Why Do Windows Steam Up?

When you are in a warm house and it is cold outside, you probably have seen the windows steam up. At one time or another, you also probably took your finger and drew a picture on the window and found that your finger came away wet. And did you ever wonder why? Did you wonder where the water came from?

The air all around you contains water, tiny droplets you can’t even see. These droplets bounce around very quickly in the warm air of a room, but when they come close to a cold window, they move more slowly. Then, instead of bouncing off the cold window, they stick to the glass. As more and more droplets cling to the glass, you can begin to see and feel them.

The same thing happens more quickly in a bathroom when you have taken a hot shower. Then, the windows and the surface of your mirrors steam up.

If the temperature is extremely cold outside on a winter day, these droplets can even freeze on the window, and you have frost on the inside and outside.