The colder a given volume of air is, the less able it is to retain water vapor. So the wintertime air outdoors has much less moisture in it than either indoor air, or summertime outdoor air. An indoor environment can dry out when cold outdoor air migrates inside effectively reducing the relative humidity to uncomfortable levels. If your furnace uses interior air for combustion, this causes lower indoor air pressure and increases the amount of outdoor air drawn into the conditioned space to replace the air being discharged with the flue gases. The use of vented kitchen fans (without make-up air), or wood burning or chimney vented gas fireplaces can also sharply increase the drying effect of winter air infiltration. A sealed combustion furnace, which draws combustion air from outdoors would eliminate this problem.