Water Vapour

• Water vapour is the gaseous form of water in the atmosphere. 
• It is invisible until it condenses into cloud, rain or ice.
• Air in the lower regions of the atmosphere nearly always contains some water vapour, varying from almost zero to some 5% by volume. 

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.

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 (Saturated) and if it did it would release it through condensation.

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.