Don't have the foggiest about foggy windscreens?

Posted by: Cal on 28/06/2013

Category: General

Have you ever found yourself driving down the road with your head down at the dashboard level, trying to peek through a tiny sliver of clear windscreen that you've just made with your sleeve?

It's one of the age-old problems associated with an early morning car ride in winter - a foggy windscreen.

Unless you've got an amazing demister capable of sorting out some clear vision for yourself in a few seconds flat, you may find that some mornings you have to wait a few minutes before heading off - even if you're in a hurry.

So what's going on with windscreen fog and what can you do to get rid of it quickly?

Let's start at the beginning!

What causes all that fog?

No surprises when it comes to what windscreen fog is - just like normal fog, it's made up of tiny water vapour droplets.

So what are they doing colonising your windscreen on cool mornings (and evenings)?

The basic version is that the air trapped in your car overnight will be warmer and more humid than the dry, cool air outside on a cool morning.

When the warm air comes into contact with your windscreen - itself cooled by the air outside - it cools, and because of the way temperatures, pressures and water interact, loses some of its moisture.

This moisture precipitates as fog on the glass of your windscreen - the annoying fog of lateness!

You're making it worse!

Guess what happens when you get in the car? You make things even worse!

That's right - unless you're a vampire, your body temperature will warm the air inside your car even more when you hop inside.

And don't even get us started about what an effect all that breathing you'll be doing is going to have on your poor old windscreen.

A plan of action

So what can you do when you need to get going quickly but are faced with a situation that you haven't the foggiest how to resolve?

One strategy is to bundle up in warm clothes and to wind down all the windows, lowering the temperature in your car and the humidity level.

This can be effective, but may be a little bit too 'back to nature' for those of us that enjoy the warmth of the car over the coolness of a winter's morning.

Instead, why not switch on the air conditioning? It's not as crazy a suggestion as it may at first seem.

You may not realise it, but one of the things that air conditioning does to cool your car on a hot day is remove the moisture in the air.

Aha - it will be perfect, but what about the coolness, you say? Well that's easily enough solved: simply turn the temperature dial to warm instead of cold.

This should set about drying the air in your car, and if you aim some vents at the windscreen it will get to work making that annoying fog nothing but a distant memory.


We can't guarantee results in every situation, simply because there are so many variables at work!

You'll need to experiment with your car and your temperature dial in order to get the best - and quickest - method to defog your windscreen down to a fine art.

Another handy tip is to clean your windscreen on the inside and the outside regularly. When dirty, windscreens can fog up more often, because the water vapour molecules latch onto the dust and grime.

You'll be safer out on the roads with a clean windscreen too - make a point of giving it a good clean next time you're at the service station, right before you check the air in your car tyres!

Remember, if you can’t see clearly, then it’s not safe (or legal) to drive!