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]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "rainfall":
   Scotch mist, blood rain, bogginess, damp, dampness, dewiness,
   drizzle, evening mist, exudation, fall, flurry, fogginess,
   gout of rain, humectation, humor, marshiness, mist, mistiness,
   misty rain, mizzle, moistiness, moistness, moisture, patter,
   pitter-patter, precipitation, rain, raindrop, raininess, rainwater,
   sheet of rain, shower, showeriness, soddenness, sogginess,
   soppiness, splatter, sprinkle, swampiness, unfrozen hydrometeor,
   wateriness, wet, wetness, wettishness