Your car boot invariably holds a snow brush & scraper like this:
You don't want to get caught out without one because clearing snow and especially ice off windows without one involves a lot of pain. Ice on glass will chew down any library or credit cards that you might try to chip away enough space to see.
Even if your windows aren't covered in ice, and you can brush back the snow and use the wipers to keep the screen clear of snow, you have to deal with the considerably fogging that a warm, breathing body makes in a cold car with a windscreen that is being chilled considerably below the ambient (usually negative) temperatures outside.
Why do windows fog?
Fogging is simple condensation. Warm, humid air inside with a cold surface you want to look through. Water vapour condenses on cold surfaces, like when you take a cold drink in a glass and watch the water accumulate on the outside of the glass. In places like Canada, fogging is inevitable and easy to understand with temperatures involved. In Australia it doesn't get nearly as cold, but given the much higher humidity, when wind and rain strike the windscreen/windows this can cool the glass enough to cause water vapour in the car to condense on the glass. To prevent fogging you can do one of 3 things: Dry out the air, warm up the surface, or wipe the condensation.
A/C and defogging
Most people here know that A/C can defog windows, and it does a pretty good job at it. However, A/C is a short-term fix for fogging and can make fogging considerably worse if used incorrectly, and can make your trip considerably less comfortable on longer trips in cooler, wet weather. A/C is a good method to initially de-fog windows in a car, however it is important to use the right vent settings to avoid making the problem a lot worse.
Most cars will have controls to change the vent options, fan speed, temperature, plust a toggle for A/C, an option for "fresh" vs recycled air, and the infamously confusing upside down hotplate.
Looking at the left dial there are 5 settings, from left to right these are: Top Vents, Top + Bottom Vents, Bottom Vents, Bottom + Windscreen Vents, and Windscreen Vents.
When using A/C to defog windows, *DO NOT* use the Windscreen Vent options. Yes, initially this will clear your windscreen quickly, but especially in a moving car this will chill your windscreen, causing condensation to form up even faster if the A/C is turned off, and can eventually start to fog up even with the A/C on. Instead, you should use a top and/or bottom vent option with the Recycle Air setting turned on.
The Upside Down Hotplate Button
This button is for the rear window defogger. If you look at most cars rear window you will see a series of horizontal lines in the glass. These are heating element wires that turn on with this button. These warm up the glass and defog the glass or melt exterior ice formed on the glass in the winter.
Proper Defogging Settings
A/C defogs windows because one of the key features of A/C is that it dries out the air. This lowers the humidity considerably inside the car, especially when on Recycle. However, A/C also cools the interior of the car, which can become rather uncomfortable, and if directed at the windows, cooling the windows can compound the fogging problem. The best way to defog windows is instead to warm the windows.
Keeping a car comfortable and defogged when driving in raining can take a bit of adjusting the climate control settings, but isn't that difficult. If the windows are initially fogged up, a quick burst from the A/C on recycle can dry out the interior of the car to clear most of the windows. While the A/C is running you can turn up the heat. Car A/C are not reverse-cycle so you don't need to adjust the temperature down to turn on the A/C. The temp settings only control the separate heating elements, not the A/C so the A/C can be drying the air while the heaters are on. As the heat starts to warm up the car, turn off the A/C and switch the vent settings to blow it at the windscreen and turn up the fan speed, switching back to Fresh air rather than recycled. If the rear window is fogged you can switch on the rear defogger button. It's best to do this when the car is stationary as the windows may initially start to fog, especially if the A/C had been chilling the window surface. Once the windscreen is clear you can turn down the heat/fan speed to a more comfortable setting, keeping it on the windscreen vent. If the side windows are fogged and you want to reduce the overall fogging you can repeat a short A/C pulse but be sure to switch the vent away from the windscreen to avoid chilling the windscreen.
Hopefully this helps anyone annoyed by fogging windows in the rain and feeling like they have to freeze themselves to keep the windows clear, or worried about using heat at the same time as the A/C. :)
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