Re: Exactly how does dew form?

I'm not sure how much you already know, or what you didn't understand, so 
I'll take it from the start.

Air contains moisture - the warmer the air, the more moisture it can hold. 
When air cools, it cannot hold as much moisture, and therefore gets rid of 
it, as cloud, fog, rain, or dew.

During the day, as the earth heats the atmosphere, the atmosphere absorbs 
moisture. At night, the earth cools, as do many things. Some things release 
daytime heat and reach air temperature quickly (soil, plants, glass, metal, 
and so on), but others hold heat well (cement, brick, etc) and so stay 
warmer than their surroundings for a longer period. As the ground cools, it 
cools the air near it. The air becomes saturated (meaning it cannot hold its 
current load of vapor) and condenses that moisture onto cooler objects, such 
as leaves, grass, metal, and glass. Objects that tend to hold heat, such as 
brick and concrete, rarely collect dew, because by the time they cool of 
enough to be good dew collectors, most of the moisture has already been 
condensed onto the things which cooled faster. 

As to your question why carports protect from dew - same story. Most 
carports have a concrete pad, which keeps the air above it slightly warmer 
and thus more capable of retaining moisture. The roof of the carport os 
usually made of plastic, metal, or fiberglass, which loses heat quickly, and 
thus becomes an excellent dew collector. So the car, sitting above a good 
heat source and below a good dew collector, is not likely to get wet.

As far as knowing when dew will form goes, you need to know the dew point. 
This is a measure of the temperature of the air versus the degree of 
moisture it contains. Cool, dry air forms dew at a much lower temperature 
than cool, moist air, simply because of what I've already mentioned - the 
warmer the air, the more moisture it will hold. Even relatively cool air 
holds moisture, just not as much. The dew point is a measure that tells you, 
given the relative humidity, to what temperature the air must fall before it 
must give up water vapor.

Hope this helps.

[note added by MadSci Admin:  There are lots of previous answers on
our site dealing with condensation, dew, humidity, etc.  Use our
search engine to find 
them!!]