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Humidity is a general term used to indicate moisture in the atmosphere. Vapor pressure and humidity are intertwined. Absolute humidity (g/m3) is the mass of water vapor contained in a unit volume of space, and can be determined from the equation of state for an ideal gas. The specific humidity (gm/gm) is the mass of water vapor contained within a unit mass of moist air.

 

Relative Humidity

Atmospheric humidity has a significant influence on evapotranspiration. The commonly used term relative humidity (RH) can be defined as the ratio (%) of the actual vapor pressure of the air to the saturation vapor pressure at the same pressure and temperature. Direct measurement of relative humidity is accomplished using a hygrograph. Indirect estimation of relative humidity is accomplished using dry and wet bulb temperature records. Wet-bulb thermometers must be carefully shielded from radiation. Further, there should be adequate ventilation to obtain a true wet-bulb temperature. The estimated humidity from the dry and wet bulb temperatures are obtained daily (equidistant and instantaneous) or twice daily at standard times at 08:30 and 17:30 hrs. The continuous record obtained from the hygrograph is tabulated at hourly intervals. Measurements of humidity are made at the same location where air temperature is measured.

 

Analysis of RH data over the country shows that RH has considerably increased over last 4-5 decades. One of the reasons behind this increase is tremendous increase in application of irrigation water over the agricultural area, a large percentage of which is evaporated.