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Moisture

The moisture content in the atmosphere is one of the major factors governing the type of clouds and other obscuring phenomena that form. The severity of any weather we encounter is also highly influenced by the amount of moisture present. Moisture content varies from a trace to 5% of the atmosphere by volume. It can exist as a solid, liquid, or gas.

The process of transforming it from a liquid to a gas is known as evaporation. Evaporation requires heat energy. The transformation from a gas to a liquid is known as condensation, which releases heat. This heat is known as the latent heat of condensation and it requires condensation nuclei for droplets to form or the possibility arises for supersaturation.  Sublimation is the transformation of a gas to solid or vice versa. This process only occurs at temperatures well below freezing. Air is said to saturated when it is holding all of the moisture that it can at a given temperature.

The dewpoint is the temperature to which air must be cooled to become saturated with water vapour. As an example of how important the moisture content is to the weather, fog is more likely when the  t emperature/dewpoint spread is less than 5 degrees C.

The relative humidity is another measure of the moisture content and is defined as the ratio of the actual amount of water vapour present to the amount of water vapour the air could hold at that temperature.