…Well, it was this morning. Those unfortunate people like us who have two cars and a lot of stuff and only a double garage, meaning one car has to sit uncovered on the drive, will have noticed that the ice on the car windscreen is generally thicker than the ice on the side windows. Why is that? Surely the temperature of the air is the same on all surfaces of the car?
The reason is that it isn’t just the temperature of the air that controls the temperature of a surface. Convection isn’t the only form of heat transfer. In this case we have radiation too. When you sit in front of a fire, you feel nice and warm, but if someone put a screen between you and the fire you would immediately feel the difference. It’s because heat is transferring from the fire to you radiatively (as well as through heating the air which moves around the room – convection).
In the case of a clear winter’s morning, the opposite is happening. The sky has very little means of radiation. [Amendment 2 July 2010 – The NIGHT sky has little means of radiation – the day sky has lots of scattered sunlight…] There’s a bit of air in it, which will radiate some energy, mostly the longer infra-red wavelengths, but this air is both cold and thin, meaning there isn’t a lot of heat coming from it. Meanwhile, your windscreen will happily radiate energy in the infra-red being glass (it’s partly emissive and partly reflective to infra-red), and the net effect is that it will cool down to a lower temperature than the the ambient air around it.
However, your side windows are facing more horizontally, and are receiving energy from the ground, hedges, nearby buildings, which are all going to be at a rather higher temperature than the sky. Consequently, they are going to be a bit warmer. Parking in a covered carport helps, even though the air might be below zero, because now the windscreen faces the inside roof of the carport, which is going to be radiating a whole lot more infra-red than the clear night sky will.
Incidently, when I lived in Bedford, in the UK, we had a winter where the temperature failed to go above zero for three weeks (the river froze – you could tell because there was a shopping trolley in the middle of it with a set of footprints across to it – rather stupid if you ask me – but someone did it and survived). I used to measure the temperatures in the mornings in ‘minutes’ – being the time it took to get my car into a state where I could see out of the windscreen. I think the lowest temperature was 12 minutes of frost. (N.B. A blanket over the windscreen overnight didn’t help much – as soon as you took it off in the morning the frost would start forming…)