Pretty Cool!

There is a little bit of parallax in this view, but even if we grant that the temperature is 45 degrees, this is a lab thermometer and it reads in Celcius. That makes it 113F. Pretty warm considering that the air temperature is right around the freezing point. Also, by the time I took the second shot, the temperature had eased upwards to just below 50C.

All of the excess warmth is from decay of the leaves, but there are a couple of effects which explain why the leaves in my bin are hot while ones on the ground are not. First, insulation; the shredded leaves hold the heat in. Also, there wouldn't be a lot of decay happening at close to zero Celsius, but at elevated temperature the reactions go faster and thus produce more heat. It is a kind of synergistic effect.

I make these leaf storage bins by just cutting a bit of wire fencing and then making a cylinder out of it.

Added: The smell of the decomposing leaves is really nice: Somewhere between the Fall leaf smell and good tobacco and fresh pine. The source material is Oak, maple and pine leaves and branches.