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Tuesday, March 13, 2012

ATMOSHPHERIC HUMIDITY


ATMOSPHERIC HUMIDITY: Some amount of water is always present in the atmospheric air. It is due to evaporation of water present on the earth surface in the form of seas, ponds, lakes, damp soil etc.
The branch of physics which deals with the study and measurement of water vapour present in the atmosphere is called hygrometry. In the study of atmospheric humidity, the following terms are used: Absolute Humidity:  the absolute humidity of air is defined as the mass of water vapour actually present in one cubic meter of air. It is measured in gram per cubic meter.
Relative humidity: The relative humidity of air is defined as the ratio of the mass of water vapour required to saturate the same volume of air at the same temperature. If m is the mass of water vapour actually present and M, the mass of water vapour required to saturate the same volume of air at the same temperature, then relative humidity is given by, R.H.=m/M……..(1)
The mass of water vapour in a given volume of air and at a given temperature is directly proportional to density. Therefore, R.H.=d/D………..(2)
Where d is the density of water vapour actually present and D is the density of water vapour required to saturate the same volume of air and at same temperature.
The unsaturated  vapour obey Boyle’s law up to the point of saturation. Therefore, mass (or density) of the water vapour present in air will be directly proportional to its pressure. Therefore, R.H.=p/P…….(3)
Thus, the relative humidity of air room temperature may be defined as the ratio of the actual vapour pressure at the room temperature to the saturated vapour pressure at the same temperature.
Usually, the relative humidity is expressed as a percentage and therefore equation (3) may be written as
R.H. = (p/P)*100% ……………(4)
Dew point: IT is defined as the temperature at which the water vapour actually present in the certain volume of air becomes just sufficient to saturate it.
Ordinarily, the amount of water vapour present in air is insufficient to saturate it. However if the temperature of the air is decreased, it requires lesser amount of water vapour to saturate it. At dew point, the amount of water vapour actually present becomes just sufficient to saturate it. Obviously, the actual vapour present at room temperature will be equal to saturated vapour pressure at dew point. Hence, the relative humidity of air at room temperature may be defined as the ratio of saturated vapour pressure at dew point to the saturated vapour pressure at room temperature. Thus,
R.H. =[ (saturated vapour pressure at dew point)/( saturated vapour pressure at room temperature )]

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