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Relative Humidity

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Its easy to tell when it's sticky outside: You have to peel yourself off vinyl car seats and plastic lawn furniture, and the laundry you hang on the line never seems to dry. Now you can get a more accurate measure of the mugginess, or relative humidity, by using the following charts and a simple tool called a sling psychrometer that you can make yourself.

Absolute humidity is the amount of water vapor in a given volume of air, usually measured in grams per cubic meter. However, because air can hold varying amounts of water vapor at different temperatures (the warmer the air is, the more moisture it can hold), a more useful measurement (and the one used most often in general weather information) is relative humidity. Expressed as a percentage, this is the amount of water vapor in the air compared with the total amount that the air can hold at that temperature. For example, at 60 percent relative humidity, the air contains only 60 percent of the total water vapor that it can hold at that temperature before it will be saturated, causing the water vapor to condense.

When the glowworm lights her lamp,
the air is always damp.

–Weather proverb

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