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

CC BY-SA 4.0
2020-09-25T06:20:53.287Z
2020-09-25T06:20:53.287Z
LazyReader
<p>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 <a href="http://www.bbc.com/news/science-environment-40877099" rel="noreferrer">peat dries out</a>.<br>
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).  </p>

<p>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 <a href="https://en.wikipedia.org/wiki/Lightning" rel="noreferrer">Wikipedia</a>:</p>

<blockquote>
  <p>The term <a href="https://en.wikipedia.org/wiki/Dry_thunderstorm" rel="noreferrer">dry lightning</a> 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.</p>
</blockquote>

CC BY-SA 3.0
2018-01-07T18:51:32.483Z
Spontaneous combustion is a well-know phenomena (https://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 (http://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.
Camilo Rada
CC BY-SA 3.0
2018-01-07T18:57:26.413Z
Yes that is one way I overlooked, although here once again, human activity is a dominant factor - it involves humans piling up stuff.
dtech
CC BY-SA 3.0
2018-01-07T19:10:46.523Z
@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: https://www.researchgate.net/publication/226942189_A_review_of_major_storm_impacts_on_coastal_wetland_elevations/figures?lo=1
Camilo Rada
CC BY-SA 3.0
2018-01-07T19:13:47.677Z
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 **wet**lands and all.
dtech
CC BY-SA 3.0
2018-01-07T19:28:10.953Z
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.
jamesqf
CC BY-SA 3.0
2018-01-07T21:17:11.223Z
I guess it does vary quite a lot. I am from eastern Europe, and I cannot recall more than a dozen lightning strikers that were not accompanied by rain in my 35 years of life, and not more than a couple or so that were not followed by heavy rain shortly after. And there is plenty of lightning here, a couple of routers get routinely fried every year.
dtech
CC BY-SA 3.0
2018-01-08T22:10:00.453Z
@dtech I recently moved to Australia and I was amazed by the amount of storms with plenty of lightning and very little rain. I now understand why it’s such a flammable country.
Gimelist
CC BY-SA 3.0
2018-01-09T00:21:49.977Z
Every year in Florida you'll see articles about the dangers of lightning. Most people who are hit by lightning were hit long before the rain falls, if it falls at all where they were hit. People assume that they are safe until the storm actually reaches them, not true.
Dunk
CC BY-SA 3.0
2018-01-07T17:17:22.113Z
2018-01-07T22:25:49.373Z
2018-01-07T22:25:49.373Z
f.thorpe
<p>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?</p>

CC BY-SA 3.0
2018-01-07T16:57:29.410Z
I suspect a lot of forest fires *are* caused by lightning strikes but I might be wrong.
bon
CC BY-SA 3.0
2018-01-09T21:46:04.933Z
We had a wildfire when an extreme windstorm blew down power lines, but I guess that's human - don't blame Mother Nature.
Keith McClary
CC BY-SA 3.0
2018-01-10T09:34:55.420Z
Birds can also apparently start fires, on purpose: https://twitter.com/newscientist/status/950334881145507840
Gimelist
CC BY-SA 4.0
2020-09-28T10:30:10.783Z
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.
Ken Fabian
<human-influence><forest><fire>
With humans out of the equation, what could cause forest fire?
dtech
# With humans out of the equation, what could cause forest fire?

(Asked by: username_0 on 2018-01-07)

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?

**Tags:** human-influence, forest, fire

> **Comments:**
> **username_1**: I suspect a lot of forest fires *are* caused by lightning strikes but I might be wrong.  
> **username_2**: We had a wildfire when an extreme windstorm blew down power lines, but I guess that's human - don't blame Mother Nature.  
> **username_3**: Birds can also apparently start fires, on purpose: https://twitter.com/newscientist/status/950334881145507840  
> **username_4**: Falling rocks making sparks seems possible, but I have not heard of any actual instances. Like username_3 says, birds have been observed spreading fires - picking up sticks burning from an existing fire and dropping them to start fires further away.  

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## 2 answers

### Answer by username_6 on 2018-01-07. Score: 6 | Accepted answer

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](http://www.bbc.com/news/science-environment-40877099).  
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](https://en.wikipedia.org/wiki/Lightning):

> The term [dry lightning](https://en.wikipedia.org/wiki/Dry_thunderstorm) 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.

> **Comments:**
> **username_7**: Spontaneous combustion is a well-know phenomena (https://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 (http://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.  
> **username_0**: Yes that is one way I overlooked, although here once again, human activity is a dominant factor - it involves humans piling up stuff.  
> **username_7**: @username_0 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: https://www.researchgate.net/publication/226942189_A_review_of_major_storm_impacts_on_coastal_wetland_elevations/figures?lo=1  
> **username_0**: 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 **wet**lands and all.  
> **username_8**: 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.  
> **username_0**: I guess it does vary quite a lot. I am from eastern Europe, and I cannot recall more than a dozen lightning strikers that were not accompanied by rain in my 35 years of life, and not more than a couple or so that were not followed by heavy rain shortly after. And there is plenty of lightning here, a couple of routers get routinely fried every year.  
> **username_3**: @username_0 I recently moved to Australia and I was amazed by the amount of storms with plenty of lightning and very little rain. I now understand why it’s such a flammable country.  
> **username_9**: Every year in Florida you'll see articles about the dangers of lightning. Most people who are hit by lightning were hit long before the rain falls, if it falls at all where they were hit. People assume that they are safe until the storm actually reaches them, not true.  

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### Answer by username_5 on 2020-09-25. Score: 2

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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