I don't know about weather, but there is a well-documented difference in mileage depending on whether you're buring winter gas or summer gas (the refineries switch formulations in winter, and a gallon of winter gas does not have as much energy as in summer gas, so mileage goes down).

There is a switch in winter vs. summer formulations of gasoline (where you live depends on how much of a difference you'll experience and what the "switch" is). You also have physics coming into play, with cold air being denser than warm air and causing more drag on the vehicle, something with the rolling resistance of tires against a cold road vs against a warm road, running the climate control full bore instead of occasionally, bringing the gas engine up to temp more quickly so that it runs more efficiently, warmer weather generally having clearer roads (free of snow/slush/salt) which lessens the resistance of the tires as well. Yes, the ambient temp does affect your hybrid battery, but EVERY car on the road, hybrid or not, gets worse fuel economy in the winter than in warmer weather -- most people don't keep accurate enough records to recognize it, but hybrid drivers have a readout staring them in the face and driving the point home.

I didn't believe a lot of this when I first got my first hybrid, but when engineers and oil company employees are posting about it and others are doing careful testing of various conditions, I see the pattern emerge depending on where in the U.S. (or elsewhere) you live. The Yahoo! groups are great for this kind of information, even if you only read the archives to see what's been tested, and so is greenhybrid.com and cleanmpg.com. In the case of cleanmpg.com, the Insight hypermilers in Oklahoma last year noted that with the same air temp, it was the temp of the *road itself* that made a difference in the MPGs achieved on different legs of their marathon.

Your hybrid battery has an intake vent into the passenger compartment so that it's kept at comfortable passenger temperature while you're driving the car, whether that's winter or summer. (In a 2002 Prius, it's that funny looking thing that looks like a speaker in the center of the rear deck.)

peace,
Linda (2001 Prius, 2003 Civic Hybrid, 2006 Insight)
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HYBRIDFEST 2007 -- www.hybridfest.com
Last year, we had attendees from 21 states and Canada. This year, it's even better!