Wood ignites at temperatures in the hundreds of degrees. How are some wildfires supposedly started by sunlight without human action or lightning?
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It’s generally dry tinder like stuff that goes up first, old moss balls, dried up old twigs, dead grass, that sort of thing. Then if there’s enough fire it will spread to the less flammable things like living trees
Dewdrops.
Could be anything from morning condensation, to some animal taking a leak on a leaf at exactly the right angle that the drops act as a natural magnifying glass.
It may seem ridiculous in small scales, but over a course of months in an area thousands of square miles with trees, shrubs, plants, and animals in uncountable numbers, one in a billion chances are being rolled all the time every second.
There’s probably a ton of other factors that help, but the one i’ve heard of most is just ‘water in the wrong place, at the right time magnifying just a tiny bit of sun till it heats up a single point hot enough to start a fire.’
I know that parts of California are considered a chaparral. Those are known to be flammable. Plus its filled with plants known to purposely catch fire. There are plants that need to catch fire to spread their species. One of them is the Chamaerops humilis. It’s not really the trees catching fire, it’s these dry or oil covered plants that want to catch fire, or have adapted to thrive in environments that catch fire often.
The sun only fires don’t start on the trees.
But they do/can start on the underbrush. The conditions that have shown to reproduce it, are a very dry pine needle layer of 6+ inches. Where the bottom layers are breaking down.
Then a rainy day or so in the hot summer. That dryes quickly, but the water activates the decomposition in the underlayers, which is an exothermic reaction. With the right winds and the right heat the next day (90+)and a low humidity to remove all the rain from air. The pine needles can spontaneously combust, and they burn fast and hot. And they catch down branches that are dry, and then the trees eventually burn.
It’s very rare, but has been seen and reproduced.
Lightning is much more common
In short it’s not the wood starting the fire. There are however countless other sources that can ignite way sooner and so in fact ignite. Some are mentioned here. Decomposition can raise temperatures to the point of starting fire. Fungi and other things can ignite at way lower temperatures. And once they catch fire the reaction releases enough heat to start igniting actual wood.
They aren’t started just by sunlight. You just have a wild misconception of what the actual ignition source of these wildfires are.
Former Canadian forest fire fighter here.
The sun does not cause trees to ignite unless there are very unusual circumstances. (I once saw a video from Dubai of a resinous palm tree start to smoke in the heat, I wasn’t able to verify the truth of it though)
You have to remember, that on the ground in forests is dead wood, leaves, tiny branches, dried mosses and lichen, etc. Those catch fire a lot easier.
When we keep track of fire hazard, we look at three numbers. The drought code (how much rain we have had in the past and the current temperature), the FFMC (Fine fuel moisture code. small branches, leaf litter, dried grasses, etc) and the DMC (Duff moisture code. The organic layers under the surface litter, larger branches)
Up here, we only see trees as the ignition point through human interaction (We once had someone soak a tree in gasoline to get rid of hornets, then later that day someone else built a fire at the base to ‘smoke them out’) or lightning strikes.
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