What people have neglected to mention for cold weather are the other gas guns that we use on a semi-regular basis.
MrJon and Penguin can add in their information here with this as well.
When playing in colder weather (below -10c), you should look into buying a brown-gas and beige-gas gun. These gas (pistols) are the same as any other green gas gun, they just happen to have modified internals meant for the brown and beige gas. However most airsoft retailers here don't sell these gasses, so you'll need to go to Canadian Tire and Wholesale Sports to pick up the bags of beige gas.
Beige gas, unlike green gas, comes in a soft polymer container that has a fill nozzle on one end. The bags look like over-inflated sandwich bags and that are in their respective colors, beige and brown. While the gas is combustible, it takes a lot of heat to ignite, so don't worry about these bags of gas blowing up when you keep them in your trunk.
Similarly, there is also pink gas which is available at some retailers. This is a relatively new propellant and is often used in JG AEG rifles which is injected directly into the magazines. This gas not only coats each bb, but also adds a slightly pink 'vapor trail' behind each bb that you have coated. Typically these will be used throughout Ontario as the gas of choice. Pink Gas also is the standard gas used at the Urotsuki game held each year in Nunavut which brings an estimated 2,200 players
One final gas you could use with your pistol in winter will be Mauve Gas. This has very similar properties to those of helium, which allows the bbs to go even further. This is, while not the easiest to find, relatively common and can be purchased from the user "Kusai Neko" here on the forums. It works with all AEG's in lieu of battery power and is contained in re-fillable charger 'sticks' which are the same shape as batteries. You might consider picking up some of these if you are worried about playing in winter.
Best of luck!