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How to de-fog your windscreen

image A steamed up winscreen is the age-old drivers nightmare.

Finally, a sure fire way to stop your windscreen fogging up....if that's what you really want.

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.


Comments on this story

Displaying 3 of 10 comments

  • A cut potato also works on the outside of a screen and can be used at a drive-in rather than the wipers if it’s raining.

    Doug of Werribee Posted on 19 June 2010 3:33pm
  • Err… it’s not that the water vapour from your lungs has nowhere to go and so the “solves this problem by condensing on the nearest and coldest surface”.

    What happens is that your breath makes the air in the car warmer and wetter. This warmer air will generally hold on to the increased water vapour, BUT when the warm wet air hits the cold windscreen that warm air is suddenly cooled upon contact and so condensation occurs upon the cool surface.

    The distinction is that the vapour is not rushing over to the cold surface as if saying “OMG, I need to get out of this air! Where’s a cold surface I can form on?” any more than the condensation droplets on a cold can of coke on a summer’s day being formed by the water vapour in the entire surrounding atmosphere ‘needing’ to condense and ‘seeking out’ the coke can.

    What actually happens with the coke can is the same as with the windscreen. When the warm moist air happens to come in contact with a significantly colder surface, that air is cooled as it wafts over the cold surface and hence leaves some of its water content on that surface.

    Alex of Sydney Posted on 07 June 2010 11:06am
  • For those unfortunate to own cars without air cond, Rain X works fine but it does not last. And it also leaves a residue which causes bad glare in bright sunlight. Try using those anti misting agent that you apply on your specs before you play sports. Those works great without the side effects.

    Peng Wong of Melbourne Posted on 20 December 2008 2:43am
  • There’s a better way to not get foggy windows..

    On a good day, get the driest bathroom towel you can find. Wipe the screen of any oils from the windows. You might need a lot of elbow grease to do this. Then no matter what happens, don’t touch the window as the oils from your hand makes it worse and will only smear if you wipe it when it’s fogged up.

    By removing all the oils from the inside of the windscreen, it will be very hard to fog up. Whenever it rains, just have the fan going blowing on the windscreen and it’d be as clear as having the A/C on but without using the A/C. Only if it becomes really foggy, then use the A/C as using the A/C constantly creates an oily film on the windscreen. This will become looking like fogged up windscreen even when humidity is relatively low.

    When the windscreen becomes foggy again easily, then it’s a sign to wipe the windscreen again. No chemicals required.

    The other thing with using heat on a cold winter’s night to be aware of is to not have the occupant area too warm because if you happen to drive into an area of cold air, it will freeze your windscreen and any moisture on the outside in less than a second. Quite scary at 110km/h on the freeway.

    ST Posted on 13 May 2008 10:43am
  • Thanks for a great article.  Have been doing the air conditioner thing for years, but difficult to convince others that it will work.  Never tried the aircond on heat.  With winter now almost here, I’ll have to give it a go.
    People amaze me when they will foresake safety just to keep warm by driving around peering through a small ‘hole’ in the mist on their windscreen. 
    Also good to see Peter, formerly of Stanthorpe, used metho to clear the outside of the windscreen of ice.  Luckily, my cars down here are garaged, but when a visitor stays over in winter, a quick spray of metho on the ice in the morning and the ice melts pretty quickly.  This is followed by a quick wipe with the blades and the screen is clear.  Water just thickens the ice, warm water a little too hot can crack the screen and scrapping with your credit card is not a good or satisfactory option.  (Hopefully the people reading this page are bright enough to know that you don’t ‘light-up’ a cigarette as you spray the metho on the screen).

    Mike O'Connor of Canberra Posted on 12 May 2008 10:55pm
  • Hi,
    At last another person that actually knows how to use the aircon/ vents in a car!
    I had recently bought my 1stt car in Oct. 06, but I use to observe my dad for a couple of decades & remembered that the easiest way was to use the aircon & rear defroster (that took a little longer). I couldn’t believe that my aunt & uncle that have been driving for decades use to wind down their windows & getting cold until about a year ago I rode with my aunt in some bad weather & asked me to roll down the window! So I told her to switch the air con on & then close the windows. Well… guess what, the rest is history as they say. Now she does that in both of their cars & also taught my uncle & cousins the same thing too!
    I never really knew people still don’t know about this!
    Good to see this info on a frequently visited web-site for cars! Please teach them more, like slow acceleration from red lights saves fuel. Always stay on the left lane while driving unless overtaking. Go JUST under the speed limit, example: 60Km limit, go with 57-58Km if you are still in a rush, that way you can also avoid the speeding fines! (unless the camera is faulty). Leave much earlier, so you won’t be in a rush if there’s traffic! If driving alone, turn your mobile on silent & ignore it!

    Boris of Chelsea Posted on 07 May 2008 10:46am
  • It also helps if you have the inside of the windscreen clean in the first place (get rid of the film that builds up). I put hot water in a bowl, then add a good measure of metho (the water stops the metho from drying off too fast). Apply with a cleaning cloth, the wipe off with an old tea towel or similar.

    Jim of Melbourne Posted on 06 May 2008 4:12pm
  • Thomas, I agree that aircon and heat work wonders to clear condensation during wet, cold weather.  I didn’t know that it also worked when chickens were falling from the sky.  Thanks for the tip!

    Wordsmith Posted on 26 April 2008 4:40pm
  • As a former resident of Stanthorpe we carried a bottle of metho to de-ice the outside of the windscreen.  There were no such things as aircon then or even a heater! Reading my user manual on my Saab, it too said aircon and warm air, and it works very quickly.
    Rain-X on the inside of a windscreen can be dangerous as it tends to blurr, and oncomming car lights smear light everywhere, blinding the driver.

    Peter of Brisbane Posted on 25 April 2008 5:10pm
  • When I used to drive taxis in fowl weather, the air con and heat was always the way to clear the mist quickly. Your article was a great explanation. Good stuff!

    Thomas Torok of Pagewood Posted on 24 April 2008 10:53am
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