Relative Humidity

If you listen to weather forecasts regularly, you must have heard 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, which determines how humid the atmosphere is in a place.

Relative Humidity Defined

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

The absolute humidity of air 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, absolute humidity depends on the volume of air and temperature, which makes it difficult to be measured when it comes to measuring it for the atmosphere.

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, 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.

Formula

Here are two formulas for calculation:

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

or

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 air and hence it cannot absorb the perspired water vapor from our body that efficiently. This causes us to feel stuffy, stuffy, hot and 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.

Calculation

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 thermometer bulb is covered with a wet cloth. The temperature of the wet bulb tends to be lower than the dry bulb, due to the heat absorbed for evaporation of air. Using a humidity table or chart, one can determine the actual value, according to the dry and wet bulb thermometer temperature. You may write a computer program to function as a calculator, based on tabulated data of temperature dependent humidity values.