Weatherwatch: what is the dew point?

The dew point is the temperature at which a certain area of air must be cooled to for dew to form. It is also commonly interpreted as a measure of the air’s moisture content. As air cools, the amount of the gaseous form of water it is capable of holding decreases, and as such, cooling will eventually result in it reaching saturation point, beyond which condensation into liquid water droplets occurs. This is the main process behind cloud formation in the atmosphere.

Of course, dew can also form on surfaces on earth. When the surface (for example grass or cars) cools below the dew point temperature of the air adjacent, condensation droplets from the air deposit onto that surface. If surface temperatures fall below 0C and below the dew point temperature, instead of water vapour gas condensing to water droplets, it transitions directly to crystals of ice, in the form of frost.

While frost formation in winter months is rather common, surface temperatures in autumn or in spring are less likely to fall down to the air’s dew point, aided by the shorter overnight cooling periods. Hence, dew and frost do not form as readily, especially on objects which can have larger stores of heat in them, such as roads.

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