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What is Relative Humidity?

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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 14 March 2014
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Many people notice that hot muggy days seem much warmer somehow than days with dry heat. One of the reasons for this is humidity, the amount of water the air holds. When the air holds more water, the basic act of perspiration or sweating is less efficient in cooling down the body. More water in the air means less water can evaporate from the skin, and most people will end up feeling warmer. Some of the ways that humidity is measured is by measuring absolute humidity, and relative humidity, which is often how people tell how “wet” a day will be.

First, it’s important to understand that air can only hold a certain amount of water at any given time. This measurement is absolute humidity, and absolute humidity is dependent on the temperature of air. Under many circumstances, the actual air holds far less water than it technically could, so the term relative comes into play. When meteorologists discuss relative humidity they usually do so in percentage amounts, and this percentage is a ratio of how much water the air does hold in comparison to how much water it could hold. The actual formula is water amount (actual vapor density) divided by total possible water amount (saturation vapor density) times 100%. Most people will see the expression of this formula often in relative humidity counts when they watch or read weather reports.

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That percentage or the relative humidity of a day can tell people how warm they may feel under given circumstances. Air that is drier may not feel as hot in warmer temperatures. Air at about 45% relative humidity is going to feel most like the temperature that is outside. Anything above this level may make the day feel warmer than it truly is at certain temperatures.

Temperature perception may be affected by relative humidity in the reverse. On cold days, usually those below 53 degrees F (11.67 degrees C), higher humidity can actually make people feel colder than they normally would. Though other determinants like wind chill may affect “temperature feel” and perception, relative humidity in cold weather may also be an important factor. Freezing weather with a near 100% relative humidity may be much chillier in feel than freezing weather with a lower level of humidity.

The relative measurement of humidity cannot be the only determination of how weather will feel; the amount of wind, especially in colder temperatures and other factors come into play. Moreover, individuals can be more or less sensitive to certain temperatures. However, it is one good way of determining just how hot or cold a day might feel.

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