The Collaborative International Dictionary of English v.0.48:

Isonephelic \I`so*ne*phel"ic\, a. [Iso- + Gr. ? a cloud.] (Phys.
   Geog.)
   Having, or indicating, an equal amount of cloudiness for a
   given period; as, isonephelic regions; an isonephelic line.
   [1913 Webster]