Where does the water for frost formation in the fridge come from?
As Daniel says, it comes from the surrounding air. But your question is perhaps, why the frost comes on the walls, right? First, the colder the air, the less humidity it can contain. At 15 C, a cubic meter of air can’t contain more than 13 grams water. The air is saturated and the relative humidity is 100 percent. At freezing point, 0 C, the same cubic meter of air can’t contain more than about 5 grams water. It is also 100 percent saturated. So, what happens when the air is further cooled, beyond its so-called dew point temperature? Well, it must condense into droplets of water or ice crystals. The problem is: the tiny water molecules have virtually no mass and in order to condense or freeze, it has to give away energy in form of heat. The gaseous water is then under-cooled until it meets anything with a mass. In the atmosphere, clouds appear when under-cooled water molecules meet just anything like pollen, dust or smoke particle. If near the ground, then it is a tree leave, your car
