Lecture
Sunshine duration is the time during which direct sunlight illuminates the Earth's surface. Sunshine duration is one of the most important climatic elements. It depends on geographic latitude, season, cloudiness, and day length. It is measured either in hours per year or as a percentage of the maximum possible day length. In the Arctic it does not exceed 25%, in Italy it is 50%, and in the Arizona desert it is 97%. The annual total of sunshine duration in Moscow is 1600 hours, and in Central Asia it is 3000 hours.
Air often appears turbid due to the presence of various impurities and minute condensation products. These impurities scatter passing light and lead to a deterioration of visibility.
Haze is a turbidity of the air caused by microscopic droplets, crystals, or dust particles; visibility in haze ranges from 1 to 10 km.
If the turbidity of the air is caused not by condensation products but by solid particles (dust, particles of burned fuel), it is called mist (dust haze).
If visibility is less than one kilometer and the turbidity of the air is caused by an accumulation of condensation products (droplets, crystals), such turbidity is called fog. In some cases, visibility in fog decreases to just a few meters.
Fog occurs when favorable conditions for the condensation of water vapor arise at the Earth's surface. Approach to the saturation point occurs mainly as a result of the cooling of the air. A secondary role is played by an increase in air humidity due to evaporation from a warm surface into cold air.
Depending on the causes of formation, all fogs are divided into two types:
Cooling fogs – associated with radiative cooling of the air.
Evaporation fogs – evaporation of water vapor into cold air.
The first type of fog predominates.
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