THE MEXICAN MONSOON

The pronounced maximum in rainfall during the warm season over southwe stern North America has been noted by various investigators. In the Un ited States this is most pronounced over New Mexico and southern Arizo na; however, it is but an extension of a much larger-scale phenomenon that appears to be centered over northwestern Mexico. This phenomenon, herein termed the ''Mexican monsoon,'' is described from analyses of monthly mean rainfall, geostationary satellite imagery, and rawinsonde data, In particular, the authors note the geographical extent and mag nitude of the summer rains, the rapidity of their onset, and the timin g of the month of maximum rainfall. Finally, the difficulty in explain ing the observed precipitation distribution and its timing from monthl y mean upper-air wind and moisture patterns is discussed.