting through water loss is a better candidate to explain the increased flammability than does an increase in air temperature or air humidity.

(2) Now this difference in shaded and sunlit fuel temperatures as opposed to say just average fuel temperature or air temperature near the ground or something like that ... is pretty hard to swallow at first glance.

But now apply what you heard above as you answer the question, "Does 1000-hour fuel logs dry from the inside to out or does the outside dry out and the drying move progressively inward??? And now, does a wet piece of fire wood on your campfire have to dry to the center before it burns? So here is the answer...The sunlit side of a "1000-hr log" acts a whole lot closer to a "1-hour" fuel than it does to "1000 hour" fuel. And its got to be the surface that counts in a fast moving fire. On the other hand the shady side of a "10-hour stick" may effectively be a 100 hour fuel. SO what I'm saying is that is the fuel moisture of just the surface of sunlit, driven to dry out as in (1) above, that I think explains Doug's observations.