A windscreen mists up when the air inside a vehicle contains moisture that condenses on glass. The condensation occurs when the inside of the glass is at a different temperature to the outside of the glass. The interior of the car heats up when people are inside, breathing warm air out of their lungs, and when the heating is on. However, heating the car will reduce mist on the windscreen if you do it properly.
Click on your rear window demister first. The rear window can be demisting while you are clearing the front windscreen.
Turn on your air conditioning, if your car has air conditioning, to the windscreen demister setting. The air conditioning will alter the humidity and temperature of the air inside the car and begin to clear the windscreen.
Aim the air vents on either side of the car at the side windows. This will direct the blast of air onto the glass and clear the side windows.
Turn the air conditioning, or the heating, if your car lacks air-conditioning, up as high and warm as possible to clear the glass quickly. Do not use the cool setting or the recirculated air setting, as your aim is to remove the moisture from the air inside the car. Use the setting that takes in new air from outside the car instead.
Open the windows if the moisture is taking too long to clear from the glass, but keep the heating on. This has the effect of removing the humid, colder air from inside the car and replacing it with warmer, dryer air. Leave the windows open for as short a time as possible or you'll waste fuel in heating.
When you are in a rush, dry the windscreen thoroughly with a dry cloth or some newspaper but avoid breathing on it. Then proceed to heat the windscreen with the air conditioning or heater with the windows open, to prevent the moisture from your breath misting up the windscreen again. Clean the windscreen in cold weather with a preparation to reduce fogging.
Do not drive with the windscreen, or any other windows, fogged up.