Happened to me this morning. Usually it happens once my vehicle warms up just enough to start melting snow inside the car and increasing the humidity. Normally the inside of the glass isn't cold enough to freeze the condensation/fog, but with this weather the inside of the glass is colder than usual. If this happens after you park, it's probably because you were running the defroster which also acts to dry the air. Once you turn that off, the humidity increases, condensates and then crystallizes.
Coles notes: Keep the inside of the car snow/moisture free and run defroster (air conditioner) as much as possible to decrease humidity. Don't track more snow inside the car.
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