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   Clouds, fog & water droplets 




  
Spring Cloudscape NW England.   Photo ©Les Cowley

  Cloud droplets and two raindrops. The tiny cloud droplets are shown here close together but in reality they are on average ~1 mm apart, some 100X their diameter.

Surface tension forces keep the droplets fiercely spherical.

They fall so slowly that air currents keep them aloft almost indefinitely.
  

Cloud, fog and mist droplets are very small. Their mean diameter is typically only 10-15 micron (1 micron = 1/1000 mm) but in any one cloud the individual drops range greatly in size from 1 to 100 micron dia. Cloud droplets are 10 to 1000X smaller than raindrops.

A cloud typically contains a few hundred droplets per cc. They are mostly far apart from each other and those smaller than 20 micron diameter rarely collide and coalesce.

In all but thunder clouds, the droplets make up less than a millionth of the volume. Most water is actually present as water vapour in the air surrounding the droplets.

Clouds appear substantial and opaque because the total number of droplets is immense.  Light rays are scattered after travelling just a few metres and most emerging rays will have been scattered several times.  Sunlit clouds appear white because, overall, their droplets scatter the different wavelengths of visible light equally and absorb little light in the process. Colourful coronae, glories and ghostly fogbows show up when the cloud or fog has similar sized droplets and when the cloud is thin and lighting conditions are such that light rays are only scattered once.