Water Vapour

Water vapour is the gaseous form of water in the atmosphere.

• Water vapour itself is invisible.
• It becomes visible when it condenses or freezes to form cloud, fog, rain or ice.
• Water vapour makes up only a small and variable amount of the atmosphere.

Temperature and Water Vapour

The amount of water vapour that air can contain depends strongly on its temperature.

• Warm air can contain more water vapour.
• Cold air can contain less water vapour.

Therefore, if moist air is cooled sufficiently, it will eventually become saturated.

Relative Humidity

Humidity refers to the amount of water vapour in the air.

Relative Humidity (RH) compares:

• the amount of water vapour actually present;
• with the maximum amount that could be present at that temperature;

and expresses it as a percentage.

For example:

90% RH means the air is holding about 90% of the water vapour it could contain at that temperature.

At 100% relative humidity, the air is saturated.

If saturated air is cooled further, condensation can occur.

Remember:

High relative humidity = air is close to saturation.

Dew Point

The dew point is the temperature to which air must be cooled for it to become saturated.