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Fog

Fog is a stratiform cloud very near the ground as opposed to haze which is a very fine dust or salt particles suspended in the atmosphere. Fog requires high humidity, light wind, and condensation nuclei. Fog forms when is cooled to its dewpoint, or moisture is added to the air near its surface. Formation is aided when the dewpoint/temperature spread is less than 2.8 degrees C, light winds are present to provide mixing action to spread and thicken fog, and if an abundance of condensation nuclei such as dust, pollen, salt, smoke etc. are present.

There are several types of fog. Radiation Fog, also known as Ground Fog, forms after clear evenings, near sunrise as this is the lowest temperature period. Advection Fog is formed when warm, moist air flows over a cold surface, most commonly in coastal areas. Upslope Fog forms as moist air flows uphill and is cooled. It is common in mountain areas and is associated with anabatic winds. Evaporation Fog results when moisture is added to cold air such as when cold air moving over water. The cold air moves over a warm surface, therefore this air is unstable. Turbulence and icing conditions may also be present. Frontal Fog forms as moisture is added via rain or drizzle to cold air. Finally, Ice Fog results when ice crystals form by sublimation when cold air passes over water. Other obscuring phenomena are smoke, smog, haze (problem only in stable air), and Whiteouts (snow).