When the outside temperature falls below the inside temperature, the cold and moist air that enters a heated building becomes hot and dry.
This happens due to a phenomenon by which equilibrium is established between the moisture in the objects inside the building and in the air that these are in contact with.
The reduction in the moisture in the air may cause well known problems, such as:

• dryness of the upper airways
• cracks in wooden structures
• electrostatic discharges.

Air-conditioning or refrigeration systems may also cause an increase in the dryness of the air, as they cool the air below its dew point, remove moisture (dehumidification) and subsequently reheat the air.

dew point: this is the temperature that air must reach (while maintaining a constant pressure and humidity) for the moisture to condense (saturation).
dehumidification: process by which the moisture in the air can be decreased or removed