In polluted cities, why do the skies above the clouds still look clear and clean? Does polluted air do not reach above the clouds?

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In polluted cities, why do the skies above the clouds still look clear and clean? Does polluted air do not reach above the clouds?

In: Physics
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Visible pollution consists of soot and nitrogen dioxide, which are both heavier than the oxygen and nitrogen molecules that make up most of the air, so they tend to settle to the bottom. Other forms of pollution are not visible and not constrained in that way.

Also, when you see photos of extreme pollution, it is often caused by temperature inversions, where warm air sits on top of cold, trapping pollution in the lowest layers. Denver, Colorado sits in a valley and experiences frequent temperature inversions in the colder months, leading to its famous [Brown Cloud](https://live.staticflickr.com/7562/16015425212_d9827b793d_b.jpg), which is actually pretty atypical.