Photo: PinkMoose (flickr)
Say a man has an open air car port, covered by a roof, but with walls on only two sides. He notices that the cement floor is sometimes damp, even if it hadn’t rained for a long time. This would sometimes happen when his driveway and lawn were perfectly dry. Where did this moisture come from?
Actually, it came from the air. Even on a dry day, air always has a certain amount of moisture in it. In this way, air is like a sponge that you can never completely squeeze dry.
It’s a fact of nature that warm air can hold more evaporated moisture than cold air can. If you took some warm, moist air and you cooled it down, some of that evaporated moisture would have to come out of the air as a liquid. This happens frequently on summer nights, when the hot, moist air of the day cools off. You see the results of this when you wake up and your lawn is covered with dew.
The temperature evaporated moisture begins to turn into liquid is called the dew point. Dew point explains why a cold object, say a can of cold soda at the beach, will bead with moisture. The can’s temperature is below the air’s dew point, so the can gets covered with dew.
A similar thing happens inside a car port. Because it’s covered by a roof, the floor isn’t warmed by the sun the way the driveway and lawn are. If the car port floor dips below the dew point, it will bead with moisture, even if the warmer driveway stays dry.