Dumb question? (optics and dew: physics?)

How can optics dew up? How can anything dew up? doesn't the surface have to be colder than the air? If the air is what is cooling the surface, shouldn't the surface always be warmer?

(obviously things can dew up, but what am I missing on how this works?)

Thanks!
Ron

Not such a dumb question. I asked exactly the same thing about 18 months ago. As Jim stated, the answer lies in the fact that all bodies radiate heat to the sky in the form of infrared radiation. When the sky is really clear you can loose a lot of heat to outer space. So much heat in fact, that surfaces can easily get colder than the ambient air and then condensation occurs.

Some surfaces are better radiators than others, that's why some things dew up faster.

dan