When fog appears, it is mainly in the form of two types, which appear and disappear in very different ways. The first is advection fog and the second is radiation fog. A third type - frontal fog - can be caused by stratus clouds close to the ground during warm fronts.
Advection nebula
In the case of advection fog, the temperature is lowered by air movement (= advection), i.e. wind. The air is transported from warm, humid areas to colder regions, where it then cools down. This usually happens when air masses are carried by south-westerly winds from the warm and humid Atlantic towards the north and north-east.
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The water is still very cold there, especially in spring and early summer. The incoming air cools down more and more to the dew point and the condensed water droplets gradually reduce visibility. The fog occurs at the boundary layer of warm and cold water and is therefore also known as sea fog. It disproves the assertion that fog can only exist when there is no wind.
Sometimes all that remains is to wait and see
Advection fog does not necessarily disappear on days with a weather situation that is usually characterised by low-hanging stratus clouds with generally poorer visibility. As south-westerly winds are often observed after the passage of a warm front, recreational boaters must wait until the subsequent cold front passes so that a right-hand wind from south-west to north-west can put an end to the fog. This is because cold, dry polar air then flows in from the north-west, which warms up en route and can still absorb moisture. Visibility after a cold front passage is therefore often good.
There are stretches of coastline in Europe, for example in the west of the Iberian Peninsula along the Atlantic, where a shift in the wind causes the fog to come and go within a short space of time. If the wind is westerly, fog can occur as long as it is slightly southerly. If, on the other hand, the wind shifts just a little to the north, the fog disappears again immediately. This interplay can repeat itself several times, much to the annoyance of pleasure boaters.
The good news is that although an advection fog situation may persist for days, visibility is often not quite as poor as with radiation fog (which is explained on the following page), so it is still possible to drive with caution.
Radiation fog
Radiation fog is also known as ground fog. It typically occurs in autumn and winter at high pressure, when a clear, cold sky allows the heat of the day to escape into space during a long, windless night.
The resulting drop in temperature mainly occurs over land, as it cannot store heat as well as water and releases it much more quickly. In inland areas, a cold, thin layer of fog forms near the ground, particularly over damp lowlands, depressions, valleys and river courses. Higher geographical locations and tall buildings often protrude from this sea of fog.
The sun burns away the fog
As cold air is heavier than warm air, it slowly flows down to the sea together with the fog during the late night and early morning hours. There, the fog bank can drift one to two nautical miles further out to sea until the water has warmed the flowing air to such an extent that the fog slowly dissipates.
The sun gaining strength in the morning will also contribute to this, meaning that visibility will usually be good again by late morning and a sunny, windless day can develop. If the high pressure situation persists, the process is likely to repeat itself in the same way the next night.
Front fog
There is a third form: frontal fog. It occurs during the passage of a warm front, when the lowest stratus clouds reach down to the water surface. It dissipates again immediately after the passage - unless it is directly replaced by advection fog.
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Leon Schulz
Freier Mitarbeiter
