The fog can be thicker in certain places than in others due to topography, the presence of colder water, or the proximity of sources of water vapor, such as lakes or rivers.
The steep valleys and depressions often promote the accumulation of fog because cold air naturally descends into these lower areas, causing what is known as a temperature inversion. Due to this inversion, the cold air becomes trapped at the bottom, with humidity easily condensing into thick fog. In contrast, the tops of hills or wind-exposed slopes tend to be less affected. The relief also influences how air currents circulate: behind certain terrains or obstacles, there can be a stagnation of humid air, forming a localized and persistent light fog. The steeper the terrain, the more pronounced these differences can be, making the fog particularly dense in specific areas, while being almost absent just a few hundred meters away!
Small-scale weather changes play a huge role in fog thickness. For example, particularly cold air near the ground promotes faster condensation, resulting in thicker fog. Similarly, a complete lack of wind is ideal: it allows humid air to remain in place and gives the fog all the time it needs to become denser. Conversely, even a light breeze quickly disperses water particles and leads to a significant reduction in its intensity. Additionally, under a clear and open sky, the ground cools much faster at night, strongly emphasizing the phenomenon of radiative cooling responsible for dense fog in the morning. This particular combination of temperature, humidity, and air circulation makes certain places significantly foggier than others right next to them.
Cities often create their own fog due to pollution. Exhaust gases from cars and industrial smoke release fine particles into the air, which promote the formation of water droplets and thus thick fog. This phenomenon is even called smog. Power plants, especially those running on coal, significantly exacerbate this polluted fog, further reducing visibility. In contrast, green spaces in urban areas often help to slightly limit the thickness of the fog. Some agricultural practices also strongly exacerbate the problem: burning plant waste in the fields releases a lot of small particles that make fog layers denser.
Places with a lot of dense vegetation like forests retain coolness more effectively and often increase the ambient humidity levels. Trees, shrubs, and grasses constantly release water into the air through evapotranspiration, much like their own perspiration. This additional humidity promotes the formation and persistence of fog. The same idea applies to areas where the soils are particularly wet or marshy: the water contained in the ground gradually evaporates, enriching the air with moisture, which makes the fog even more persistent and thick in those areas.
Lakes, rivers, ponds, or even the sea significantly influence the thickness of fog. This phenomenon occurs mainly due to evaporation: when the water is warmer than the air just above, it releases a lot of humidity that cools rapidly as it rises. As a result, the vapor condenses and forms a thick fog just above or near the body of water. This effect is often more intense in autumn or early winter, when temperatures drop rapidly while the water remains relatively warm. Certain regions close to large bodies of water like lakes or seas almost consistently experience particularly dense foggy mornings, unlike areas far from water.
Do you know about fog nets? Used notably in Chile and Morocco, these devices capture the moisture contained in fog to supply drinking water to communities located in arid regions.
The densest fog ever recorded occurred in London in December 1952. Known as the 'Great Smog of London', it was primarily caused by pollutants and specific weather conditions, and it resulted in thousands of deaths in just a few days.
Did you know that even in the desert, fog can form? Coastal fogs, like those in the Namib Desert, bring enough moisture to support typical fauna and flora that depend on this precious resource.
Have you heard of the phenomenon called 'radiation fog'? It occurs when a moist ground cools at night under a clear sky, forming layers of fog that are more pronounced in valleys or topographical depressions.
Generally, fog is densest early in the morning or during the night, when temperatures drop and the condensation of humid air becomes significant. As the sun rises higher, the air gradually warms up and the fog tends to dissipate.
Yes, large cities can indeed promote the formation of denser fog due to several factors: air pollution acting as condensation nuclei that facilitate droplet formation, temperature differences caused by concrete and buildings, and the presence of human activities that increase humidity in the air.
Vegetation, particularly in wooded areas, contributes to higher atmospheric humidity through evapotranspiration. Thus, green and humid areas are often associated with denser and more frequent fog than other regions.
Yes, certain landforms such as narrow valleys, topographic depressions, or mountainous areas can promote the trapping of cool, moist air, leading to the formation of thicker and more persistent fog in these specific locations.
Fog forms more easily near rivers, lakes, and other bodies of water due to the constant evaporation that moistens the air near the surface. When this humidity-saturated air cools down, the water condenses, resulting in a thick layer of localized fog.
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