Sunday, October 12, 2008

Fuel Moisture...

A couple of people wrote today to ask what I was talking about when I said “fuel moisture” in an earlier post.

First, there's a graph of the past week's fuel moisture on Lyons Peak at right. Click to enlarge it. On the sidebar at right, in the local information, is a link that will get you the most current graph.

The “fuel moisture” measurement being made here is the percentage of water (by weight) in dry, dead wood. The chaparral brush has a high percentage of such materials – I've pruned as much as 100 pounds of it out of a single manzanita tree. The water content is one of the primary determinants of fire danger. Wet or moist wood is harder to ignite, and burns cooler and slower. Extremely dry wood (less than 5% or 6% water) is more like gasoline in combustability – especially for the small diameter twigs that dominate the chaparral.

In the October 2007 fires, the fuel moistures were less than 3%. Combine that with hot, dry Santa Ana winds and you've got a recipe for disaster – as we all saw.

Incidentally (for you geeks out there), the sensor for fuel moisture is kind of interesting. You might expect it to be some kind of exotic semiconductor, or rare earth ceramic, or some such thing. Nope. It's a stick of wood, of standardized species, with no knots or flaws, attached to a sensitive electronic scale. As the wood absorbs moisture, it gets heavier, and vice versa. A somewhat fancier version measures the speed of sound through the wood (it gets faster as the moisture content goes up). In both cases, the stick must be replaced every year or two – apparently it wears out somehow!


Now all you folks from chillier climes can start laughing right now. But by our Southern California standards, it's starting to get cold!

You can see from the chart at right that just four days ago we had very warm daytime high temperatures – the actual high was 97°F (36°C). Last night we got down to 41°F (5°C) – that's a pretty big swing in just a few days!

I've walked the dogs a couple times this morning, and I'm still shivering from the experience. We're actually having a Santa Ana wind right now, though it's a very mild and cold one. The winds haven't gotten over 25 MPH (40 KPH), so the fire danger hasn't been extreme. It is very dry, though – just 3% RH – but our fuel moistures are still above the extreme danger level.

But it's just the start of fire season...