thunderhead

from WordNet (r) 3.0 (2006)

thunderhead
    n 1: a rounded projecting mass of a cumulus cloud with shining
         edges; often appears before a thunderstorm

from The Collaborative International Dictionary of English v.0.48

Thunderhead \Thun"der*head`\, n.
   A rounded mass of cloud, with shining white edges; a cumulus,
   -- often appearing before a thunderstorm.
   [1913 Webster]