The Collaborative International Dictionary of English v.0.48:

Hypaethral \Hy*p[ae]"thral\, Hypethral \Hy*pe"thral\, a. [L.
   hypaethrus in the open air, uncovered, Gr. ?; ? under + ?
   ether, the clear sky.] (Arch.)
   Exposed to the air; wanting a roof; -- applied to a building
   or part of a building. --Gwilt.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hypethral
    adj 1: partly or entirely open to the sky [syn: hypaethral,
           hypethral]