The Collaborative International Dictionary of English v.0.48:

Rainfall \Rain"fall`\ (r[=a]n"f[add]l`), n.
   A fall or descent of rain; the water, or amount of water,
   that falls in rain; as, the average annual rainfall of a
   region.
   [1913 Webster]

         Supplied by the rainfall of the outer ranges of Sinchul
         and Singaleleh.                          --Hooker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rainfall
    n 1: water falling in drops from vapor condensed in the
         atmosphere [syn: rain, rainfall]