line connecting points of equal cloud cover
• (a.) Having, or indicating, an equal amount of cloudiness for a given period; as, isonephelic regions; an isonephelic line.
I`so·ne·phel'ic adjective [ Iso- + Greek ... a cloud.] (Physics Geology) Having, or indicating, an equal amount of cloudiness for a given period; as, isonephelic regions; an isonephelic line. 
isonephelic An indication of the equality of cloudiness.