Relative Humidity indicates how moist the air is and may be defined as the ratio of the water vapour density (mass per unit volume) to the saturation water vapour density, usually expressed in percent. Relative humidity is also approximately the ratio of the actual to the saturation vapour pressure.

Actual vapour pressure is a measurement of the amount of water vapour in a volume of air and increases as the amount of water vapour increases. Air that attains its saturation vapour pressure has established equilibrium with a flat surface of water. That means, an equal number of water molecules are evaporating from the surface of the water into the air as are condensing from the air back into the water.

Approximate relative humidity from dry and wet-bulb temperatures

Td = td + 273.2 where td = dry-bulb temperature in º C
Tw = tw + 273.2 where tw = wet-bulb temperature in º C
A = (5418 - 2.2 td), but A = (6141 - 2.2 td) if wet-bulb < 0 º C
P = approximate atmospheric pressure in hPa
e= mathematical constant (base of natural logarithms)