How Much Moisture Is in the Air? A Heating Contractor’s Perspective on Humidity

How much moisture does outside air possess? This question has a few answers; however, the first question that should be asked before giving an answer is, “What is the temperature outside?” A given quantity of gas will occupy a certain volume at standard atmospheric pressures and temperatures. Double the temperature and allow the volume to increase so that the pressure remains at standard atmospheric values, and the space the gas occupies will approximately double. This is the reason unconditioned air in your home (no humidification) is relatively dry in the winter.

Let’s discuss this in more detail.

Very cold outside air at standard atmospheric pressure contains much less moisture per given volume of air than warmer air. This is specifically true when cold and warm air at the same relative humidity levels are compared. Relative humidity is a measure of how much moisture a given quantity of air contains compared to how much moisture that same quantity of air could potentially hold. As an example, say a 100 cubic foot volume of air at 70 degrees has a relative humidity of 50% and contains 1 cup of water. At 100% relative humidity this same volume of air would contain 2 cups of water. Now lets take a look at why our homes are so much drier in the winter.

Consider a day when the outside air is 0 degrees F and at 20 percent relative humidity. If we take a 100 cubic foot sample of this air and assume it holds 5 ounces of water when it is at 0 degrees and 20 percent relative humidity, what will happen to that air when we bring it into our house and raise the temperature to 70 degrees? The answer is, the 100 cubic foot space this air occupies will increase dramatically when it is heated to 70 degrees. Instead of occupying 100 cubic feet this same quantity of air will occupy a much greater space. If air is allowed to expand, heated air will always occupy a much greater space than the same quantity of colder air.

Let’s go back to the outside air that contained 5 ounces of water at 0 degrees F. Now that we’ve brought the air inside the home and raised it to 70 degrees lets assume it occupies 200 cubic feet of space. As you can see, five ounces of water contained within a much larger 200 cubic foot space thins the solution out considerably. All of those airborne water molecules are much farther apart and the air begins to feel dry. Also the relative humidity of the air has decreased significantly, since not only has the air expanded to a much greater volume, but also warmer air is able to hold much more water at similar volumes as cold air.

Our homes are continually bringing in outside air to make up for air lost to the outside. There are many reasons inside air is lost to the outside. For example, inside air is vented outside by exhaust from hot water heaters and furnaces. When new air from the outside is brought in to replace this air it immediately expands and the relative humidity of that air goes down. Unless we humidify our homes this air will seem dry since the water molecules within this new larger area are farther apart. This is why we need humidification in the winter.

To simplify everything that has just been discussed about relative humidity try imagining two pictures. The first picture I want you to imagine is a dry desert with no vegetation. The second is a rain forest. The big difference between the two pictures you have just imagined is how close or far apart the water molecules are from each other. In dry air the water molecules are far apart. In moist air they are close together.