Fatal attraction: the murderer in the mirror

The most thrilling scene in the infamous movie Fatal attraction (1987) came when Anne Archer wiped the steamy bathroom’s window with her hand and the previously opaque surface suddenly reflected Glenn Close’s appalling face. She wasn’t there exactly to say happy Christmas. The murderer hidden in the bathroom taking advantage of the steam coming from a hot shower is a very popular horror cliché, and so are the words written on the steamy window.

But, do we actually know the reason for such an ordinary fact as the bathroom mirror or the car’s windows getting steamy? It is a matter of air humidity: air always has a certain amount of water vapour. If it gets wetter and wetter, there will come a moment when it can’t contain any more vapour (it is said then that relative humidity has reached 100 %) and so the “spare” vapour condenses, which means that it becomes liquid. If this happens over a wall then the phenomenon of “sweating” walls and ceilings takes place, if it happens over a plant that is dew, and if it happens over glass, it gets steamy.

Therefore, it is easy to understand that evaporation of water from the shower raises air humidity and then air condenses over the mirror. But why does the same thing happen to our glasses when we come from a cold street and get into a warm place? That is due to the fact that warm air can absorb more water than cold air. Unless we are inside a very dry atmosphere, when we enter in a much warmer space wearing glasses, the air inside the place has more water vapour then the air outside, where it’s cooler. Our glasses’ temperature is lower than that of the air around for some time, until a thermal balance is reached. During that time the air around the glasses can’t keep that amount of water, so vapour condenses over the glasses, making them steamy. Absolute humidity hasn’t changed, but relative humidity has: it has suddenly reached 100 % due to the temperature gap between the glass and the air around. This lasts as long as glasses reach thermal balance with the place: afterwards, when they are no longer cold, there isn’t a relative humidity gap anymore, so there is no more condensation.