WordNet (r) 3.0 (2006):

air mass
    n 1: a large body of air with uniform characteristics
         horizontally

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "air mass":
   anticyclone, cold front, cold sector, cyclone, front, high,
   high-pressure area, isobar, isometric, isometric line,
   isopiestic line, isotherm, isothermal line, low, low-pressure area,
   occluded front, polar front, squall line, stationary front,
   warm front, weather map, wind-shift line