[UPDATE 12:25 p.m.] Vegetation Fire Erupts in Northern Sonoma County Amid Heat Wave

https://en.wikipedia.org/wiki/Spontaneous_combustion#Confirmed

Hay[2] and compost piles[3] may self-ignite because of heat produced by bacterial fermentation, which then can cause pyrolysis and oxidation that leads to thermal runaway reactions that reach autoignition temperature. Rags soaked with drying oils or varnish can oxidize rapidly due to the large surface area, and even a small pile can produce enough heat to ignite under the right conditions.[4][5]Coal can ignite spontaneously when exposed to oxygen, which causes it to react and heat up when there is insufficient ventilation for cooling.[6]Pyrite oxidation is often the cause of coal’s spontaneous ignition in old mine tailings. Pistachio nuts are highly flammable when stored in large quantities, and are prone to self-heating and spontaneous combustion.[7] Large manure piles can spontaneously combust during conditions of extreme heat. Cotton and linen can ignite when they come into contact with polyunsaturated vegetable oils (linseed, massage oils); bacteria will slowly decompose the materials, producing heat. If these materials are stored in a way so the heat cannot escape, the heat buildup increases the rate of decomposition and thus the rate of heat buildup increases. Once ignition temperature is reached, combustion occurs with oxidizers present (oxygen). Nitrate film, when improperly stored, can deteriorate into an extremely flammable condition and combust. The 1937 Fox vault fire was caused by spontaneously combusting nitrate film.