The Collaborative International Dictionary of English v.0.48:

Alto-cumulus \Al`to-cu"mu*lus\, n. [L. altus high + L. & E.
   cumulus.] (Meteor.)
   A fleecy cloud formation consisting of large whitish or
   grayish globular cloudlets with shaded portions, often
   grouped in flocks or rows.
   [Webster 1913 Suppl.]