How do clouds form in the sky?
Every cloud starts as a tiny speck of dust, salt, or smoke floating in the air.
Clouds form when water vapor clings to microscopic particles called aerosols. These tiny seeds provide a surface for water to condense into liquid droplets. Without them, clouds could not easily form even in humid air. When billions of these moisture-covered specks gather together, they create the visible clouds we see in the sky.
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Cloud formation requires Cloud Condensation Nuclei (CCN), which are microscopic particles typically measuring only 0.2 micrometers in diameter. These particles can include sea salt from ocean spray, volcanic ash, forest fire smoke, or even organic compounds from trees. Without these 'seeds,' air would need to reach a relative humidity of about 400% before water droplets could form spontaneously through a process called homogeneous nucleation.In the natural atmosphere, humidity rarely exceeds 101%, so aerosols are essential for cloud development. Research conducted by NASA and the National Oceanic and Atmospheric Administration (NOAA) shows that the concentration of these particles directly affects cloud brightness and lifespan. For example, the 'Twomey effect' describes how higher concentrations of aerosols lead to smaller but more numerous water droplets, making clouds appear whiter and more reflective.The study of these particles is a major focus of the Intergovernmental Panel on Climate Change (IPCC) because they influence the Earth's energy budget. Human activities, such as burning fossil fuels, have significantly increased the number of aerosols in the atmosphere since the Industrial Revolution. This increase can lead to 'cloud brightening,' which reflects more sunlight back into space and partially offsets global warming, though it also alters local rainfall patterns.
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FP-0001649 · Feb 15, 2026