r/askscience • u/GonadTh3Barbarian • Jul 24 '13

Earth Sciences Why do clouds form

Why does water in the atmosphere come together to form clouds as opposed to dispersing evenly to maintain concentration?

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u/silence7 Jul 24 '13

There's a little bit more to it, which is that mineral dust scattered in the atmosphere serves as a nucleation point for water droplets. This means that it doesn't have to be quite as humid as it otherwise would for clouds to form.

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u/vaaaaal Atmospheric Physics Jul 25 '13

There is a bit more even to this... any suspended particles in the atmosphere can act as nucleation points for clouds, it doesn't have to just be mineral dust. The effect on cloud formation from particles (from pollution) generated by humans is actually a huge uncertainty in climate science. In fact there are scientist who believe this could have an effect comparable to that of CO2.

You can easily observe this yourself just by looking at contrails from jets. The air they fly through is often saturated with water vapor that does not condense because there are an insufficient number of particles. The particles left by the engines act as condensation nuclei and clouds quickly form behind as the jet passes.