Condensation occurs when the air cools

How is fog created?

Autumn time is foggy time. The phenomenon that sometimes blocks our sight can occur at any time of the year. For the water vapor in the air to condense into droplets, the air must either cool down or the humidity increase.

In principle, fog is nothing more than a cloud on the earth's surface: countless water droplets float in the air and reflect light. In this way, they “obscure” our view. Fog always looks the same, but it can arise in different ways. In any case, the relative humidity - the ratio between the current and maximum possible water vapor content - must reach one hundred percent so that the water vapor condenses more intensely.

In autumn and winter, the nights are longer than the days, so the air can cool down for a long time. The relative humidity then increases, because cold air can absorb less water vapor than warm air. When the air reaches a characteristic temperature at night - called the dew point - the water vapor begins to condense. The fog is there promptly. Fog can also form if the temperature drops for other reasons. For example, after a hailstorm in summer, it takes a while for the grains of ice to melt. This is how long they lie on the floor and cool down the humid ambient air. In doing so, they often create flat clouds of fog.

A special form of the cooling mist is linked to the wind. If the wind brings moist air over a colder surface, so-called advection fog can occur. The phenomenon can be observed, for example, in autumn on the German North or Baltic Sea coast, when the wind is blowing from the sea towards the coast and the land area has cooled down significantly beforehand. The reverse case occurs on the Pacific coast of Peru and Chile. There, warm air from the continent often flows over the cold water of the Humboldt Current. Another special form of cooling mist occurs in the mountains: If moist air glides up on a mountain flank, the air pressure and temperature drop. As soon as condensation occurs, uplifting fog is created.

If water evaporates from a lake or a river into cold air, fog can also form. This phenomenon, which is typical in autumn, is known as sea smoke. Evaporation mist can also be observed in summer, for example over the surface of a still warm street that has been dampened by a brief rain shower. In addition, weather fronts can trigger this type of fog: If, for example, rain falls on a warm front into an air mass close to the ground that is still cold, characteristic clouds of fog often form due to evaporation of the rainwater.

Mixing mist

Another form of mist is created when two humid air masses mix. The maximum amount of water vapor that air can hold increases roughly exponentially with temperature. Assuming the two clashing air masses have a temperature of 10 and 20 degrees Celsius and the relative humidity is below one hundred percent, then there would be no fog. As soon as the air masses mix, however, fog forms. Because for the temperature of the mixed air - namely 15 degrees Celsius - the maximum absorption capacity of water vapor is exhausted. The air is saturated or even oversaturated and condensation occurs.

On a small scale, everyone can produce mixture mist themselves. It is enough to breathe out into the frosty air on a cold winter day. The water vapor condenses immediately in the mixture of breathing air and ambient air. So the little cloud that arises is nothing more than a whiff of mixed fog. In normal weather conditions, however, mixed fog is very rare. It is most likely to be observed on the coast of the sea. There, warm, humid air from the sea occasionally mixes with cold air from the land.

In all the forms of fog described so far, it is droplets of liquid water that cause the phenomenon. Even below freezing point, the water droplets in the air initially remain liquid. One speaks of undercooled water. At moderate minus temperatures, the droplets can only freeze on solid surfaces. When it is very cold, however, fog can also consist entirely of ice crystals. To do this, the temperature has to drop below minus 35 degrees Celsius. Ice fog therefore occurs mainly in the polar regions.