资料来源 : Webster's Revised Unabridged Dictionary (1913)

Cloudless \Cloud"less\, a.
   Without a cloud; clear; bright.

         A cloudless winter sky.                  --Bankroft.
   -- {Cloud"less*ly}, adv. -- {Cloud"less*ness}, n.

资料来源 : WordNet®

cloudlessness
     n : the lightness of a sunny day when there are no clouds in the
         sky