What they are

Clouds are formed by microscopic drops of water or by small ice crystals. The size of the droplets generally range from a couple of microns to 100 microns: this is the limit beyond which cloud drops become rain drops. The shape of the drops is usually spherical, but it can vary, especially in bigger drops that are deformed by gravity.
Water in the atmosphere: evaporation and condensation
The evaporation of water bodies and evapotranspiration from soil and vegetation supply the atmosphere with great quantities of water vapour (vedi sezione acqua). Even though it is abundant, the amount of water vapour supplied to the atmosphere by evaporation processes generally is not sufficient to make the air reach saturation point. Saturation and the consequent condensation of vapour into drops of liquid water are therefore generally caused by the cooling of air masses at a temperature below condensation, also known as dew point. When condensation takes place, a part of the excess water vapour changes from the gaseous state to the liquid state, forming microscopic drops of water. For condensation to occur, however, it is not sufficient to reach the dew point: it is necessary that the so-called condensation nuclei should be present. These are small particles (whose dimensions range between 0.001 and 10 micron) on which the tiny drops of water can condense. The condensation nuclei normally present in the atmosphere are sodium chlorides, sulphates, carbonaceous particles and atmospheric dust and can be present as a result of natural causes (as for example, marine aerosols transported by the wind or ashes coming from volcanic activity)  or due to human activities (the result of the combustion of fossil fuels, for example). The bigger and more hygroscopic (i.e. capable of attracting water molecules) the nuclei are, the more effective is their role in favouring condensation processes.
If the temperature is low, water condenses in the form of ice crystals, but it still requires the above-mentioned nuclei that, in this case, are called crystallization nuclei.

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