Why does the mirror in the bathroom fog up after a shower?



The fog on the bathroom mirror is caused by condensation. Condensation is the term we use to describe the process which occurs when moisture in the air is extracted. Condensation happens when the temperature of air containing moisture decreases. Falling air temperatures cause condensation because warm air can hold more moisture than cold air. This is the exact same process that takes place when a hot air mass encounters a cold air mass and we experience a rainstorm.

When warm air in the bathroom is heated by the shower to temperatures as much as 100 degrees F, the air becomes saturated with moisture. When this saturated hot air comes in contact with the bathroom mirror, which may be as much as 30 degrees F less than the air temperature, the moisture begins to come out of the air and be deposited on the mirror surface. The clouds of steam in your bathroom are basically raining on your mirror.

By raising the surface temperature of the mirror to be the same as or greater than the temperature of the air in the bathroom, condensation is prevented.

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