Humidity is measured using an instrument known as a sling psychrometer. A psychrometer is made up of two thermometers; one thermometer measures the air temperature while the other one measures the wet-bulb temperature. The difference between these two temperatures is known as humidity.
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Last edited at 9:12PM on 7/4/2013
If u dont understand it properly and want to calculate the presence of humidity or whatever go to this site.as there are diagrams which will help u and makes u understand clearly. http://www.bom.gov.au/lam/humiditycalc.s?
Humidity can be measured indirectly with dry and wet-bulb thermometers. The dry-bulb is an ordinary mercury-in-glass thermometer and measures the air temperature (see Figure 1).
The bulb of the wet-bulb thermometer is covered by a muslin bag which is kept moist. It gives a lower reading than the dry-bulb because of the cooling effect of water evaporating from the muslin bag (see Figure 1).
The two thermometers may be placed in a Stevenson screen, or whirled together in a sling (sling psychrometer), or ventilated by drawing air past them with an electric fan (aspirating psychrometer).
The actual readings of the two thermometers and the difference between them enable us to calculate all the humidity quantities defined in the 'Humidity Definitions' given below.
The difference between the wet and dry-bulb readings is a good indication of the efficiency of evaporative-type coolers and the wet-bulb reading is a reasonable indication of human comfort.