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Humidity of the Air

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  P-272

2025-11-24

550

The relative humidity of the air is the ratio between the amount of moisture present in the air and the amount that would be present if the air were saturated.

If the temperature of the air is taken at the same time that the dew-point is determined, the relative humidity can be found by the help of the table in .

Suppose, for example, that at a time when the temperature of the air is 23° C. the dew-point is 17° C. From the table the amount of moisture present when the air is saturated at 17° C. is 14.339 g. per cubic meter. But at its temperature of 23° C. it could contain 20.355 g.; hence the relative humidity is 14.339: 20.355, or about 70 per cent.

Instruments used in determining relative humidity are called hygrometers. The wet-and-dry-bulb hygrometer consists of two similar thermometers, the bulb of one being covered with a wick, the end of which dips into water. This keeps the covering of the bulb wet, and the rate of evaporation affects the temperature of the bulb. If there is little moisture in the air, the evaporation takes place rapidly, and the wet-bulb thermometer will read considerably lower than the other. Tables are provided, by the use of which the relative humidity can be determined from the readings.