Definition

• a cloud that is on the surface
• usually occurs when the temperature of air near the ground is cooled to the air’s dew point

Types of fogs

• Radiation fog : On clear nights, with little to no wind, radiation fog develops. It usually forms in mountain valley. It occurs when the ground cools rapidly due to terrestrial radiation, and the surrounding air temperature reaches its dew point. 
• Advection fog : When a layer of warm, moist air moves over a cold surface, advection fog occurs. Wind(up to 15knot) is required to form advection fog. It is common in coastal areas where sea breezes can blow the air over cooler landmasses.
• Upslope fog : When moist, stable air is forced up sloping land features like a mountain range, upslope fog occurs. Wind is required to form it. 
• Steam fog : When cold, dry air moves over warm water, steam fog occurs. Steam fog is common over bodies of water during the coldest times of the years. 
• Ice fog : When the temperature s much below freezing and water vapor forms directly into ice crystals in cold weather, Ice fog occurs.