Normally, water vapor can only condense onto condensation nuclei—tiny particles that serve as kernels around which drops can form.
Condensation nuclei are often nothing but natural dust. But soot particles from automobile exhaust or other types of pollution can also serve the purpose. One study has found that changing levels of air pollution cause different rates of cloud formation (and rain) on weekends and weekdays, at least in humid climates with lots of cities.
Photo Gallery: Clouds
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Photograph by John Leslie/Alamy
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