Phase changes


As the temperature of an air mass cools, less water vapour is needed for the air to become saturated. Any parcel of air containing water can be cooled to the point at which it becomes saturated. Further cooling results in condensation. The condensation process releases latent heat as explained previously.

The freezing process

Pure liquid water, if undisturbed, can be cooled to temperatures well below freezing point and still remain liquid. It is then said to be supercooled water. The introduction of an ice crystal or a freezing nucleus will cause freezing to take place in supercooled water. Freezing will also occur if the supercooled droplets are disturbed, for example by an aircraft flying through them. In the atmosphere, there are many types of condensation nuclei. Dust particles over land and salt particles over oceans are examples. As a general rule, there are fewer condensation nuclei available at higher altitudes than near the earth’s surface.

The fact that water can remain in the liquid state at temperatures below freezing has important ramifications for aircraft flying in cloud. These will be discussed later.


The process by which water vapour changes directly to ice (frost) without passing through the liquid state is called deposition. A deposition is not as common in the atmosphere as condensation because nuclei on which deposition takes place are less numerous than condensation nuclei. If moist air is cooled below the freezing point, water vapour may be deposited as ice (frost) on certain solid objects, including other ice surfaces. The frost in the freezer compartment of fridge forms in this way.

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