4.3. Relative Humidity

Relative humidity is a better way of expressing the level of humidity in the air. It is the ratio of actual amount of water vapour present in air at a given temperature to the amount of water vapour air can hold at the same temperature. The Relative Humidity is expressed in percentages.

𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐻𝑢𝑚𝑖𝑑𝑖𝑡𝑦

𝐴𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝐻𝑢𝑚𝑖𝑑𝑖𝑡𝑦

(𝐴𝑐𝑡𝑢𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑢𝑟 𝑝𝑟𝑒𝑠𝑒𝑛𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑎𝑖𝑟 𝑎𝑡 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒)

= 𝐻𝑢𝑚𝑖𝑑𝑖𝑡𝑦 𝑅𝑒𝑡𝑒𝑛𝑡𝑖𝑣𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦

(𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑢𝑟 𝑡ℎ𝑎𝑡 𝑐𝑎𝑛 𝑏𝑒 ℎ𝑒𝑙𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑎𝑡 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒)

Generally capacity to hold water vapour increases with increase in temperature and decreases with decrease in temperature. Thus, the relative humidity of the air decreases with increase in temperature and vice versa Changes in the Relative Humidity of Air1

Changes in Relative Humidity can occur in the following three ways:

I. The temperature remaining the same and amount of water vapour in air increases. Its relative humidity will also increase.

II. When the temperature of air rises its humidity retentive capacity also rises correspondingly and the Relative Humidity decreases.

III. If the temperature of air decreases its humidity retentive capacity also decreases and Relative Humidity increases.