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PPL » Meteorology » Water Vapour

8.16 Water Vapour

8.16.2 Explain how the temperature of air influences its capacity to hold water vapour.

  • The warmer the parcel of air, the more vapour that parcel can hold.


8.16.4 Define the term ‘relative humidity’.

Humidity refers to the amount of water held in a mass of air.

Relative humidity gives the ratio between;

  • the weight of the actual amount of water held,
  • to the maximum weight of water that could be held as a vapour,
  • expressed as a percentage.

Therefore 90% RH means the air mass is holding 90% of the water vapour that it could hold.

100% RH would mean that the air mass could not hold anymore water vapour and if it did it would release it (provided condensation nuclei where present).


8.16.6 Define the term ‘dew point’.

Dew point gives us a good idea of the amount of water vapour present in the air.

It is defined as “the temperature at which a parcel of air saturates under constant pressure”.

Further cooling beyond the dew point will result in condensation.



8.16.8 Explain the effect of moisture content of air on the dew point.

A constant moisture content = a constant dew point temperature.

Dew Point temperature will only change with a change of moisture content.

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