Here's what happens in winter to make it feel so dry in our houses. Let's say that the outdoor temperature is 0 degrees C, or 32 degrees F. The maximum amount of water that a cubic meter of air can hold at this temperature is 5 grams. Now you bring this cubic meter of air inside and heat it to 25 degrees C or 77 degrees F. The relative humidity is only 23 percent:
= 23 percent relative humidity
It gets worse as the temperature outside falls lower. This is why the air inside any heated building in the winter feels so dry. Any time the temperature outside is below freezing, relative humidity inside will be below 20 percent unless you do something to increase the humidity.
The outside air might have a comfortable level of humidity, but when that air is heated, the relative humidity drops, causing the air to be very dry inside the house.
During the dry months, a humidifier can help maintain a comfortable level of humidity. Let's take a look at a simple humidifier.