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Fog Types
Fog is classified according to the manner in which it forms and is dependent upon the current temperature and the amount of water vapour in the air.
- Radiation Fog: On clear nights with relatively little to no wind present radiation fog may develop. Usually it forms in low-lying areas like mountain valleys. This type of fog occurs when the ground cools rapidly due to terrestrial radiation, and the surrounding air temperature reaches its dew point. As the sun rises and the temperature increases, radiation fog lifts and eventually burns off. Wind will also cause radiation fog to dissipate.
- Advection Fog: When a layer of warm, moist air moves over a cold surface, advection fog is likely to occur. Unlike radiation fog, wind is required to form advection fog. Winds of up to 15 knots allow the fog to form and intensify. Above wind speeds of 15 knots, the fog usually lifts and forms low stratus clouds. Advection fog is common in coastal areas where sea breezes can blow the air over cooler landmasses.
- Upslope Fog: Upslope fog occurs when moist, stable air is forced up sloping land features like a mountain range. This type of fog also requires wind for formation and continued existence. Upslope and advection fog, unlike radiation fog, may not burn off with the morning sun but instead can persist for days. They can also extend to greater heights than radiation fog.
- Steam Fog: Steam fog, or sea smoke, forms when cold, dry air moves over warm water. As the water evaporates, it rises and resembles smoke. This type of fog is common over bodies of water during the coldest times of the year. Low-level turbulence and icing are commonly associated with steam fog.
- Ice Fog: Ice fog occurs in cold weather when the temperature is much below freezing and water vapour forms directly into ice crystals.
- Fontal Fog: With light winds, fog may form instead of stratus cloud in the area of continuous precipitation preceding a warm front or TROWAL. The fog will advance with the system and dissipate as the front passes.