146 THE FUTURE OF ARID LANDS 
Precipitation in mm. 
o 
fo} 
(eo) 
1 
500 
1870-79 1880-89 1890-99 1900-09 1910-19 1920-29 1930-39 1940-49 
YO @ & 
Figure 2. Ten-year overlapping means of annual precipitation in 
Tacubaya, D.F. (1878-1953). 
5. Middle latitude westerly troughs passing over the northern 
part of the country in winter. 
In winter when the intertropical convergence zone 1s far to the 
south, the easterlies influence only the southernmost part of the 
land area while the northern parts are under the influence of the 
middle latitude westerlies, which give very little precipitation 
except for the Gulf Coast area in connection with the so-called 
nortes. In summer the whole country, except for the northwestern 
very dry regions, is under the influence of the tropical easterlies, 
and rainfall is frequent owing to convectional instability and even 
more to perturbations in the easterlies and to hurricanes. 
Long-Term Fluctuations in Mexican Rainfall 
Smoothing of the fluctuations in annual and monthly precipita- 
tion was made by calculating 10-year overlapping means. Figure 
2 shows the Io-year overlapping means curve for annual rainfall 
in Tacubaya, D.F. The curve indicates that there was a decrease 
of rainfall in central Mexico from the period of 1878-87 to the 
period of 1892—1go1 and then an increase up to around 1935. 
Since then there has been a decrease again up to recent years. 
The fluctuations are rather great and the maximum amount of 
