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 how to demist a car, how to remove fog from a windscreen, 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
How to defog car windows. 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 car demister 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 demister vents will also evaporate the water condensed on your windscreen
Many drivers, will press the 'front demister' 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! Want to know how to stop your windscreen fogging up when you don't have air-con? Hit the car windscreen demister button without air-conditioning and the ventilation system 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 windshield glass can make some difference
Commercial anti-fog spray 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
Do you struggle with de-fogging your windscreen? Had any nightmares with the window demister in your car? Tell us your experience in the comments below
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