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Radiation fog occurs at night and usually does not last long after sunrise, but it can persist all day in the winter months especially in areas bounded by high ground. Radiation fog is most common in autumn and early winter. Examples of this phenomenon include tule fog. [19] Ground fog is fog that obscures less than 60% of the sky and does not ...
For example, in the range of normal temperatures, air at 68 °F (20 °C) and 50% relative humidity will become saturated if cooled to 50 °F (10 °C), its dew point, and 41 °F (5 °C) air at 80% relative humidity warmed to 68 °F (20 °C) will have a relative humidity of only 29% and feel dry.
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A given value of relative humidity causes larger increases in the heat index at higher temperatures. For example, at approximately 27 °C (81 °F), the heat index will agree with the actual temperature if the relative humidity is 45%, but at 43 °C (109 °F), any relative-humidity reading above 18% will make the heat index higher than 43 °C. [5]
Tule fog is a radiation fog, which condenses when there is a high relative humidity (typically after a heavy rain), calm winds, and rapid cooling during the night. The nights are longer in the winter months, which allows an extended period of ground cooling, and thereby a pronounced temperature inversion at a low altitude.
Another way fog forms in deserts occurs when a desert is close to an ocean which has a cold current. When air is heated over desert land and blows towards the cool water in the ocean, it condenses and fog is formed. The cool fog is then blown inland by the ocean breeze. Fog is mainly formed in the early morning or after sunset. [5]
Thus, fog and mist are not precipitation; their water vapor does not condense sufficiently to precipitate, so fog and mist do not fall. (Such a non-precipitating combination is a colloid .) Two processes, possibly acting together, can lead to air becoming saturated with water vapor: cooling the air or adding water vapor to the air.
Clouds and potentially fog can form within a marine layer when the water-saturated air is cooled and reaches a humidity of 100%, where it will then condense and turn into water droplets. [3] Stratus and stratocumulus can form at the top of a marine layer in the presence of these conditions. A marine layer can often be dispersed by sufficiently ...