Fuel Moisture and the Dragon’s Breath


Fuel moisture content equilibrates with atmospheric humidity on a scale of hours to weeks depending whether it is dead or alive and how thick it is. There is widespread superstition that drier fuels from climate change can be blamed for the rash of fires recently. Here we explore the relationship between atmospheric temperature and humidity to show that temperatures have not increased enough to reduce fuel moisture more than 3%.

The United States Forest Service has been studying fires for a long time. A graphic from USFS research paper INT-359 below shows the relationship between atmospheric temperature and humidity in an Idaho forest.

It can be seen that the temperature changes about 25 degrees F and humidity changes about 25%. Santa Rosa, CA has a similar diurnal range. We can generalize that the relationship is symmetrical and inversely related at 1:1 degrees F to %humidity.

Santa Rosa and nearby Windsor have lost houses each of the last three years. How much has Santa Rosa’s climate changed? Below is a graphic from Jim Steele showing average high temperatures for Santa Rosa.

It can be seen that average maximum temperatures in Santa Rosa have declined since the 1930’s like most of California and the American West. Maximum temperatures control fuel moisture low points. Atmospheric humidity and fuel moisture must have increased from the 30’s, although they have been decreasing since 1980.

The average temperature of the entire state of California is a fatuous metric for fires, but even if we could come to believe the average somehow controls the many areas like Santa Rosa where temperature has decreased, the increase of 2.5 degrees F from 1930 would only yield a 2.5% decrease in fuel moisture. The dragon’s breath needs more than that.

Furthermore, NASA has come out with a global fire area assessment.

Seems the dragon just likes California.

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