Sorry, bandmanjoe, but frost certainly will occur indoors if conditions are right. All you have to do is look inside a freezer to see this phenomenon occurring indoors.
The reason frost is more common when surfaces are exposed to the night sky is that that position maximizes radiative cooling,which is the process by which warm bodies emit heat in the form of long-wave radiation. Radiative cooling is more common on cloudless nights and in the open, but it can happen any time a warm body is in a colder location.
In order for frost to form on the inside of the window, the air inside the structure must have some water vapor suspended in it. When an individual water molecule bumps into a cold window, the molecule may give some of its kinetic energy to the glass (which, being cold, has less kinetic energy). This process of heat transfer by contact is known as conduction. Now the water molecule has less energy, so it changes phase from vapor to water, and clings to the glass. If the glass is cold enough, the molecule may lose even more energy and become a solid. Frost is made up of many water molecules that accumulate this way. Since water is a polar molecule, subsequent molecules will stick to the first ones in set sequences, which can result in crystals and attractive patterns in the frost.
Frost occurs when water vapor in the air condenses and appears on a window pane, as ice crystals. This happens when the surface, the window pane, is cooled to below the dew point of the water vapor that is available in the air. The water vapor condenses, and is deposited directly on the window pane in the form of ic crystals, commonly known as frost. The amount of frost that is formed depends upon the amount of water vapor in the air, as well as the temperature the air is cooled down to. Frost is typically translucent, meaning it will allow some light to pass through, but scatters light rays as well, giving a distorted, fuzzy image of what is on the other side of the window pane. Frost, like it's liquid relative, dew, must have a surface upon which to condense, if the condensation is going to materialize. The surface must be exposed to the outside air; this is why frost will not materialize on a car parked under a carport with a roof over it.
It is because cold air causes the water to harden, thus creating frost