Is cold weather driving impacting range because the occupants use heat to warm the interior of the car and use heat warmers? Or is cold weather just hard on the batteries? I am curious on how the heater and/or seat warmers have impact on the car's batteries.
Seat heaters use a trivial amount of energy, so that is the way to go if it works for you. (I find that a heated steering wheel is even more important for comfort; why all Teslas don't come with it standard, instead of as a $1000 option, is a mystery to me.)
The cabin heater is a huge power hog. Unlike an ICE car, where most of the energy from the fuel is lost to heat, leaving plenty to warm the car, all the cabin heat in an EV comes from the battery, at a significant loss of range. The way around this is to preheat the car while it is still plugged-in before leaving. Then you get into a warm car and don't need to heat it as much, or at all for short trips. [A serious design defect of the current Model S is that there is no way to use the climate control system and turn the heater off. So, no circulating unheated air around the car in cool or cold conditions.]
As to why cold weather has such an impact on range, it is three fold:
1) Cold air is more dense and causes increased aerodynamic drag, so it takes more energy to push the air molecules out of the way, and cold tires have more rolling resistance, thus reducing range.
[By the way, driving at high altitude has less drag than at sea level, at a given temperature, so cars in my state — where there is no "low" altitude — get better gas and electric mileage than those in the more dense air at sea level.]
2) A cold battery has less energy capacity and a Tesla uses energy to heat the battery to help it work more efficiently (driving also heats the battery).
3) Cabin heater use is a drain on the battery and reduces range.
There is also one more small factor: a cold battery is limited in regenerative braking use, so you might recover a bit less energy from regen braking.
In addition to simple cold effects, if you throw in rain or snow the range can be reduced significantly. Much as with aerodynamic drag, having tires push water on the road out of the way takes energy; it is even worse in the case of slush or snow.
FWIW.