What we call humidity is actually Relative Humidity (RH). When we say that it is 50% humidity we don't mean that the air is half water and half air. What we mean is that the air is holding 50% of the water it is able to hold at that given temperature. As air gets hotter, it is able to hold more water in suspension. As it gets colder, it is able to hold less. Bear with me, this can be very important to us.
When we change the temperature of air with a heating or cooling system without adding or removing water, we change the Relative Humidity or RH. Here are a few examples.
1. If we take 100 degree air from outside when the outdoor RH is 35% and bring that air inside and cool it to 75 degrees without removing water from it, we have now created an air mass that is about 77% RH. Since our bodies most prefer between 40% and 60% we would not be comfortable.
2. If we take 25 degree air from outside when the outdoor RH is 50% and bring it in and heat it up to 75 degrees without adding water to it, we have now created an air mass that is about 7% RH. (The Psychometric chart is very helpful here.)
Most all air conditioners with the exception of "Swamp Coolers" do remove humidity as they cool, and generally keep the RH at about 50%. Most heating systems, however, do not address this problem, thus causing many people to get colds as their mucus membranes dry out allowing germs to enter their bloodstreams. Please take time to read how HUMIDIFIERS SOLVE THIS PROBLEM
Weber Refrigeration Heating and Air Conditioning