Air is a mixture of gases and water vapour is one very small yet very important component of that mixture. Compared to other gases, the amount of water vapour is highly variable and is responsible for giving us that feeling of dryness or wetness in the air around us, the scientific term for which is “humidity”. However, the amount of water vapour that air can hold increases with its temperature. So at different temperatures, we are likely to feel pleasant or uncomfortable for the same amount of moisture in the air.

Meteorologists talk about humidity in many different ways. The term “absolute humidity” is a measure of the mass of water vapour in a given volume of air expressed in units of grams per cubic metre, at a given temperature whatever may be the barometric pressure. The “specific humidity” is defined similarly, but for a given pressure whatever may be the temperature.

The more commonly used term and reported in the midea, however, is “relative humidity”. This is a ratio of the actual amount of the water vapour present in a given volume of air to the maximum (saturated) amount of water vapour that the air could hold at its temperature. The ratio is expressed as a percentage. Relative humidity would be zero for an atmosphere devoid of any moisture, which in real life is unlikely to happen. It would be 100 % when the air is fully saturated with moisture at its prevailing temperature, such as when it is raining. 

The relative humidity is measured at an observatory with the help of two thermometers, called “dry bulb” and “wet bulb” thermometers. The wet bulb thermometer is identical to the dry bulb thermometer but its bulb containing the mercury is wrapped around with a small piece of muslin cloth. This muslin is kept moist by means of a wick dipped in a small water bottle. As the moisture in the muslin evaporates, the temperature of the wet bulb falls since the energy required for the evaporation process is drawn from the mercury.

The difference in the temperature readings of the dry bulb and wet bulb thermometers is a measure of the relative humidity and is calculated from a set of tables. When the air is dry the difference is large, when the air is very humid the wet bulb temperature comes close to the dry bulb temperature. There is a third temperature called the “dewpoint temperature” which is not measured but calculated from the tables.  This is the temperature at which air would get saturated with water vapour. The wet bulb temperature lies between the dry bulb and the dewpoint temperatures except at a relative humidity of 100 % when all three of them become equal.