8.16 Water Vapour

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

The warmer the air, the greater the amount of water vapour that can exist in it before saturation occurs.

The colder the air, the less water vapour can exist in it before saturation occurs.

This is why cooling air often leads to condensation, cloud, mist, fog, or dew.

8.16.4 Define the term ‘relative humidity’.

Relative humidity is the amount of water vapour actually present in the air compared with the maximum amount of water vapour the air could contain at that temperature.

It is expressed as a percentage.

For example:

• 50% relative humidity means the air contains half the water vapour needed for saturation.
• 90% relative humidity means the air is close to saturation.
• 100% relative humidity means the air is saturated.

At 100% relative humidity, the air is saturated. If the air is cooled further, or if more water vapour is added, condensation may occur, provided condensation nuclei are present.

8.16.6 Define the term ‘dew point’.

Dew point is the temperature to which air must be cooled, at constant pressure and moisture content, for saturation to occur.