House windows serve as insulators. They let light through while trapping heat. In the summer, they keep the inside cooler than the outside; in winter, they keep the inside warmer than the inside. Windows are not great insulators, however, and do allow some thermal exchange. That is why if you touch a window from the inside on a cold day, it feels cold to the touch even if it is warm inside.
Windows and Insulation
Heat and Humidity
Warm air can carry much more moisture than cold air can. That is why, when the temperature drops overnight, dew settles on the grass. The air can't hold onto all of its moisture as the temperature drops, so the moisture condenses onto plants and anything else around. When the temperature increases during the day time, the air is able to contain more moisture again, and the dew evaporates.
If the air outside is colder than the air inside, the windows will be colder too. When inside air comes in contact with the windows, it cools down and can't hold onto its moisture. It leaves this moisture on the windows as condensation or "fog." Your windows may also fog up on the outside at night, when the temperature drops and the dew falls.
Reducing Window Fog
Your windows will not fog up unless the air inside is warm and moist. Central heating systems tend to dry up the air, so unless you have a humidifier running during the winter you may not see your windows fog up at all. If your windows do fog a lot, it may be a sign of poor insulation. Try installing windows with double paned glass. The inside pane will not conduct as much heat so it will not get nearly as cold. This will solve the fog problem and save on your heating bill.