Smoothaccel,
While it's possible for moisture to condense on the road surface and for ice to form that way, this is not very common, and would only happen with a fairly unusual combination of atmospheric conditions. Moisture usually condenses out of the air when temperatures are dropping, because colder air cannot hold as much moisture as warmer air. This is why dew forms overnight, as temperatures cool, and goes away in the morning as temperatures warm.
In the winter, the pavement temperature usually stays slightly warmer than the air above it. When the air temperature is 30 degrees F, as in nsx4fun's example, the roads generally don't freeze at all; they only accumulate moisture if snow falls on them. At colder temperatures (below 25 F) ice can form on the roads, but it commonly forms from precipitation (rain or snow) that re-freezes, not from condensation. When there has been no precipitation, the roads are usually dry. However, if there is enough humidity in the air for moisture to condense - in other words, close to 100 percent humidity - then it condenses in the air as snow (precipitation), and this is what makes the roads wet.
And moisture certainly does not automatically condense from the air when the temperature goes below freezing, unless the humidity is close to 100 percent. Which, unless it's snowing, it isn't.
That's correct - and that's why ice rarely condenses on pavement - because the pavement is rarely significantly colder than the air above it. On those rare occasions that conditions are right for this - relatively humid air and air temperatures that drop fast - snow is likely to be falling on the ground so you don't even notice condensation on the pavement.
[This message has been edited by nsxtasy (edited 04 March 2003).]
