When you need to de-fog your windscreen, why is it harder on a cold morning?

Understanding WHY it happens will help you to understand why some ‘solutions’ just won’t work. And why using your air conditioning is actually the BEST way to de-fog your windscreen.

When it’s cold and raining, the outside air AND the air inside your car are both heavy with moisture. So when you get in your car and start breathing, your breath is full of water vapour. Combined with your body temperature, it means the air can’t hold any more moisture. The air’s capacity to hold water also varies with temperature and pressure – humid and warm means higher air pressure, so it can hold more moisture. A dry and cold climate means the air pressure holds less water.

Combine these factors with the warm and moist air from your lungs meeting the cold air inside your car, and the water vapour settles on the nearest surface – your cold windscreen. Voila, you need to de-fog your windscreen.

Some things NOT to do

DON’T rub the windscreen with your sleeve or a cloth. It will mist right up again with your next breath

DON’T wind down the window and let in more cold air. A strong breeze will de-fog your windscreen but you’ll freeze in the process

DON’T turn on the recycled air function. You may think you’re keeping the cold air OUT, but the interior water vapour will still need to settle somewhere – yup, on your windscreen.

What you should do

The best solution to de-fog your windscreen is to use the air conditioning set to blast on the windscreen. The air conditioner will start to remove the water vapour. But check the vents are directed to the windscreen, not your body; the last thing you need is a blast of cold air on a cold morning.

Running the air conditioner has an impact on fuel economy so turn off the air-conditioner or reduce the heat when you feel comfortable, and can clearly see through your windscreen.

If you don’t have air conditioning then you probably have a de-mister button. This is a better solution than opening the windows and getting blasted with cold air. However, if you have neither air conditioning or a de-mister, make sure you wrap up well in warm clothes. Open the windows, grin and bear it, clear the windscreen, and drive safely.