Absent human activity, what does cause forest fires? I mean aside from lightning strikes, which are usually accompanied by water in the form of rain, and very rare events like volcanic eruptions or meteor impacts?

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    $\begingroup$ I suspect a lot of forest fires are caused by lightning strikes but I might be wrong. $\endgroup$ – bon Jan 7 '18 at 16:57
  • $\begingroup$ We had a wildfire when an extreme windstorm blew down power lines, but I guess that's human - don't blame Mother Nature. $\endgroup$ – Keith McClary Jan 9 '18 at 21:46
  • $\begingroup$ Birds can also apparently start fires, on purpose: twitter.com/newscientist/status/950334881145507840 $\endgroup$ – Gimelist Jan 10 '18 at 9:34
  • $\begingroup$ Falling rocks making sparks seems possible, but I have not heard of any actual instances. Like Gimelist says, birds have been observed spreading fires - picking up sticks burning from an existing fire and dropping them to start fires further away. $\endgroup$ – Ken Fabian Sep 28 '20 at 10:30

Wildfires are mainly caused by humans (or their technology) and lightning. Lightning is the most prolific natural source. There really aren't many other sources in most areas, unless there is volcanic activity or a meteor strike as you mention. There are also occasional cases of wildfire when peat dries out.
During a hot-dry summer, it can take several days of rain to moisten vegetation enough so that the fire danger is curtailed. If there is a little rain accompanied by lightning, it is probably much less than what is needed to lower the fire potential (dryness of vegetation).

Furthermore, lightning strikes are not "usually accompanied by rain". In the Pacific Northwest, for example, there are several lightning-caused wildfires every summer in areas where summer rain is quite scarce. This is discussed a little in Wikipedia:

The term dry lightning is used in Australia, Canada and the United States for lightning that occurs with no precipitation at the surface. This type of lightning is the most common natural cause of wildfires. Pyrocumulus clouds produce lightning for the same reason that it is produced by cumulonimbus clouds.

  • $\begingroup$ Spontaneous combustion is a well-know phenomena (en.wikipedia.org/wiki/Spontaneous_combustion). And also less known (at least to me) is the self-combustions observed by some plants, like the one of the genus puya (botanyphoto.botanicalgarden.ubc.ca/2010/01/puya_sp). I don't know to what extent these phenomena can contribute to wildfires, but that's the question: What can cause a wildfire BESIDE human activity, lightning, volcanic activity and meteor impacs. And I doubt the answer is NOTHING. $\endgroup$ – Camilo Rada Jan 7 '18 at 18:51
  • $\begingroup$ Yes that is one way I overlooked, although here once again, human activity is a dominant factor - it involves humans piling up stuff. $\endgroup$ – dtech Jan 7 '18 at 18:57
  • $\begingroup$ @dtech Not necesarily, the piling can be natural due to a landslides, storms, a large tree falling, beavers lodges, underground seed depots by squirrels, termites colonies, and the list go on and on. A perfect recipe could be marsh folded and piled by storms like this one: researchgate.net/publication/… $\endgroup$ – Camilo Rada Jan 7 '18 at 19:10
  • $\begingroup$ I meant most cases, not all cases. Folded marshes in particular, while likely candidates for spontaneous combustion, are not that likely as sources of forest fires, being wetlands and all. $\endgroup$ – dtech Jan 7 '18 at 19:13
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    $\begingroup$ In the western US, at least, rain from a summer thunderstorm tends to be pretty localized. So even when there is rain, it may not fall in the same place that the lightning strikes. $\endgroup$ – jamesqf Jan 7 '18 at 19:28

The same thing that typically cause them,.... Lightning. a very small percentage started by spontaneous combustion of dry fuel such as sawdust and leaves.


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