Temperature and humidity

Unit of thermodynamic temperature: kelvin (K)

​The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10−23 when expressed in the unit J K−1, which is equal to kg m2 s−2 K−1, where the kilogram, metre and second are defined in terms of h, c and ∆νCs.

Information on the practical realisation of the definition of the kelvin can be found in the ​Mise en pratique for the definition of the kelvin in the SI.


​Humidity is the amount of water vapor present in the air (or any other gas). There are three main ways the express humidity: relative, absolute and and specific of which relative humidity is the most commonly used.

  • Relative humidity is the amount of water vapor in the air, expressed as a percentage of the maximum amount that the air could hold at the given temperature; the ratio of the actual water vapor pressure to the saturation vapor pressure.
  • Absolute humidity is the total mass of water vapor present in a given volume (g/m3) or mass of air (g/kg).
  • Specific humidity (or moisture content) is the ratio of the mass of water vapor to the total mass of the moist air parcel.


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