Re: How is frost formed?

Air always has a certain amount of water vapor in it, even in the driest 
places in the world. When air is cooled, it cannot hold as much water vapor 
as it can when it is warm. You can see what happens when air cools off just 
about any day, by looking into the sky - clouds are places where the air has 
cooled so much that the water vapor in the air has been released as fog. 

When it gets cold at ground level, the air around the ground cools off too. 
So do things near the ground, such as cars, trees, and other objects. If 
they become cool enough, the air around them may cool even more, and if it 
gets cooled so much that it cannot hold as much water vapor, it leaves the 
water on the surface of the cool object it touches. In warmer times of the 
year, this is how dew is formed. But when the temperature of the object 
drops below freezing (0 degrees Celsius or 32 degrees Fahrenheit), the water 
freezes onto the cold surface instead, forming frost.

One interesting thing to note about frost is that it doesn't form as quickly 
on some things as others. Metal and glass things cool off a lot faster than 
wood or stone, so that's why some days you might find frost on the cars, but 
not on the outside of your house or on the trees. Of course, if it gets 
really cold, and air with a lot of moisture comes by, then you get frost on 
everything. Since you live in Tacoma, you may have seen the very heavy 
frosts that form when it gets cold and foggy at the same time. This "hoar 
frost" turns everything white, and is one of the most beautiful things 
winter has to offer.

You can find more information by going here:
 http://www.iitap.iastate.edu/jhodson/idot/frost/forms.html