Today@Dal

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.

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