Questions & Answers - Humidity
This quarter our “Question & Answer” segment discusses how moisture and humidity have a great deal to do with comfort.
What exactly is humidity?
Humidity is the amount of water vapor contained in the air. When discussing humidity, there are two terms that are pertinent. The first is absolute humidity. Absolute humidity is the amount of water vapor actually contained in the air. The second term is relative humidity. Relative humidity is the amount of water vapor contained in the air compared to the maximum amount of water vapor the air is capable of holding. For example, if a sample of air is capable of holding 80 grams of moisture but is actually holding only 40 grams of moisture, the relative humidity would be 40/80 or 50%. If the air is fully saturated with moisture, the relative humidity is 100%.
When weather forecasters talk about humidity, are they talking about absolute or relative humidity?
When weather forecasters refer to humidity, they are talking about relative humidity.
Where does the moisture come from and how much moisture can the air hold?
Water is constantly evaporating from the earth and from the bodies of water contained on its surface. This constant evaporation releases moisture into the air. The amount of moisture the air is capable of holding varies depending on the temperature of the air. The warmer the air, the greater its moisture carrying capacity.
The graph below shows the maximum amount of moisture the air is capable of holding at various temperatures. If the outdoor temperature is 50°F, the air can hold approximately 8 grams of water per kilogram of air. However, if the temperature increases to 86°F, the air can now hold approximately 28 grams of water per kilogram of air, more than three times as much as it could hold at 50°F. It’s no wonder warm summer days with high humidity can feel so uncomfortable!
What part does relative humidity play in comfort?
Relative humidity is very important in determining how comfortable you feel in your home. The ideal relative humidity for human health and comfort is between 35% and 55%. Levels of humidity below 30% can cause dry skin, dry noses and throats and damage furniture and wood floors. On the other hand, relative humidity levels consistently above 60% can cause the formation of mold and mildew.
Why does it tend to feel dry in the winter and humid in the summer?
This relates back to the fact that cool winter air holds much less humidity than warm summer air. As the air in your home is heated during the winter, it expands, reducing the relative humidity and making it feel dry. During the summer, air conditioners cool the air, reducing the amount of water vapor the air is capable of holding. This water vapor condenses into water and is captured by your air conditioner in the form of condensate.
How can I tell the humidity level in my home and how can I control it?
Humidity levels are measured by a device called a hygrometer, which can be purchased at any hardware store. A good rule of thumb is, if you notice static electricity when folding clothing, combing your hair or walking about your home, your humidity level is too low. In the wintertime, humidifiers can be used to increase the humidity level. If you have a warm air heating system, this can be accomplished by installing a central humidifier. If you have a hydronic heating system, portable warm mist humidifiers can be used. In the summertime, air conditioning reduces the humidity level at the same time that it cools your home.
