Relative Humidity

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Relative Humidity

One of the most important factors which affects the weather in a location is its relative humidity. The concept is elucidated in detail, in the following lines.

If you listen to weather forecasts regularly, you must have heard of something called the relative humidity being quoted for a location, along with the local temperature. It is one of the most frequently used meteorological terms used in rainfall and thunderstorm predictions. It is a characteristic of the air in a given region, which determines how humid the atmosphere is in a place.


What do we mean when we say that it’s too humid? The term Humidity refers to the amount of water vapor in a given air volume or to put it crudely, the amount of wetness in the air.

Absolute humidity is the ratio of the mass of water vapor and mass of dry air present in a given volume of air, at a specific temperature. However, it depends on the volume of air and temperature, which makes it difficult to be actually measured.

Relative humidity is the ratio of absolute humidity of air at a specific temperature to the maximum possible absolute humidity at the same temperature, expressed in percentage. Absolute humidity is also known as vapor density. So, one could define it as the ratio of actual vapor density of air at a given temperature to the saturation vapor density at the same temperature.

Vapor pressure is the partial pressure of water vapor in air. Relative humidity can also be defined as the ratio of actual vapor pressure, to the saturation vapor pressure, expressed in percentage.


Here are two formulas for calculation:

Relative Humidity (%) = (Actual Vapor Density)/(Saturation Vapor Density) x 100


Relative Humidity (%) = (Actual Vapor Pressure)/(Saturation Vapor Pressure) x 100

The significance of this humidity ratio can be understood, when you know how it’s connected with our ability to cool our body through sweating. When the relative humidity of air is high (around 70%, 80%, or higher), air can only hold a very limited amount of water vapor and hence it cannot absorb the perspired water vapor from our body that efficiently. This causes us to feel stuffy, hot, and makes the atmosphere humid. On the other hand, when it is down (less than 50%), sweat from the body is easily absorbed by the atmosphere in vapor form, making us feel much cooler and comfortable.


One way of directly measuring relative humidity is through the use of dry and wet bulb thermometers. This instrument consists of one thermometer, whose bulb is directly exposed to air, while the other one is covered with a wet cloth. The temperature of the wet bulb tends to be lower than the dry one, due to the heat absorbed for evaporation of air. Using a humidity chart, one can determine the actual value of relative humidity, according to the dry and wet bulb thermometer temperature.

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