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radiation cooling. [For more on radiational cooling, see my background piece: Laying Some Groundwork:
Balancing Radiation.] A typical scenario for forming radiation fog would proceed in this manner.
Start with a clear day with moderate or high humidity and light winds. As the solar radiation diminishes at dusk, the surface begins to radiate heat away faster than it can gain it, and thus its temperature drops. The cool surface, in turn, cools the air in contact with it. Throughout the night, the surface and air will continue to cool; the degree of cooling depends on many factors including the day's maximum temperature and number of hours of darkness. If the air reaches its condensation, or dewpoint, temperature during that nocturnal cooling, fog will form. A wet surface -- moist soil or standing water -- significantly increases the chances of radiation fog formation, so the radiation fog potential is high after a rainfall, particularly if followed by a cold front which clears the skies and lowers the air temperature. Once formed, the fog itself becomes a heat radiating surface, which further cools the surface air layer and thus deepens the fog. The fog layer may vary in extent from shallow scattered patches formed in surface depressions to a general blanket as much as 300 metres (1000 feet) in depth. Visibility within the fog is less than 1 kilometre (5/8 mile) and further cooling makes the fog layer denser, often reducing visibility to the proverbial "hand in front of face." Often, and particularly in autumn when long nights are conducive to rapid and deep cooling, we see patchy fog in the countryside first forming in low areas of the terrain. These are preferred fog locations because cold air, being denser than warmer air, flows like water toward the lowest point in the terrain. Therefore, if there is any slope to the land, such as water drainage channels or hillsides, the cooler air will flow into the lowest elevation points. Fog will form there first because this air is cooler than the surrounding air. Fog forming on mountain slopes spirals and puffs like the woods are on fire. The dissipation of radiation fog occurs when the air is reheated (usually by the morning sun), or the wind increases and mixes the foggy air with warmer or drier air around it. Where fog is thin, morning solar radiation (excluding the weak mid-winter sun) can penetrate to the ground and heat the underlying surface, thus evaporating the fog from below. In summer, the short nights usually prevent nocturnal radiation fog
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