Bathroom mirrors have a tendency to get misty or foggy when you are taking a shower or using hot water in the sink. This is because warm water vapor, suspended in the air, transforms back into water droplets as it cools. These small droplets collect on surrounding surfaces and are especially noticeable on materials such as glass since they distort light waves, making the mirror and windows appear foggy or misty
When you take a shower, tiny warm water particles are suspended by the air in the bathroom. The hotter the water is, the further apart its molecules are. When the air in the bathroom is warmed by the heat of the shower, the air will suspend the tiny water droplets until the air is saturated and the droplets fall to the floor.
As the water droplets come into contact with cooler surfaces such walls, the toilet or the mirror, they are cooled and condensed back into liquid form, creating a film of water on the cooler surfaces. If the bathroom gets steamy enough and if the mirror is cool enough, you will see fog or mist on the mirror. If the mirror is much cooler than the water and if there is a significant amount of vapor in the air, you may actually see streams of condensed water running down the mirror. Light traveling through the film of water bends, creating a clouded appearance on the mirror.
Condensed vapor on a tile floor can become very slippery, creating a falling hazard. A bath mat or rug provides a safer place to stand when in a steamed-up bathroom. Construction materials continuously exposed to water vapor may become weakened and begin to fall apart. These can be expensive repairs.
Preventing Vapor Build-up
Using an exhaust fan while you shower may help prevent vapor build-up. This may not only keep your mirror clear, but also prevent mildew problems. If you don't have an exhaust fan, showering with the bathroom door open will dissipate the steam just as well.
Clearing the Mirror
To clear the mirror, first open the bathroom door or turn on the exhaust fan to vent the steam. Blow warm air from a hair dryer onto the mirror to clear the steam quickly. Water droplets will not be able to condense on the heated mirror. Fog-free mirrors have a heating element just for this purpose.