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Dew on the Window

This experiment got its start while I was reading Craig F. Bohren's "What Light Through Yonder Window Breaks." It is a book on atmospheric physics, and is written so that you don't have to be a physics professor (or even a physics student) to understand and enjoy it. He writes about the dew that forms on your house windows in the winter, which made me think of other questions about dew drops.

To try this, you will need:

  • your house
  • your car
  • an early morning after a cold night

Mr. Bohren points out something that is so common that most people never stop to think about it. Look at your windows on a cold morning, and notice the
dew. Is it on the inside of the glass, or the outside? On the inside, right? Why?

First, we have to understand why dew forms. On any surface that is wet, water molecules are constantly moving from the water into the air. Other water
molecules are constantly moving from the air into the water. With water molecules always coming and going, the important thing is which is happening faster.

If more molecules move from the water to the air, then the water dries up. Evaporation has won.

On the other hand, if more water moves from the air to the water, then the amount of liquid water grows, as condensation takes the lead. If both actions are balanced then, the amount of liquid water stays the same.

OK, so what controls which happens the most? It is a balance between the temperature and the amount of water in the air. Higher temperatures cause more evaporation. Evaporation is also easier in dry air. At colder temperatures and with humid air, condensation will be dominant.

Knowing that, think about the windows on your house. On the outside, the glass is warmer than the surrounding air. The air near the glass is being warmed, and we just learned that warmer air is good for evaporation.

On the inside of the window, things are different. The air in your house is warmer than the window pane, so the air that is near the glass is being cooled. As the air gets cooler, more water moves from the air to the glass, and condensation forms dew drops on the glass.

You will also notice that most of the dew drops are at the bottom of the window. As the air near the glass cools, it becomes denser, and it moves downwards, cooling more along the way, so the air near the bottom of the window is colder than the air at the top. At the same humidity, colder air gives you more condensation, so most of the dew drops form at the bottom.

Now lets look at a different situation. While you have dew drops on the inside of your house windows, look at the dew drops on your car windows. They are on the outside! Why?

Think about it a minute, and you will probably figure it out. Your house is heated, so the temperature inside is warmer. Unless you have been out late driving, your car will be cold. As the sun comes up, the air outside the car will warm up quicker than the window glass and the air inside, so you wind up with the air outside being warmer than the glass, and the air on the inside being colder. Again, the warmer side winds up with the dew drops, so the dew forms out the outside. If your car windows keep fogging up on the outside, turn on the heater to warm the glass. This will clear them quickly. If the windows keep fogging on the inside, turn on the air conditioner, which make the inside air cooler and drier. Again, the fog will clear quickly.

If it is not a cold day, you can form some dew drops by filling a glass with some ice cream. As the glass gets cold, you should start to see condensation happening on the cold glass. Sounds like a good idea to me.