My friend was complaining that the inside of her 2014 VW Golf's windows are always getting frosted at night – she has to scrape the inside and outside if she doesn't have time to warm it up to melt it. Why? – D., Halifax.
Here's the condensed answer: the weather inside your car is humid with a chance of frost.
"The best way to put it, is air at any given time can hold a certain amount of water vapour – the warmer the air , the more water vapour it can hold," says Dr. Rachel Chang, Canada Research Chair in Atmospheric Science at Dalhousie University. "So if you start off at a warmer temperature with a certain amount of water vapour and cool the air down, the air can't hold as much water vapour and it has to turn into a liquid or a solid."
When it gets cold, that moist inside air forms frost on the windows. But why doesn't it form on, say, the plastic over your speedometer?
"It cools down the quickest on the windows because they're directly in contact with the outside," Chang says.
The trick is figuring out where the humidity is coming from. One possibility, Chang says, is that it's just humid air from outside when it was warmer.
"If you had your car doors open and come in and out at, say late afternoon, and then overnight it cools down – then the air can no longer hold the water vapour it held before."
Worst case scenario: there could be water leaking into your car. But, it could also just be that your floor mats and carpet are wet from rain or melted snow. Carpeted mats or rubber mats with deep grooves can hold a lot of water.
"I would suspect that on days of heavy rain, followed by a sub-zero night, it is possible that enough moisture could end up either on the floor mats or in the ventilation system to produce this effect," says Thomas Tetzlaff, Volkswagen Canada spokesman, in an e-mail.
How to stop it
The Internet is saturated with advice. If you're parked inside, you could leave a window partially open so the moisture escapes. Other advice includes using baking soda or silica gel (the stuff in those little packets inside vitamins) to soak up moisture.
But Volkswagen's advice is to bring wet mats inside to let them dry .
Then, you should drive with the defroster on for the next few days to dehumidify your car's interior, Tetzlaff says.
"This will cause the condenser to run – not the entire A/C system but only the condenser," Tetzlaff says. "Our cars automatically engage the condenser to dehumidify cabin air whenever the defrost setting is selected."
The defroster's job is to clear windows that fog and frost up from your breath, or from any other humidity in the air.
When people see that A/C light on in the winter, they worry that the air conditioner is on – but it's not. Sometimes people, including this writer, will even manually turn it off. That will keep the defroster from doing its job, Tetzlaff says.
"The function of the A/C system in defrost mode is simply to dehumidify and not to cool the cabin air," he says. "A temperature sensor in the system will prevent any possible damage to the A/C system in the event of cold outside temperatures."
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