Frost on car windows

[Mac attack]

What determines whether there is frost on my car window or not? Not just temperature as it was below freezing the other day but no frost.

[logman]

I noticed that too, i put it down to the amount of moisture in the air and maybe wind direction has a say in the matter also. I think the worst weather combo is freezing especially for road users.

[wacky racer]

Dew point and a very cold windscreen.
A car will cool down much more on a cloudless night as it radiates heat to space.

[Gef]

It also depends on where the car is parked. If I park just outside my flat (within a couple of feet of the building) I rarely get frost but the cars parked at the end of the street, a wide kunction, do.

[logman]

Huors later i decided to see what other answers had been posted, and realised i missed the word fog next to the word freezing. Hope that makes some sence now.