Perhaps not as big a hit as you've noticed overall, but I have actually noticed more temperature dependence than what you've mentioned, but perhaps it is a result of using mpg or a result of what we each experience as cold weather. I believe I have a fairly consistent driving style, which is more conservative than most people.
I monitor this closely with my car (2010/Prius); in conditions below 0 C, it drops to about 4.8 L/100km, in conditions below -15 C, the L/100km drops to around 7.0, when it gets around -30 C it drops to about 8.5 (note this is with winter tires, which seem to effect roughly 0.5 L/100km). With my driving style I usually get around 4.2 L/100km in the city during summer (more than 5 C) on my regular all season tires.
I usually make short 10-30 minute trips without idling my car significantly beforehand. In winter, I need defrost on the entire time or I can't see through the window.
On the highway and for much longer trips (around 3 hours), I get around 4.2-4.4 L/100km. In conditions around -20 C I get around 4.8-5.2 L/100km.
That being said, in cold weather, with the heater (which is almost always on), snow/ice, and winter tires; the mileage drops to what an average (not tuned to efficiency) car gets in the best conditions, which is arguably a pretty good result.