Vapour pressure is that part of total atmospheric pressure which is attributable to its water vapour content. Like other gases, water vapour exerts a pressure. As water molecules evaporate from the water surface, there is a small increase in pressure in the air above.
This increase is the result of the motion of water vapour molecules supplied to air through evaporation. Because of increasing vapour pressure in the air above, more and more of these escaping molecules are forced to return to the liquid.
At last, the number of water molecules leaving and returning to the evaporating surface becomes equal. Under such a condition the air is said to be saturated, and the vapour pressure is then called saturation vapour pressure.
However, any increase in the temperature of air would require more moisture for saturation. It shows the amount of water vapour required for saturation at different temperatures.
At any point of time, the difference between the moisture holding capacity of air and its actual humidity is referred to as the saturation deficit. The temperature to which air has to be cooled in order to reach saturation is called the dew point.
There are occasions when in the absence of condensation nuclei condensation does not start at the dew point. The air in which the process of condensation starts at much lower temperature than the dew point is called supersaturated.
If the air were cooled below this temperature, then the excess vapour would condense. The dew point of ascending parcel of air decreases with altitude at the rate of about 2°C per kilometer.
Since the water vapour holding capacity of air increases with rising temperature, the air in the equatorial belt has a higher dew point than that in the higher latitude.