| Clouds are an important component of the | | | | The more the air cools down, the closer it gets to |
| hydrological cycle. The heat of the sun | | | | the temperature at which the water vapour in |
| evaporates daily enormous amounts of water | | | | the air turns into water droplets, that is to say, |
| from both the oceans and the continents. This | | | | the condensation occurs. However, it occurs only |
| water rises in the form of vapour into the | | | | if the air contains condensation nuclei on which the |
| atmosphere. In this way, over 400 billion tonnes | | | | water can settle. The nuclei are fine particles, |
| of water are moved every year. | | | | which are either marine crystals transported |
| The shape and the altitude of the clouds are | | | | across the land by wind, or particles originating on |
| indicators of air temperature and humidity. The | | | | the surface of the rock or in exhaust gases. |
| movement of the clouds indicates the direction of | | | | The most important water-absorbing particles |
| the wind and its speed. It is often possible arrive | | | | (hygroscopic particles) are salts, sulphuric acid, and |
| at an accurate weather forecast by correctly | | | | nitric acid. The number of condensation nuclei |
| interpreting the behaviour of the clouds. | | | | differs from region to region. Above the ocean, |
| The cloud consists of accumulated water droplets | | | | we find only several hundred in a cubic |
| and ice crystals. The diameter of the liquid | | | | centimetre, while above an industrial city as, for |
| particles is between 0.005 and 0.05 millimetres. | | | | example, Essen, there may be as many as |
| When the temperature cools down considerably, | | | | several hundred thousand. During condensation, |
| these particles freeze. | | | | every one of these particles is covered by a thin |
| Clouds form only when the water vapour | | | | film of water, forming water droplets. If the |
| condensates. Humid air rises from the earth's | | | | vertical current of the air is strong or weak, small |
| surface. This occurs during the uplift of the air | | | | cumulus clouds will form. When there is a strong |
| against mountain ranges, at low air pressure, or | | | | horizontal air movement, extensive layered clouds |
| when the surface of the earth is heated by | | | | will form. |
| sunlight The warmer air becomes lighter and rises. | | | | We distinguish a total of ten types of clouds, |
| In higher altitudes, the air pressure decreases. The | | | | which are divided into three cloud layers. They are |
| rising air expands. It cools down during the | | | | found in the troposphere, which is the part of the |
| expansion at a rate of about 0.6 degrees Celsius | | | | atmosphere where most of the meteorological |
| every 100 metres. | | | | phenomena occur. |