altostratus

from WordNet (r) 3.0 (2006)

altostratus
    n 1: a stratus cloud at an intermediate altitude of 2 or 3 miles
         [syn: {altostratus}, {altostratus cloud}]

from The Collaborative International Dictionary of English v.0.48

altostratus \altostratus\, Alto-stratus \Al`to-stra"tus\, n. [L.
   altus high + L. & E. stratus.] (Meteor.)
   A cloud formation similar to cirro-stratus, but heavier and
   at a lower level; a stratus cloud at an intermediate altitude
   of 2 or 3 miles.
   [Webster 1913 Suppl. + WordNet 1.5]