As I helped my wife and daughter out to our car, I noticed that all the cars had icy windshilds except the car I drive, a Suzuki Samurai.
Reasons I thought of why it wasn’t:[list=1]
[li]My Samurai is open-topped.[/li][li]Its windshield is completely flat.[/li][li]Its windshield is coated with something to prevent frost.[/li][li]It was parked under a small tree.[/li][/list=1]
I can’t think of why exactly any of these would prevent frost (except #3). Any thoughts on which is right and why, or if they’re all wrong, what is the reason?
First, let me say that i don’t know the answer to your question.
Now, let me mention that parking under at tree can actually be a reason why it doesn’t get frost on it. The tree radiates heat more slowly and can keep a small area near by a “touch” warmer. (Note, you can’t bake a cake in the residual heat.) If the air is just below the frost point and you are under the tree, it is feasible that the tree can keep the ambient temperature a scoche above the fp.
The tree doesn’t keep the air temperature warmer, but it prevents the ground or other surfaces beneath it (such as your car) from radiating heat to the distant stars.
The temperature of the clear sky is not the 3K of empty space (the atmosphere acts like a blanket). Actually I can’t remember what it is, but I think it’s between 200K and 250K, which is pretty cold.
Anyway, if your car is losing heat by radiation, it will lose a lot more heat if it is exposed to the sky than to a nearby tree that’s at, say, 275K. It’s not because the tree is storing heat from the daytime; you would get the same effect from a blanket suspended between the car and the sky. Or from a cloudy sky.
Similarly, you will feel a lot colder in a room with the curtains open to a starry sky than with the curtains closed, even if it’s the same air temperature inside.
I’m really not explaining this well at all. I’m sorry.