How to de-fog your windscreen

CarsGuide ·

22 April 2008

How to de-fog your windscreen
A steamed up winscreen is the age-old drivers nightmare.

A few cool, wet mornings into the season and I've hopped into my friend's passenger seat, he's started the car, pulled away, and immediately the front windscreen begins to mist up.

Like countless other drivers I've travelled with, he then proceeds to try almost everything to stop it fogging up. Everything, that is, except the one or two things that actually work. This is particularly common amongst drivers who live in generally warm, temperate climates, such as Sydney, Melbourne and Brisbane.

Driven to help my fellow drivingkind, I'll now explain what causes the windscreen to fog up on cold and/or wet mornings. Armed with the science, I'll then show you what you can do to make it go away and stop it coming back.

On a cool, rainy morning, the air inside your car is similar to the air outside your car: heavy with moisture. When you enter the car and close the door, before you even start the engine you start inhaling and exhaling. That first breath you exhale is full to the brim with water vapour - at body temperature and this air pressure, it literally can't hold any more water.

Which is a problem, because air's capacity to hold water varies with temperature and pressure. That's why it's humid in the tropics (warm, higher pressure air can hold more water) and dry in the mountains (cold, lower pressure air holds less water.)

When the warm, moist air from your lungs meets the cold, moist air already in the car, the air from your lungs cools down, which leaves the water vapour from your lungs with nowhere to go. The water vapour solves this problem by condensing on the nearest and coldest surface, which in a cold car on a cool morning, is the cold windscreen in front of you.

The worst demisting solutions

Now you understand why the windscreen fogs up, it's much easier to see why some demisting strategies work better than others. For instance, if you rub the screen with your sleeve to remove some of the condensed water vapour, more vapour will condense from your next breath and mist it right back up again. 

Wind down your windows and let more cold, moist air into the car, and it will still fog up since you're probably opening it just a little. If you open the windows enough so that the breeze is strong enough to stop your warm, moist breath reaching the windscreen, that'll work, but you'll wish you brought your scarf, and you'll still have those first few breaths stuck in place on the windscreen.

Likewise, if you just open an air vent or two, you're not doing enough to shift the warm, moist breath and doing nothing to remove the water already on the screen. I've seen some people turn on the recycled air function on the dash, thinking they're doing the right thing by keeping the cold, moist air out. While this slightly increases the air pressure inside the car, slightly increasing the water-carrying capacity of the air in the car, any water vapour that 'needs' to condense will still seek out the cold windscreen.

The best demisting solution: got air-conditioning? Use it!

If you have air-conditioning in your car the best and fastest solution is to turn it on and set it to the windscreen outlets. Air-conditioners cool your car on a hot day by removing water vapour from the air inside the car, which reduces the temperature of the air. So operating your air-conditioner will immediately start removing water vapour from your car on a cold morning too, so that your warm, moist breath will disperse into the air inside the car rather than condensing on the windscreen. 

Turning the air-conditioner up high for a little while and directing it through the windscreen vents will also evaporate the water condensed on your windscreen.

Many drivers, will press the 'demist' button on their dashboard and then suddenly stab at the air-conditioning controls to turn off the rush of cold air they weren't expecting. Do that and you're actually working against yourself.

If you don't like the cold, dry air blowing on you from the air vents, turn off the vents facing you, point them at the side windows (which will start demisting the side windows) or try this: turn up the heat on the temperature control.

Because we're so used to thinking of air-conditioning as a way to cool things down, many drivers aren't aware that their air-conditioner is also capable of producing hot, dry air. It most certainly can, and this warm, dry air is just about the nicest way to face a cold, wet drive to work, as it'll also start drying out the rain your clothes absorbed on the run to the car.

If you're concerned about fuel economy and you're already warmly dressed, don't forget to reduce the heat and turn off the air-conditioning when you and the windscreen are dry. Air-conditioners increase your fuel consumption, and producing warm, dry air uses a little more fuel than producing cold, dry air on a cold morning.

If you stop to pick up passengers, be prepared to turn on the air-conditioning again for a little while, since the amount of warm, moist air will be doubled by your first passenger and by another third when your second passenger hops in. 

Don't have air-conditioning? Get breezy!

The demister button in a car without air-conditioning will try to warm up the windscreen, by blowing warm air at it and/or by running an electric current through a thin wire stuck onto the screen (more common in rear windscreens) to warm up the wire, and eventually, the window. While less effective than air-conditioning, these solutions will work eventually because they force the water vapour to find another place to condense once the heated portion of the windscreen becomes warmer than your breath. 

If you need to get moving and can't wait for that to happen, pull that scarf tight around your neck, grit your teeth, and open enough windows to ensure your warm, moist breath is unable to reach the windscreen. Only then use your sleeve, a bit of newspaper, or anything absorbent to wipe the condensation off the windscreen, since only now will your next breath not fog it up again.

Extra bonus points: what do nucleation, Preparation X, shaving cream and potatoes have in common?

"Nucleation" is the word for the way liquids, gases and crystals (such as water, water vapour and ice) form in some places and not in others. The beads of bubbles in your champagne form threads like that because tiny imperfections in the glass surface encourage bubbles to form (liquid into gas.)

So if you suffer from a foggy windscreen regularly, putting a coating over the tiny imperfections in your windscreen, keeping the screen clean, and even avoiding cheap replacement windscreen glass can make some difference.

Commercial preparations such as Rain-X, available from auto accessory stores, may reduce condensation in many cases by applying a smooth film to the surface, making it harder for the water vapour to find nucleation points, though I haven't tried it myself and 'your mileage may vary.' There are also two old folk remedies I have tried: shaving foam and a cut potato (believe it or not) and neither seem to work.

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Written by

Alan Jones

Published 22 April 2008