Humidity

Humidity refers to the water vapour in the air. The air absorbs it from the water evaporated from water bodies or plants and animals. Two types of humidity are absolute humidity and relative humidity.

The actual amount of water vapour that the air holds at any given time is absolute humidity while the ratio of absolute humidity to the maximum humidity that the air can hold is relative humidity.

Thus, absolute humidity divided by maximum humidity multiplied by 100 gives the Relative Humidity of the air.

⇒ \( \scriptsize Relative\: Humidity = \normalsize \frac{Absolute\:Humidity}{Maximum\:Humidity} \scriptsize \: \times \: 100\%\)

For example, if some air is capable of holding, say 500 grams (500 g) of water per cubic meter, and it has only 300 grams (300 g), its relative humidity can be computed using the above formula:

Thus: \( \frac{300}{500} \scriptsize \: \times \: 100\%\)

⇒ \( \frac{300}{500} \scriptsize \: \times \: 100\%\)

= \( \frac{300}{5}\)

= 60%

Measurement of Relative Humidity:

A hygrometer is an instrument used for measuring relative humidity.

The Hygrometer is made up of two thermometers: Wet Bulb and Dry Bulb Thermometers. The two are put side by side in the Stevenson Screen. While the Dry Bulb Thermometer is outside and measures the shade temperature, the Wet Bulb Thermometer has a muslin (wick) dipped into a distilled water container (beaker) which cools it and, hence, reads a lower temperature than the dry bulb thermometer.

When the difference in the two thermometer readings is high it means that the humidity is low, and if the difference is low, it means the humidity is high.

If there is no difference in their readings, it means that the air is saturated, and therefore the relative humidity would be 100%.

For example, if:

• Dry bulb temperature = 30ºC
• Wet bulb temperature = 28ºC
• Depression in wet bulb = 30 – 28 = 2ºC 

Therefore, the Relative Humidity = 90%