Fog is a phenomenon that occurs in the layer of air just above the earth’s surface. We talk about fog when water vapor condenses in the form of droplets (ice crystals may also appear in frost) limits horizontal visibility to less than 1 km. If visibility is limited, but we can still see objects located more than 1 km away, then we are talking about fogging. Water droplets in fog are of a similar size to those in clouds, i.e. from 5 to 20 micrometers.
Fog occurs when the air temperature at the earth’s surface drops below the dew point (temperature at which the gas liquefaction process can begin). It is worth remembering this term, it will appear several times below. Basically, we can distinguish six types of fog: radiative, advective, frontal, mixing, evaporative and orographic.
Radiation Fogs they arise as a result of heat radiation from the ground, usually at night or in the morning. They are favored by cloudless weather, high water vapor content in the air and low wind speed (but not its complete absence). Their thickness may range from several to even several hundred meters. These high (and at the same time long-lasting) ones occur in mountain valleys and valleys. These fogs are the most picturesque when they occur over wet meadows and forest clearings, and in the mountains they create a “sea of fog”.
Advection fogs are created as a result of the inflow (advection) of wetter, warmer air over the cooler ground. The air then cools down and reaches the dew point. It reaches at least 10 m above the ground, and sometimes 1-2 km. They form mainly in winter. Then the warm air from the sea (which releases heat for a long time after summer) flows over the quickly cooled land.
Front fog they arise at the surface of atmospheric fronts (usually warm ones). After passing through them, due to turbulent mixing of air, it is cooled down from the ground. Humidity is approaching saturation. When the dew point is reached, condensation begins and fog forms.
Mists from mixing they form regardless of the fronts, often where there is a large temperature difference (e.g. in the region of subpolar seas, on the border of the Labrador Current and the Gulf Stream). Due to its characteristic appearance, this type of fog is sometimes called sea or lake smoke.
Evaporation mists they arise over relatively warm open waters (lakes, but also rivers). Water evaporating from their surface rises as water vapor and then condenses in the warmer air. Their method of formation is similar to the genesis of advection fogs. In this case, the main role is played not by advection (inflow) of air, but by the heat capacity and thermal inertia of water reservoirs (i.e. how quickly they lose the temperature supplied to them from the atmosphere by solar radiation). There may also be “smoking of the seas” here, which also looks extremely picturesque and attracts many photographers.
Orographic fogs they are created when air rises up a mountain slope. The expanding air then cools down (often intensified by cold ground, e.g. covered with snow, with northern exposure or cooling during the night).
In most cases, fogs form in high latitudes as well as oceanic climates. They are more often found in mountain valleys and valleys, lake districts or in the humid tundra zone. We also see them more often at night and in winter, and less often in summer and during the day.
On the one hand, rising fog can be a beautiful phenomenon that encourages you to capture it in a photograph and is an inspiration for painters. On the other hand, it can make driving extremely unpleasant for drivers and make it easy to have an accident.
In Poland, fog most often appears in autumn. Then, on the one hand, we have a supply of moisture from water reservoirs still heated in summer (including the Atlantic; remember that water has a large heat capacity, i.e. it heats up more slowly, but also loses this heat more slowly), on the other hand – heat radiation, resulting in cooling. near-surface layer of the atmosphere. The greatest number of days with fog (from 4 to 12) is recorded in our country in Octoberoccur least often (d 1 to 3 days) in June. As for the regions, one of the most foggy places are: Nizina Wielkopolska (on average about 300 hours a year) and Śnieżka (115 days a year with all-day fog).
Source: K. Kożuchowski (ed.), 2007, Meteorology and climatology, Ed. Science. PWN
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