Although it's not mentioned in the story, the clouds which
appear in this story are "nimbostratus".  Thus:


   The nimbostratus, formerly called nimbus, is any thick, 
   extensive layer of formless cloud from which rain 
   is seemingly on the point of falling.

   -- W. J. Humphreys _Physics of Air_ (1940)

They look like this:

   http://www.nxdc.com/weather/wip/nimbostratus.htm



Maybe I'll include them in the director's cut of the story.



--nev