If you look closely you can see the lightning strike which started the fire. Sometimes it’s like finding a needle in a haystack. Sometimes it’s obvious. We usually labeled fires with no identifiable cause as lightning fires simply because it is so hard to find the epicenter. Most human caused fires have a clear start point and usually some kind of evidence of an ignition source or an accelerant. If you can't find either you're probably looking at a lightning strike.
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From @driptorchpress