Why do windows fog up?

Posted on Mar 9, 2022      260

If on a sunny day you put a clean empty glass upside down on a hollowed bed or on a lawn with grass, after a while you can notice an interesting phenomenon: the dry transparent inner walls of the glass will fog up, but the outside glass will remain completely dry.

What happened and how can we explain the appearance of water droplets visible inside the glass? The water molecules that are on the surface of the soil and in the plants are all the time rising with the air and, cooling down, are deposited on the inner walls of the glass, dampening it.

Experiment with fogging up the inside of the beaker

Water molecules evaporate from the grass and the surface of the soil and form vapor in a glass that stands upside down on the lawn.

If the air contains as much water vapor as is at all possible at a temperature, it is called saturated. And if it cools below that temperature, some tiny vapor droplets will stick together. The temperature at which this occurs is called the dew point. Air whose temperature is higher than the dew point is called unsaturated air. When air cools below the dew point, the movement of moisture molecules in it slows down and the droplets settle on the icy walls inside the glass. Outside, the molecules continue to rise upward. They make their invisible way to the sky - they have evaporated.

The process by which water goes from gaseous to liquid again is called condensation. We often see it in everyday life, such as when we fog up our window panes, eyeglasses, or car windows.

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