Relative Humidity

The relative humidity of the air affects how comfortable we feel. But what is humidity, and what is "relative humidity" relative to?

Humidity is defined as the amount of moisture in the air. If you are standing in the bathroom after a hot shower and can see the steam hanging in the air, or if you are outside after a heavy rain, then you are in an area of high humidity. If you are standing in the middle of a desert that has not seen rainfall for two months, or if you are breathing air out of a SCUBA tank, then you are experiencing low humidity.

Air contains a certain amount of water vapor. The amount of water vapor any mass of air can contain depends on the temperature of that air: The warmer the air is, the more water it can hold. A low relative humidity means that the air is dry and could hold a lot more moisture at that temperature.

For example, at 20 degrees C (68 degrees F), a cubic meter of air can hold a maximum of 18 grams of water. At 25 degrees C (77 degrees F), it can hold 22 grams of water. If the temperature is 25 degrees C and a cubic meter of air contains 22 grams of water, then the relative humidity is 100 percent. If it contains 11 grams of water, the relative humidity is 50 percent. If it contains zero grams of water, relative humidity is zero percent.

The relative humidity plays a large role in determining our comfort level. If the relative humidity is 100 percent, it means that water will not evaporate -- the air is already saturated with moisture. Our bodies rely on the evaporation of moisture from our skin for cooling. The lower the relative humidity, the easier it is for moisture to evaporate from our skin and the cooler we feel.

You may have heard of the heat index. The chart below lists how hot a given temperature will feel to us in various relative-humidity levels.

If the relative humidity is 100 percent, we feel much hotter than the actual temperature indicates because our sweat does not evaporate at all. If the relative humidity is low, we feel cooler than the actual temperature because our sweat evaporates easily; we can also feel extremely dry.

Low humidity has at least three effects on human beings:

• It dries out your skin and mucous membranes. If your home has low humidity, you will notice things like chapped lips, dry and itchy skin, and a dry sore throat when you wake up in the morning. (Low humidity also dries out plants and furniture.)

• It increases static electricity, and most people dislike getting sparked every time they touch something metallic.

• It makes it seem colder than it actually is. In the summer, high humidity makes it seem warmer than it is because sweat cannot evaporate from your body. In the winter, low humidity has the opposite effect. If you take a look at the chart above, you'll see that if it is 70 degrees F (21 degrees C) inside your home and the humidity is 10 percent, it feels like it is 65 degrees F (18 degrees C). Simply by bringing the humidity up to 70 percent, you can make it feel 5 degrees F (3 degrees C) warmer in your home.

Since it costs a lot less to humidify the air than to heat it, a humidifier can save you a lot of money!

For best indoor comfort and health, a relative humidity of about 45 percent is ideal. At temperatures typically found indoors, this humidity level makes the air feels approximately what the temperature indicates, and your skin and lungs do not dry out and become irritated.

Most buildings can not maintain this level of humidity without help. In the winter, relative humidity is often much lower than 45 percent, and in the summer it is sometimes higher. Let's see why this is.