Humidity

Definitions

Absolute humidity

... is the mass of water vapour present in a given volume of air

Relative humidity

... the mass of water in a given volume of air as a percentage of the mass required to saturate that given volume of air at the same temperature.

Thus,

• Relatively humidity
  = actual absolute humidity/saturated absolute humidity at that temperature

From the ideal gas equation,

• Relative humidity
  = actual VP/SVP
  * VP - vapour pressure
  * SVP - saturated vapour pressure
  * Assumes constant V, R, T

Normal values

Maximum absolute humidity @ 0 C
= 4.8 mg/L

Maximum absolute humidity @ 20C
= 17 mg/L

Maximum absolute humidity @ 37C
= 44 mg/L

Absolute vs relative humidity

Absolute humidity doesn't change with temperature.

But,
Saturated absolute humidity varies with temperature

Thus,
Relative humidity also varies with temperature