Atmospheric air is a mixture of various gases and water vapour. The invisible water vapour in the air is called humidity.
The amount of water that can be contained in a pound of air depends upon ……
1. Temperature of the air
2. Pressure of the air
At a constant pressure, the warmer the air the more water vapour it can retain. If air at a certain temperature has absorbed all the moisture it can hold at that temperature, it is said to be saturated.
The amount of water present, or the degree of saturation is evaluated in terms of Relative Humidity or Saturation ratio.
Hence, Relative Humidity is the actual water content in % of that of a saturated volume.
When air is 50% saturated, it contains only one half the amount of water than it can contain at the same temperature and pressure. As the relative humidity approaches 100%, the air can take on less and less moisture and at 100% relative humidity, that air cannot hold more water.
If air is saturated with water vapour at a given temperature, a drop in the temperature will lead to condensation of water in the form of droplets. The temperature at which moisture condenses out is the dewpoint temperature.