What makes this happen is that the mulch rots which is an exothermic process and it insulates very good as well. So the core temperature of pile will slowly cre

What makes this happen is that the mulch rots which is an exothermic process and it insulates very good as well. So the core temperature of pile will slowly creep up until you reach the ignition point. Absolutely true. But while I wouldn't even be able to begin quantifying the degree to which the radiant heat from the sun is a factor, I strongly suspect that it is a factor, if for no other reason that the observation that we only went to those fires in summer months. But, to be fair, there could be a million other confounding factors. Whether or not a given system ignites on a given day depends on many things: ambient temperature, wind, humidity, presence of accelerants, yadda, yadda, yadda. I suspect this is one reason why it's so hard to get people to take fire safety seriously: that pile of oily rags in the corner? It's not going to catch fire. This time. Or next time. Or next time. And so on... until it does. So people get complacent because "hey, the oily rags never caught fire before, why would I expect them to today?" Disconnect the sprinkler system? Sure, why not - the building isn't going to catch fire. This time...