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Defog your windows with Science

One of the worst things about winter is getting into a car and not being able to see out of the windows. Some cars have a single button to defrost or defog your car windscreen, but a lot of older or more basic models just have basic functions. Since you’re probably in a hurry to drive off, we ask what are the best settings to defog your window fast?

Most people will just turn their fan speed to max, their heat to max and direct it to the windscreen, but what about cold air? Should you use recirculated air? What even is recirculated air? What about the A/C? What about opening windows? Does it speed it up and by how much?

So let’s test Air Temp, Flow, Circulation, A/C and Windows and see if there’s a combination we can use to defog our windows quicker.

Before we start, lets understand why windows fog up in the first place. Windscreen fog, if you zoom in, is just tiny droplets of water, also known as condensation. It comes out when we breathe, it develops overnight in the car as temperatures and humidity levels inside and outside of the car change. It’s the exact same effect you see when you pick a cold can out of a fridge, or what happens to your mirror after a shower, or even how clouds are formed.

All of the air around us contains tiny bits of water, or water vapour. The more water vapour in the air, the higher the humidity. Once humidity reaches over 100%, that’s when vapour starts clinging to itself, and water droplets and condensation forms.

It’s interesting that people are quick to head to heat to remove condensation, probably because the heat could evaporate the water droplets off the window. However, warm air has more potential to hold on to water vapour than cold air.

In fact, at sea level, a meter square of air that’s 40 degrees C which is 100% humid will hold 53ml of water. But if the water was only 2 degrees instead, it’d only contain 5ml of water, or around 10% that of warm air.

So if we think of the air like a towel, how much water we can soak up depends on two things - The size of the towel and how wet it already is.

The size represents the temperature of the air, as warm air holds more, so would be a bigger towel. However, the cold air would be a drier towel, the warm air would already be damp because it contains more water at the same level of humidity.

So what we want is a big, dry towel. Here’s the break down:

Step 1: Temp up to max - Like we said, warm air holds more water. Blasting cold air across the window has less chance that the water droplets will be picked up, because the cold air just doesn’t want them. Blast warm instead, fans and heat on max. This is increasing the size of our towel.

Step 2: A/C On - This pulls moisture out of the air as the air passes over the cold coils, so it’s like we’re ringing out our towel.

Step 3: Circulation off - Winter air is cold and we know that cold air contains less moisture, so if we bring cold air into the car and heat it up we’re increasing its absorption capacity

Which then helps suck up more condensation.

Step 4 - Crack the windows - this helps initially by quickly swapping the humid air inside the car with the cold, low humidity air outside which will speed things along even quicker!

In short, if you skipped this whole post just to get the instructions:

- Temp/speed up

- A/C On

- Circulation off

- Open the windows a bit

Here's some extra tips:

If you're still having issues with your windows fogging up, make sure when you turn off your car that your vents to the windscreen aren't open - wet air from rain or snow outside gets pushed through the vents by the wind. You can also stick some cat litter into a sock, tie a knot in the end and throw that up on top of your dash, this'll help absorb some moisture.

Another little trick is to get some shaving foam, sticks it on a rag and wax your window with it. It's a lot cheaper than the "anti fog" waxes you can get for windows and works just as well!

We hope this helps you get into work or drop the kids off on time. If you’re having any issues with your heater then its possibly the worst time of the year for it to happen. Bring your car down and let’s take a look at it.

Stay safe this winter, if you have any concerns then book a visit in, or drop by:

Brendon Garage

Windsor Street

Haverton Hill


TS23 4EY

01642 564120

Special thanks to Matti from MWM Digital for helping us with the blog!

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