Water in the Air

Water in the Air

We have all witnessed water in the air by either looking at clouds or noticing the mist rising over a boiling kettle.

The air is constantly full of water. When you look into the sky and see a cloud, it is actually moisture in the form of tiny water droplets that you are seeing. Most of the time, water vapor in the air cannot be seen unless it collects and condenses to form a cloud.

Warm air can hold more water vapour than cold air.

When there is as much water vapour in the air as possible at a given temperature, we say that the air is saturated.

Water in the Air

The table to the right shows the amount of water vapour in saturated air at standard atmospheric pressure.

For example at 20oC, 1.0 kg of saturated air contains 15.0 g of water vapour.

Condensation

Water droplets form around tiny particles, such as dust and salt, in the air. This is how some clouds form.

Particles that stimulate droplet formation are called condensation nuclei.

Droplets also form on the surface of solids.

The Dew Point

The temperature to which air must be cooled for saturation to occur, without any change in air pressure or moisture content, is called the dew point.

At that temperature dew begins to form, and water vapor condenses into liquid.

We have all seen this when water condenses on a water glass or on grass in the early morning.

The Dew Point

Humidity is the amount of water vapour in the air. Absolute Humidity is the actual amount of water

vapour in the air expressed in units of grams of water per kilogram of air.

Relative Humidity is the percentage of water vapour in the air compared to the amount of water vapour that the air would hold if it were saturated.

Dew Point can also be defined as the temperature at which the relative humidity is 100%.

%100Air of kg 1in Water of Mass Maximum

Air of kg 1in Water of Mass Actual.. HR

Example: What is the relative humidity on a 26 oC day if the mass of water in 1 kg of air is 7.2 g?

Example: How many grams of water would be in 1 kg of air if the room temperature was 18 oC and the relative humidity was 80%?

Example: What is the dew point on a 30oC day when the relative humidity is 93%?