When water evaporates, water vapour enters into the atmosphere. The air can only take up a certain amount of water. As soon as the dew point (maximum water absorption capacity of air) is reached, the relative humidity reaches 100%. Important: The warmer the air, the higher the dew point (the more water vapour can be taken up by the air). The dew point is usually expressed as temperature (°C).
If the air temperature falls below the dew point, the water vapour begins to condensate. A practical example: after a long warm shower in the bathroom, often the windows and mirror become "wet". Air cannot take up all the water vapour, which then settles in such a way on colder surfaces. The formation of clouds is not much different. In the air, many small so-called "condensation nuclei" (or "Cloud Seeds") float around. Those are tiny small particles, which we can not see, e.g. dust, soot, at also salt particles when close to the sea.
On these cloud seeds the water droplets start settling. The resulting droplets are average 0.001 millimeters in size. If many of these droplets accumulate, clouds develop.
How does a cloud then develop: Warm air rises by heating up in the sunlight and cools down after rising some kilometers again. The dew point is lowered by the cooling of air. Since the quantity of the water vapour in the air remains the same, the condensation point is reached at some stage and vapour begins to condensate. A cloud develops.
Therefore clouds also predominantly develop with warm and cold fronts
, where larger temperature differences occur.
Clouds have an enormous effect on the weather. For example, if a thick cloud layer draws up in the summer, the air becomes noticeably colder, because the sunbeams cannot pass the clouds or air is not anymore heated. At the night, the opposite happens. In a clear nights it is colder, because the heat radiates away into space. If the sky is covered at night, then the majority of the heat between ground and cloud remains below the clouds.
Clouds are also important for the water regime of the earth, since they are the fastest route of transportation of the water.
The main cloud types, their forms and interpretations are described on this page