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Humidity is a measure of the amount of water vapor in the air and is a primary element of climate. Humidity can be expressed in a number of ways, including absolute humidity, specific humidity, mixing ratio, relative humidity, and dew point. Although water vapor rarely accounts for more than 4 percent of the total volume of the atmosphere, it is an extremely important component of the atmosphere. Atmospheric water vapor regulates air temperature by affecting the transmission of radiant energy both to and from the earth’s surface and provides latent heat energy to fuel storm systems. The maximum amount of water vapor that the atmosphere can hold is a function of the temperature and pressure of the air. The moisture content of air increases rapidly as the temperature of the air increases. When a volume of air contains the maximum amount of water vapor at a given temperature and pressure, the air is said to be saturated.
Absolute humidity directly measures the amount of water vapor in a given volume of air and is usually expressed in grams of vapor per cubic meter of air. The absolute humidity of an air parcel will change as it expands or contracts even though there is no change in the amount of water vapor. Due to these drawbacks, measurements of absolute humidity are rarely used.
Specific humidity refers to the mass of water vapor in a given mass of air and is usually expressed in grams of water vapor per kilogram of air. Unlike absolute humidity, specific humidity has the advantage of not changing as air expands or contracts. The mixing ratio is closely related to specific humidity and is defined as the mass of water vapor in the air to the mass of dry air. Relative humidity is the most common and frequently used measure of humidity. Relative humidity is the ratio, in percent, of the amount of moisture in the air compared to the amount that the atmosphere can hold at a given temperature and pressure.
The relative humidity of the air changes throughout the day, usually reaching its highest value in the early morning hours and then decreases to a minimum in the early afternoon. Thus, relative humidity indicates how near the air is to saturation rather than the actual quantity of water vapor in the air. Another useful measure of humidity is the dew point, the temperature at which air becomes saturated if cooled without a change in pressure or moisture content. The dew point is always less than the air temperature unless the air is saturated.
Several types of instruments have been created to measure humidity. Humidity measurements are increasingly being recorded by remote sensing packages that transmit upper-air observations back to ground stations. An instrument commonly used to measure humidity is the psychrometer. It consists of a wet bulb thermometer that is kept moist by a wick soaked in water and an unmodified dry bulb thermometer. When the psychrometer is swung freely in the air or aerated by a fan, evaporative cooling lowers the wet bulb temperature. The amount of moisture in the air can be determined by calculating the difference between the dry bulb and wet bulb temperature. The hair hygrometer is another instrument used to measure humidity and is based on the fact that hair expands and contracts in response to changes in humidity.
Humidity greatly affects our comfort and health and can make the warmth of the surrounding air feel as if it is warmer than the actual temperature. If the atmosphere has a high moisture content, the rate of evaporation is reduced, which impairs the body’s ability to maintain a constant temperature. Physical strength declines and fatigue occurs more rapidly in a humid environment. A heat index can be used to determine the apparent temperature caused by the combination of heat and humidity.
- Edward Aguado and James Burt, Understanding Weather and Climate (Prentice Hall, 2004);
- Howard Critchfield, General Climatology (Prentice Hall, 1983);
- Ann Henderson-Sellers and Peter Robinson, Contemporary Climatology (Longman Scientific and Technical, 1986);
- Frederick K. Lutgens and Edward J. Tarbuck, The Atmosphere (Prentice Hall, 2004);
- Joseph Moran and Michael D. Morgan, Meteorology (Prentice Hall, 1997).