Cold Weather Fuel Milage

I seem to be getting lower fuel milage during these cold months than I did back in the spring and summer. Why would that be? Is that just a characteristic of the diesel fuel? I've had my truck a little over a year now and was stock when I purchased it (also in the winter) I was getting 18 in the city. She just turned 160,000 and am extremly happy with it.

I have observed the same drop-off in MPG every winter. Other members here have advised that winter mix (#1 diesel) has significantly less energy than the normal #2.

Plus taking longer to warm up, idling in the driveway, etc, etc.