Humidity is the amount of water vapour in the air. The humidity of air is defined as the amount of water vapour which is present in one unit volume of the air.
It is measured in the units of gram per cubic meter.
What is absolute humidity?
The maximum limit of the water vapour which could be present in one cubic meter of air at any temperature..
What is relative humidity?
The actual amount of water vapour present in air is usually lower than the absolute humidity of the air at the same temperature. The wetness of the air is generally expressed in terms of its relative humidity.
If m is the actual amount of water vapour present in a certain volume of air, and ms in the amount of water vapour required to saturate the same volume of air at the same temperature, then the relative humidity is expressed as:
Hence, the percentage of the amount of water vapour actually present in a certain volume of air to the amount of water vapour needed to saturate it, is called its relative humidity.
The 100% relative humidity means that the air is fully saturated with water vapour. Therefore, when the relative humidity is 100%, then no evaporation occurs.
Difference between Absolute and Relative humidity
| S No. | Absolute Humidity | Relative humidity |
|---|---|---|
| 1. | It is the measure of the actual amount of water vapour in the air, regardless of temperature. | It measures water vapour in air but relative to the maximum vapour that the air can hold at that temperature. |
| 2. | It is expressed as grams of moisture per cubic meter of air. | It is expressed as the ratio (percentage value) between the absolute humidity of a given mass of air and the maximum amount of water vapour that it can hold at the same temperature. |
| 3. | The higher the amount of water vapour, the higher is the absolute humidity. | Warm air possesses more water vapour than cold air, so with the same amount of absolute humidity, air will have a different relative humidity depending on temperature. |
| 4. | In weather calculations, absolute humidity is generally not taken into account. | It is the essential characteristic of weather forecasts, which indicates the likelihood of precipitation, dew or fog. |
Read More: What is Evaporation and factors affecting it?
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