Dew, frost and fog

Dew, frost and fog

In order for any of the above to occur, calm, anticyclonic conditions are required, allowing for rapid cooling of the Earth's surface at night.

Dew occurs when the ground cools sufficiently for the air directly above it to condense.

Hoar frost develops if dew point is below freezing.

Several types of fog exist:

This is most commonly found in valleys and depressions where air can sink. Lower levels of the air are originally warm and contain hygroscopic nuclei, if rapid cooling of the ground occurs radiation fog then forms. The fog may become thicker leading to smog if a temperature inversion occurs, where cold air at the surface is trapped beneath warmer air lying above it (the reverse if normal atmospheric conditions).

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This forms if warm air either passes over cold air or meets cold air resulting in rapid cooling.

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