WHAT IS RELATIVE HUMIDITY?

          To Measure humidity accurately, meteorologists look at relative humidity. This is the amount of water in the air, relative to the maximum amount of water that it can hold at that temperature. To measure relative humidity, a wet and a dry thermometer are used. The wet bulb is covered with wet muslin. The water in the muslin evaporates, making the temperature around the wet bulb cooler than that around the dry bulb. The amount of water that evaporates increases along with the dryness of the air — the greater the difference in temperature, the lower the humidity. A smaller difference means higher humidity. The thermometers are housed in a Stevenson screen, to shade them from the Sun.

           Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air. A related parameter is the dew point.

              Climate control refers to the control of temperature and relative humidity in buildings, vehicles and other enclosed spaces for the purpose of providing for human comfort, health and safety, and of meeting environmental requirements of machines, sensitive materials (for example, historic) and technical processes.

              A hygrometer is a device used for measuring the humidity of air.

             The humidity of an air and water vapor mixture is determined through the use of psychrometric charts if both the dry bulb temperature (T) and the wet bulb temperature (Tw) of the mixture are known. These quantities are readily estimated by using a sling psychrometer.

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