The diameters of the droplets inside the fog are significantly lower than in a typical cloud of a few hundredths of a millimeter, but due to the different condensation nuclei they also vary greatly between individual droplets. Their size determines whether the flame cries or not. If it is slightly damp it will average droplet size of 10 to 20 microns, in dense fog it will most likely be between 20 and 40 microns.

Unwanted fog can also be eliminated: a process that is hardly used today is on-route heating, that is, simply warming up airport runways to achieve the dissolution of fog due to higher air temperatures near the ground. This is promising only with a small thickness of fog and at the same time low wind speeds, but is hardly used today due to high energy costs. Another way is to go the other way around, trying to increase the size of the droplets inside the nebula so that it drips.

For this purpose, liquid or solid propane or carbon dioxide is used, which causes heat to evaporate, lowering the air temperature and thereby increasing condensation.


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This institute organizes the training of doctors, mediating between the wishes of the pharmaceutical industry, doctors, the law and patients. In conversation we learn about his work, his origins and a world unknown to us. Geotechnical engineer Johannes Pistrol wrote his thesis on oscillating earth drums. At the same time, it measures soil conditions during operation. The goal of the study is to better understand the relationship and ultimately reduce wear and tear. He'd like to do a "Physical Evening" episode with me. What is a landslide and what are its characteristics is related in this issue of "Physical Order by Michael Lotter" of the Federal Institute of Geology in Vienna.

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In autumn you sometimes pass through clouds. Because at this time of year it is often very foggy. A white layer of cotton cloud lies above the ground, so you cannot see far. Have you ever wondered where it came from? In fact, fog is nothing more than a cloud on the ground. In autumn it is often quite warm during the day. It gets really cool at night in the fall. The soil then releases the heat it absorbed during the day. It is often cold in the morning, especially after clear nights, which cools the air above it.

It is then colder than the air layers above. There is always humidity in the air. However, cold air can absorb less moisture than warm air. For this reason, wet laundry does not dry in cold air, as well as in warm air. Therefore, the cold layers of air above the ground must be part of the water. In the form of tiny particles of water that are so light that they do not sink to the ground, but float in the air.

Cologne Cathedral disappears into the fog. You can also see condensation when you boil water: water vapor rises from the boiling water into the air. Since the windows are cooler than the warm air in the room, water vapor settles there like droplets. As cold air above the earth's surface rises and cools it, more and more water droplets and more fog form - until the sun becomes strong during the day to clear the fog. Because when the air is heated by the sun, it can again pick up tiny droplets of water.

You especially often see fog on lakes or rivers. In autumn, the water is warmer than the air above it. Cold, dry air is heated near water, so it absorbs moisture from the water and rises. At the top, it again falls on colder air layers, and some of the water vapor condenses with water droplets.

In summer, however, the sun shines more and the nights are not very cold. Therefore, the air is always warm enough to absorb moisture - there is no fog. The valleys often experience early morning fog in late summer and early autumn; in autumn, the fog season really begins.

Typical distribution of morning fog

Fog in the morning is especially common in Austria along the Mur and Mürz, as well as in the Carinthian valley and basin. Alpine valleys tend to be more susceptible to morning fog than flat, hilly country. In Germany, days are often foggy, especially in the valleys of low mountain ranges, and fog is widespread from Lake Constance along the Danube to Lower Bavaria. In Switzerland, the Alpine valleys and the Central Plateau are susceptible to fog.

In late summer and autumn, the days become shorter and the nights longer. In calm conditions and clear nights, the air cools greatly and collects in the valleys. Increasingly, it forms a kind of cold air lake; it is cooler than on the mountains. In addition, cold air can absorb less moisture and thus quickly become completely saturated with water vapor.

As a result, the moisture contained in the air begins to condense, that is, transform from a gaseous state to a liquid one. The resulting small droplets of water are called mists. As autumn progresses, the fog fields become more and more established and harsher as the sun no longer provides the energy needed to "heat up". Lakes of cold air may continue to develop, further increasing the likelihood of fogging.

Fog dissolution usually occurs only when strong winds "blow" cold air at ground level. This often occurs in the event of cold fronts or blow dryers. But even the cloud layer above the fog is enough for the fog fields to be cleared. Wind especially is the reason why flat plains are usually less prone to fog.

Briefly: the reasons why later years and autumns are more foggy in the valleys - these are large temperature differences between the surface and higher layers of air and the lack of wind. Fog is one of the most unpopular meteorological phenomena because it is perceived as a nuisance Everyday life. It has become detached from the road and often causes depressive moods due to lack of light if it comes for a long time in the fall. But what is fog and how is it created? Strictly speaking, fog is understood as atmospheric turbidity through very small drops of water when horizontal visibility drops to less than one kilometer.

If the visibility is between one and four kilometers, it is called haze. If the fog is distributed horizontally, there are fog banks or fog. The day when fog occurs is called a foggy day. To do this, visibility must drop below one kilometer at a time.

Air contains moisture in the form of water vapor. Because water vapor - also called water gas - is invisible, the air can contain large amounts of moisture without limiting visibility. This opacity of the atmosphere, which is often found on summer days, is called wet haze. Wet vapor usually occurs when the first droplets of water begin to form on aerosols in the air. Aerosols are the best particles of organic and, thanks to man, inorganic nature. Soot particles from traffic, industry and internal fire.

If visibility is between one and four kilometers, then there is fog, including fog. For fog to form, the humidity must rise to 100 percent, then water vapor begins to settle on the aerosol particles to the finest droplets and gradually the smallest drops of water rise, which naturally include the aerosol particle. The temperature required to create fog is called the dew point. Depending on how hot or cold the air is and how much moisture it contains, the dew point is higher or lower.

Warm air can contain more water vapor than cold water. So if you cool the air, the relative humidity increases. With a humid 30 degrees and 80% relative humidity, the air would only need to be cooled to about 26 degrees, so the fog falls. At a chilly 10 degrees and relative humidity of only 35 percent, the dew point will be around -5 degrees. Cooling can be accomplished by nighttime thermal loss of air by radiation into space or by introducing cold air.

But even when warm, moist air over cold water surfaces or layers of cold air glides. Accordingly, one can classify a nebula according to its origin as radiation nebula, advection nebula, marine nebula, high nebula, etc.

The following picture shows the fog in Ukrainian Carpathians, crowded with mountain range. Basically, both of them are the same. If a cloud forms covering a mountain peak, then the hiker at the top will be in fog, provided that visibility drops below one kilometer. Therefore, first of all, the question of location always arises, and then only after its development. However, some clouds have different causes than fog and also differ in the shape of the fog. Also, the size of the droplets differs from the clouds from the nebula, where it is much smaller.

Expert knowledge for slant reader

If the fog is not near the ground, but something higher, it is called high fog. It's a pleasure when moist warm air flows over cold air near the ground. The following picture shows high fog in New York. Homogeneous nucleation, i.e. the precipitation of tiny drops of water from moist air without condensation nuclei never occurs in nature. Firstly, the atmosphere is not pure, but contains many impurities, and secondly, spontaneous homogeneous generation can only be caused in laboratory conditions.