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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All by itself, sunlight isn’t hot enough. But if it’s magnified by some kind of reflective or refractive human object, like glass from windows or bottles, or shiny metal or something, it can, in very rare circumstances, be hot enough to create a tiny little spot of fairly intense heat, like when you burn ants with a magnifying glass. Researchers still aren’t sure if this has ever happened and sparked a wildfire, but it is theoretically possible.
The majority of wildfires are ignited by human carelessness (discarded cigarettes, sparks from a car dragging something on the road, abandoned campfires, bad electrical lines, etc.) or outright arson. And it’s assumed the remainder are from lightning.
Wildfires aren’t caused just by sunlight, I’m not sure where you heard that and I’m interested in seeing any source that says that. Intense sunlight leads to high temperatures and dry conditions, which are optimal for starting fires. All wildfires are caused by humans or lightning, with other natural meteorological or geographical factors being responsible for an extremely small amount. None of them are being started by just the sun alone
The leaves, branches, small plant matter ignite easier than the wood. That stuff then ignites the trees.
Then you have decomposition that produces heat. This is a big problem for farmers with hay bales as well.
And there’s coal seam, garbage dump, root systems, etc that are smoldering sometimes for over a hundred years, and when it get to the trees, they ignite.
I dont think it happens solely by sunlight alone. There are likely some human waste products (mirrors, prisms, glass, water bottles, etc) that can absolutely magnify the suns rays, however.
If you are ever on a hike, it doesnt hurt to bring a magnifying glass with you as a fire starter. While the sun alone wont light your tinder, with the help of something that can concentrate and magnify the heat, you can very easily get a fire smoldering.
I would guess that the fires you are referencing were more likely caused by a combination of factors.
Pitch pine in eastern Long Island NY exude a drop of sap that focuses the sun to ignite themselves. Fires have ignited on bright sunny days miles from nowhere.
There is no way just sun shining onto a forest can cause a fire. However, hot temperatures can dry the land, vegetation and the soil, reducing humidity and increasing the likelihood of any existing fire to spread. Fires are much less likely to spread when the air is very humid and the trees are full of water.
Source: I live in Australia. We have had recent catastrophic bushfires. We monitor humidity levels every summer to determine the likelihood of fires
It doesn’t need to get to hundreds of degrees. Very hot days and very dry conditions can lead to spontaneous combustion of other materials. “Hay can ignite if the internal core reaches as little as 130 degrees F.” That’s 55 degrees C. Once a little bit of fire starts, it spreads to the wood you mentioned.
Also, human intervention is not needed. Wildfires are normal, and a part of some healthy ecosystems. Some trees and plants have evolved over millions of years in conditions in which fire is common, and developed strategies to deal with it.
Source for hay: https://www.firerescue1.com/fire-products/wildland-fire-products/articles/from-pyrotechnics-gone-wrong-to-animal-electrocution-the-odder-ways-wildfires-ignite-v0XmrGlVY0ZMeNot/
Dry Thunderstorms/ Dry Lightning. Happens quite regularly here in Australia during summer, which often results in wildfires. I think these are also common over in the States as well. (Correct me If I’m wrong Yanks).
It’s also notable that wildfires don’t start by trees burning. Wood actually contains a very high content of water, so it’s very hard to burn it. What does burn easily though are dried out shrubs, leaves, and branches. That’s why in a drought with little rain wildfire risks climb so high. It is these things that will catch fore first then spread until it’s hot enough to ignite large trees.
That’s the combustion temperature for wood, not leaf litter. A pile of compost can spontaneously combustion on even moderate days days. It absorbs the heat and due to its size and desntiy (and relatively little surface area) it can insulate quite well and continues to accumulate heat beyond the point of combustion. Couple this with bacteria breaking down the litter adding more heat and its pretty easy to see how it can happen. It’s part of why some fire management plans include management of leaf litter
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