What causes fog on a mirror and how can we prevent it?
The science behind a mirror fogging after a hot shower is really quite simple. The technical term for this process is condensation. Water molecules expand and contract with heat and an exceptionally hot shower can produce water temperatures as high as 100 degrees Fahrenheit or 38 degrees Celsius. This is hot enough to create steam or water vapor which is then released into the air. This release of water vapor and steam does not take long to saturate the airspace of a typical bathroom especially if a fan is not circulating the air. The water must then find somewhere to deposit itself. As the vapor comes into contact with the bathroom mirror it finds a surface much cooler then the surrounding air. This causes the water particles to condense or move closer together forming water droplets and a foggy mirror. The easiest way to prevent this would be to heat up the mirror during the shower. Home remedies will often be suggested such as bathing the mirror in shaving cream, the alcohol in the cream will help to prevent steaming mirrors. A more tested method can be found in the auto section of your local store. Buy a windshield solution and simply apply it to the mirror. Unfortunately these processes will have to be repeated continually and nothing will beat the science of heating up the mirror. Interestingly, there are now very thin heaters which can be purchased and placed behind a mirror in order to heat it up. Of course the most common and easiest solution is to open a window or turn on a fan and circulate the air avoiding water saturation and a fogged mirror.