Translations of Encyclopedia about Geology

How Clouds Form

Clouds are an important component of the hydrological cycle. The heat of the sun evaporates daily enormous amounts of water from both the oceans and the continents. This water rises in the form of vapour into the atmosphere. In this way, over 400 billion tonnes of water are moved every year.

The shape and the altitude of the clouds are indicators of air temperature and humidity. The movement of the clouds indicates the direction of the wind and its speed. It is often possible arrive at an accurate weather forecast by correctly interpreting the behaviour of the clouds.

The cloud consists of accumulated water droplets and ice crystals. The diameter of the liquid particles is between 0.005 and 0.05 millimetres. When the temperature cools down considerably, these particles freeze.

Clouds form only when the water vapour condensates. Humid air rises from the earth's surface. This occurs during the uplift of the air against mountain ranges, at low air pressure, or when the surface of the earth is heated by sunlight The warmer air becomes lighter and rises. In higher altitudes, the air pressure decreases. The rising air expands. It cools down during the expansion at a rate of about 0.6 degrees Celsius every 100 metres.

The more the air cools down, the closer it gets to the temperature at which the water vapour in the air turns into water droplets, that is to say, the condensation occurs. However, it occurs only if the air contains condensation nuclei on which the water can settle. The nuclei are fine particles, which are either marine crystals transported across the land by wind, or particles originating on the surface of the rock or in exhaust gases.

The most important water-absorbing particles (hygroscopic particles) are salts, sulphuric acid, and nitric acid. The number of condensation nuclei differs from region to region. Above the ocean, we find only several hundred in a cubic centimetre, while above an industrial city as, for example, Essen, there may be as many as several hundred thousand. During condensation, every one of these particles is covered by a thin film of water, forming water droplets. If the vertical current of the air is strong or weak, small cumulus clouds will form. When there is a strong horizontal air movement, extensive layered clouds will form.

We distinguish a total of ten types of clouds, which are divided into three cloud layers. They are found in the troposphere, which is the part of the atmosphere where most of the meteorological phenomena occur.

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