Humidity, a measure of water vapor in the air, is one of the variables measured in basic meteorology. There are actually several different kinds of humidity, but what most people mean when they talk about "humidity" is relative humidity. Relative humidity is defined by Perry's Chemical Engineers' Handbook as "the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a prescribed temperature." In other words, relative humidity is an indirect way of measuring of how much water vapor is in the air at a given time, versus how much water vapor the air can hold at maximum. It is expressed as a percentage. When the relative humidity gets to 100 percent, water vapor in the air begins to condense back into liquid water: It will rain. Relative humidity is useful to know because it gives an idea of how "wet" the air feels. Low relative humidity can lead to dry skin, itchiness, and thirstiness. High relative humidity makes cold temperatures feel colder and hot temperatures hotter. When the weather is very hot, high humidity impairs the body's ability to cool down by sweating. Relative humidity also has an impact on delicate machinery such as computer circuit boards and on the development of microorganisms and fungi. Inside the home, high humidity makes mildew more likely to develop, while low humidity facilitates the spread of the flu virus. For all of these reasons, and more, it is useful to be aware of the relative humidity. Collectively, any of the instruments used to measure humidity is referred to as a hygrometer, a humidity gauge. more »