144 THE FUTURE OF ARID LANDS 
of stations makes detailed studies difficult to conduct. Neverthe- 
less it was possible to make some studies of fluctuations and 
variability of rainfall, these characteristics being of special interest 
to agriculture. 
From the scattered network of stations, with often not too 
accurate measurements, it was possible to select 52 stations 
having in most cases reliable records for more than 20 years. 
Tacubaya, D.F., with records since 1878, has also been used. 
It was generally assumed that only small errors can be involved 
in records from a certain area which all show the same general 
interannual fluctuations. When the records showed doubtful 
deviations from the general trends of fluctuation, the stations 
were not accepted. 
The northern and northwestern parts of Mexico show semi-arid 
or desert conditions with an annual rainfall ranging from 200 
millimeters in Sonora to about 500 in Durango and Coahuila 
(Figure 1). On the other hand, rainfall reaches 3,000-4,000 milli- 
meters in the southeastern parts of the country. Except for these 
extremes the rainfall map shows a large triangular area of dryness 
extending through the country southward and having its base at 
the U.S. border. 
Mexico is a country of seasonal rainfall with summer and 
autumn the rainy seasons in all parts of the country except for a 
small area in the northwest where the Mediterranean rainfall 
regime of winter rain prevails. The May—October season gives 
generally more than 80% of the annual precipitation. This figure 
is somewhat smaller along the Gulf Coast and higher in the 
southwest (4). 
The following circulation factors regulate the Mexican rainfall 
as far as we know them: 
1. Seasonal fluctuations in the position of the intertropical 
convergence zone. 
2. Location, extension, and intensity of the subtropical high- 
pressure cells, directing the influence of the trades. 
3. Perturbations in the summer easterlies (“easterly waves’’). 
4. Hurricanes, generally created in connection with the 
“easterly waves.” 
