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Clouds are collections of water droplets or ice crystals in the air. They accumulate when the air can no longer take in any more moisture because it is already saturated. This saturation point depends on the temperature in the air. Warm air can take in more moisture than cold air. Thus if warm air rises and cools down, clouds
accumulate when they are able to take up a sufficient amount of moisture beforehand.
The clouds are usually in the troposphere, the lowest layer of the earth’s atmosphere. It reaches from the ground up to approx. 8 km at the poles and 18 km at the equator. Approx. 90% of the air and almost the complete amount of water condensation are in this belt of air. The next layer at approx. 50 km is called stratosphere. While the temperature in the troposphere with a greater height falls to approx. -60° C it goes
back up to approx 0° C in the stratosphere. The result of the low temperature at the border to the stratosphere is that the air there is very dry. That is why one rarely finds clouds in the stratosphere.
A cloud is a mixture of air and very fine water droplets. Usually these water droplets only have a diameter of two to ten micrometers. Only when they reach the rain clouds the droplets become bigger and can reach a diameter of approx 2 mm. The droplets become larger when they can take up more water molecules than they give off. Due to the fact that the speed at which these droplets sink depends on their size, there is rain when the droplets have been able to grow accordingly and become heavier. Not only the air movement plays a role, because it is not only horizontal winds but up winds and down winds. When the down winds are relatively strong they can transport the droplets back up again where additional water can be deposited. Usually the
temperature is so low that ice has already formed. Then the deposited water freezes, the ice kernel becomes larger and ultimately falls to the ground as hail. That is why hail is larger than water drops.
Clouds are not only the point of origin for precipitation. They also are important in that they determine the amount of sun rays that reach the earth. Clouds also block radiation from the earth, for instance during the summer, when it is less hot when there are clouds in the sky than when there are no clouds and it is clear. They also have an impact on light; it can become as dark as night when there are thunder clouds at noon.