Why do windows fog quickly?

Question

I'm from Bogotá, Colombia. In my city the temperature is all the year between 12ºC and 25ºC (bus mostly under 19ºC) and when I drive and it's raining, there is accumulation of fog on the windows, but slowly.

However, I was driving at Miami in summer. I had the windows up and the air conditioner on. In a matter of, the windshield was absolutely foggy. I tried to clean the window with my hand but of course the vapour was outside. My question is, why could that happen so quickly? Does that generally happen? I appreciate your answers.

Answer 1

It depends on the humidity or water content in air. Humidity is a measure of how much water content is in the air. There's a limiting partial pressure of water vapour that the air can "hold" for some temperature, and the air can "hold" more water vapour with higher temperature. When it rains it is likely that the air is saturated with water vapour (100% relative humidity) at some temperature. The window is likely at a lower temperature (due to your air conditioner) than the temperature in which the air is saturated (outside) and will cool the air past the dew point and force the water to condense (what you refer to as "steaming up") on the window since the air at that temperature physically cannot "hold" that much water. The reason water condenses on glass is because there are nucleation sites (from imperfections) on the glass that makes it easier for condensation.

The reason I use quotes around the word "hold" is because air doesn't actually hold the water. The saturation/maximum equilibrium partial pressure of water vapour only depends on temperature and the presence of air doesn't matter.

We can use the psychrometric chart to determine whether water will condense and how much water vapour air can hold. Here is a link if you are interested: http://en.wikipedia.org/wiki/Psychrometrics