How Humidifiers Work | HowStuffWorks - Home And Garden

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

Humidity is 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.

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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.

Water Vapor and Temperature

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.

Relative Humidity and the Heat Index

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. This chart 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.

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