It is the same phenomenon that occurs naturally on a clear, cold night when your garden grass is covered in dew. Your garden may even be graced with a beautiful frost glittering in the morning sunshine as the dew crystallizes overnight into a magnificent icy veil.
If you are an early riser about to commute to work you may be contemplating whether to switch the engine on and wait patiently for the car windscreen to defrost or opt to vigorously scrape off the ice in order to pursue your journey.
Your house windows may be so well insulated that there is not enough heat loss to warm the outer pane above what is known as the dew point. This is not a problem with single-pane glass or older double glazing. When saturated air hosting high moisture levels encounters the outer pane when the outer pane is below the dew point temperature then moisture in the air condenses into tiny droplets on the window. If the glass is at freezing temperatures then you may see frost on your windows.
This can be frustrating as the view out to your garden is impaired. It almost certainly is not what was expected when the latest and greatest modern glazing was bought. Given time though, the morning sunshine melts the frost and the condensation evaporates restoring a clear view out.
There is a phenomenon called night-time radiative cooling. The outer pane (as well as car windscreens and the blades of grass in your garden) radiates its heat towards the inky and starry nighttime sky. Surprisingly, the outer pane can actually become colder than the outside air temperature.
In this case, it is more than likely that your windows have a good view of the sky whereas on a cloudy night this is far less likely to happen. Perhaps your home is not shielded by trees and buildings or perhaps you live on a side of a hill.
There is very little that can be done about external condensation. Maybe the next generation of stellar performing modern glazing will come with special wipers to clear the condensation similar in concept to those on a car windscreen!
