Two nights ago was the first hard freeze in College Park. I woke up at about 8am, went outside and there was frost on every shadowed patch of ground. The sun had already started melting the grass it touched. The planters around the yard were frozen four inches in, and my currently running compost batch was mostly frozen to the bottom of the bin.
When I opened it up, it seemed much more wet than it should be. Then I thought about frozen fruit. Freezing breaks the cell walls of most living organisms (most organisms that can survive freezing have some chemical in their cells besides water which keeps ice crystals from forming). So during freezing of the compost, the cells in the grass, kitchen scraps etc, break apart, releasing the water contained in those cells. I ended up adding brown matter to the mix to absorb the water, and keep the pile form going anaerobic (the stinky black slime that occurs when there isn't enough turning and therefore oxygen).
In the best compost piles, the pile stays hot enough that it doesn't freeze during the winter. Because I have a tumbler composter, the pile can freeze from the bottom and top. Also because it is well-mixed, the compost proceeds faster than a pile, so even if I added enough stuff to make it hot, it would quickly shrink in size to the point where it could no longer sustain the reactions to keep it hot. So my compost kind of goes dormant in the winter.
So I learned something, Warm weather is good for compost - it speeds up reactions. Cool weather is bad, it's like putting food in the fridge, it just stays cool, and decomposes very slowly. Cold, or freezing weather can be good because it breaks apart cells to make it easier to decompose later.