In reality, the water droplets that make up the clouds do disperse, but the cloud has a lifetime. There are a series of physical processes in the atmosphere that favor the formation of clouds and others that favor their dispersion. Clouds are generally formed by updrafts of air. They are formed when the water vapor in the air cools. This happens sometimes in the ascent and sometimes in the collision of air masses, one colder and drier and another warmer and more humid. Under these conditions, the water vapor in the air mass cools and as the water vapor particles cool, they condense on aerosol particles. Aerosols are particles that can be desert dust, pollution, smoke, or other gases that condense. These particles are sometimes hydrophilic, that is, they attract water droplets. Because the nucleation of pure water, that is to say that the pure water condenses, never occurs in the atmosphere, since enormous vapor pressures would be required, which do not occur. That is why water clouds to form need those aerosols on which the water molecules are concentrated.
Once the cloud is formed, it will continue in the area as long as there are favorable conditions: that there is humidity, that the air rise is maintained and that the water drops do not weigh so much that they precipitate because if that happens, it rains and the cloud disappears.
Clouds are not static entities like rocks that remain on the ground and are always the same molecules and the same atoms. Clouds are more of a process. If this process remains stable, depending on the type of cloud, it can have a lifetime of a few hours or even days.
As for the different types of clouds, it depends, among other things, on the height. The lowest clouds are those that look like cotton. Depending on the height, the humidity flows and the temperatures are different and that leads to different types of clouds. The highest, from 5,000 meters high, are made of ice, they are cirrus clouds, those clouds that we see very frayed. Smaller clouds form when a warm air mass rises in a limited area. When there is a generalized rise of an air mass over a wide area, then strata form and we see the sky covered with finite layers. There are different types of clouds because there are different physical processes in the atmosphere.
Clouds do not always precipitate, many times they dissipate without it ever raining. Cloud droplets are microns in size. When they pass the millimeter is when they can begin to precipitate, but if they do not reach those sizes, the cloud keeps floating. The updraft is able to push these small droplets upwards and keep the cloud in suspension. And if the humidity conditions cease, the cloud will dissipate without causing precipitation.
Arantxa Revuelta is a doctor in Geophysics and Meteorology. He currently works at the State Meteorological Agency.
Question sent via email by Carlos Jose Cruz
Coordination and writing: Victoria Toro
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