Heavy fog lingers in Switzerland, but it can still dissipate

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Much of the Swiss plateau is once again covered in gray. Fog soup affects many people's psyches, and some wish they could simply blow the fog away or dispel it in other ways. Although the number of foggy days has decreased significantly in recent decades, the situation does not appear to be improving.

What is fog?

Before discussing the question of whether and how to eliminate gray soup, we should first clarify what exactly fog is. Visible fog is actually made up of tiny water droplets from low-level clouds; these droplets are smaller in diameter than a human hair and ten to a hundred times smaller than raindrops. Because they are so small, they float in the air and then fall to the ground extremely slowly.

The gray blanket over the central region is heavy fog, which usually occurs during temperature inversions. The cold air then acts like a lake in the depression beneath the warm air, unable to escape due to low winds and high pressure. The inversion layer acts like a barrier, preventing vertical air exchange, which is why dust and soot accumulate in the cold air near the ground, acting as condensation nuclei for water vapor.

When the layer of air near the ground cools at night, tiny water droplets form on these condensation nuclei, creating fog. Its upper limit is usually consistent with the upper limit of the air-conditioned lake. When solar radiation is weak, fog is not always cleared during the day. Foggy days most commonly occur in autumn, when the air becomes significantly cooler at night and the humidity is higher.

In fact, we only talk about fog when the visibility is below one kilometer; however, non-meteorologists usually only consider visibility below 300 meters as fog. If the visibility is one to four kilometers, it is called haze.

Admire the fog-shrouded Zurich Oberland at night from Bachtel. Image: KEYSTONE

Can it be defogged?

Unfortunately, there is no antidote to the gray clock that sometimes hangs over our heads for days. Removing fog can only be done in a very limited localized area and is very costly. These methods either allow water droplets to grow to make it rain, or they evaporate them by heating them.

In the first case, the mist is “inoculated” with chemicals, which means additional condensation nuclei are injected, creating larger droplets. In the past, the second method was sometimes used when heating airport runways. However, this is only possible if the fog layer is not too thick.

Dylan Nesh's crash site.Image: KEYSTONE

If the fog could be made to evaporate over a large area, surprisingly little energy would be required. Professor Werner Eugster, a geographer and environmental scientist at ETH Zurich and a recognized fog expert who died in 2022, made a rough calculation of Watson's 2020 situation. Jurgst limited himself to a purely hypothetical question, namely, how much energy is required to evaporate fog droplets—if only the energy required to evaporate liquid fog is considered.

In his calculations, Eugster assumed an average value of 0.25 grams of fog water per cubic meter of air – by the way, the air contains ten times that amount of water in the form of invisible water vapor – and estimated the heat of vaporization to be about 2.502 trillion Joules per kilogram of water.

The fog layer will rise

Since the fog layer is 50 meters thick, the energy requirement of Yogst is approximately 31.3 megajoules per square kilometer. This is roughly equivalent to the heating energy contained in 7.4 liters of heating oil. If the fog layer is 100 meters thick, 14.8 liters of heating oil will be produced per square kilometer.

That's not much, Eggster said. Therefore, experts point out that an increase in surface energy turnover may be responsible for the high fog no longer touching the ground in some places, as was the case before. According to Jugster, it is also not possible to use the entire heating energy to evaporate the fog droplets in our calculation example. It causes a slight rise in air temperatures, which pushes the fog layer upwards slightly.

Incidentally, Jugster knows that this is exactly what is reported in the preferred residential areas of the southern foothills of the Jura, the Zugland and the Alpine foothills: 50 years ago, the views here still offered unobstructed views of the sea. Fog, people are more often caught in fog today. A height difference of 50 meters is enough.

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In this slide, there are about two basic tones: sun or vegetation giving the image a yellowish tint – like in Michael Ruby's photo – or, more obviously, fog giving the image a variety of Gray tone.