On mild days 68 degrees indoors feels comfortable, but on zero degree days, 68 degrees feels chilly?

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Dear Tom,
When we have mild winter days, say around 60 degrees, and I keep my house thermostat set at 68, I am comfortable inside. But when the outdoor temperature is around zero degrees and I keep the thermostat at 68, I have to bundle up like an Eskimo inside. Why?
Ken Mueller

Dear Ken,
It has to do with the relative humidity. A higher indoor relative humidity makes the 68 degree temperature feel more comfortable; a low relative humidity makes it feel chilly. An outdoor temperature of 60 degrees is often accompanied by a dew point reading of 50 degrees, sometimes higher. Outdoor temperature/dew points of 60/50, when brought inside and heated to 68 degrees, yields a relative humidity of 52 percent; outdoor readings of 0/-5, for example, when heated to 68 degrees indoors gives a low 5 percent relative humidity.

Increasing temperatures, if no moisture is added to the air, result in sharply lower relative humidity values. If, for example, the outdoor temperature and dew point are both at 0 degrees (in other words, the air is saturated and can hold no additional moisture) and the temperature increases to 10 degrees but no moisture is added to the air (that is, the dew point temperature remains at 0 degrees), the relative humidity will drop to 64 percent. If the temperature increases further to 20 degrees, the relative humidity becomes 41 percent. At 70 degrees, the relative humidity plunges to 6 percent.

This explains why a humidifying system should be employed in a house during the winter.

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