Water vapor in the air
Liquid water can evaporate and turn into water vapor in the air. Conversely, water vapor in the air may turn back into the water. Some are very dry, with no moisture at all, such as deserts, and others are very humid, such as the humid air of a public bath. As such, the amount of water vapor in the air is constantly changing.
Saturated water vapor in the air increases with increasing temperature. Therefore, warm air may contain more water vapor than cold air of the same volume.
Relative humidity
The degree to which the air is moist or dry is called ‘humidity.’ The humidity we usually use is ‘relative humidity,’ which is the ratio of the current amount of water to saturated water as a percentage.
Humidity in everyday life is not defined as the absolute value of water vapor but as a relative ratio because the humidity felt in real life is closer to relative humidity than absolute humidity.
\[ Relative\,humidity( \% ) =\frac { The\,amount\,of\,water\,vapor(g/kg) }{ Saturated\,water\,vapor(g/kg) } \times 100\]