Why Does a Bathroom Mirror Turn Misty After a Shower?

Mirrors get "misty" when water condenses on the surface.

Bathroom mirrors have a tendency to get misty or foggy when you are taking a shower or using water in the sink. This is because warm water vapor, suspended in the air, condenses on cooler surfaces, which the bathroom mirror happens to be.

Steam

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.

Condensation

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.

Safety

Condensed vapor on a tile floor can become very slippery, creating a falling hazard. Construction materials continuously exposed to water vapor may become weakened and begin to fall apart. This can be an expensive repair.

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. Water droplets will not be able to condense on the heated mirror. Fog-free mirrors have a heating element just for this purpose.

References

• Net Ally: Keeping Your Windshield Clear: Defrost vs. Demist
• UNI: Water Properties