What is Relative Humidity

Humidity can be defined as the quantity of water vapor held by the atmospheric air, at a certain temperature. Here, the term 'held' does not mean that air acts as a holder, rather it implies that the air is a carrier of moisture. Relative humidity is a type of humidity. Some other forms of humidity are absolute humidity and specific humidity. When the term 'humidity' is used in weather forecasts, it usually refers to the relative humidity in the air.

Definition of Relative Humidity

Relative humidity can be defined as the ratio of the actual amount of water present in the atmosphere and the amount of water that the air can sustain at a given temperature. As the units of the numerator and the denominator of its ratio are the same, relative humidity does not have any unit. Generally, it is expressed in terms of percentage. This is obtained when the above given ratio is multiplied by 100.

The quantity of water molecules present in air depends on its temperature and pressure. If the temperature is high, it can contain greater amounts of water vapor. The actual moisture content of the air is often termed as the mixing ratio and is calculated as grams of water in every kilogram of dry air. When it is said that air gets saturated, it means that the air cannot hold any additional amount of water vapor. At this point of time, relative humidity becomes hundred percent.

Calculation of Relative Humidity

An easy method of relative humidity calculation is by the means of dry- and wet-bulb thermometers. A dry-bulb thermometer is a simple thermometer that we use for measuring temperatures. The heat content of the air measured with the help of a dry-bulb thermometer is referred to as the Dry-Bulb reading. On the other hand, a wet-bulb thermometer is always kept under the cover of a moist muslin bag. Thus, it shows a much lower reading as compared to a dry-bulb thermometer as a result of the cooling effect of evaporation of the water-content in the muslin bag.

Here, the effect of evaporation is utilized to measure the humidity. Air is blown over the wet-bulb thermometer, until all the water from its moist muslin cloth evaporates. As we know, the process of evaporation requires heat, which will decrease the temperature reading shown by the wet-bulb thermometer. Thus, there will be a difference in temperatures of the two thermometers which are kept in the same place at the same point of time. When these two different temperatures are combined with a Mollier chart (a chart that represents the relation between the air temperature and its moisture content), the relative humidity can be evaluated. In case the air has reached full saturation (which means relative humidity is 100 percent), the moisture in the cloth will never evaporate and the temperature shown by both thermometers will remain the same.

On any given day, if the relative humidity is quite high, we tend to feel warmer as it takes longer for the perspiration to evaporate from the surface of our skin. Hence, we feel very uncomfortable on these days. In hot weather, relative humidity plays a major role for human comfort. This is because our body is sensitive to the changes in relative humidity.