148 THE FUTURE OF ARID LANDS 
rainfall, 958 millimeters, was reached in 1925, the minimum value, 
440 millimeters occurred in 1894. 
Studies of the fluctuations of monthly precipitations have 
revealed that the increase from 1900 until around 1920 was 
caused by an increase of both summer and winter precipitation, 
counterbalanced by a decrease in fall precipitation. The rapid 
increase from 1920 until 1935 was caused by an increase of 
precipitation in both summer and fall. The decrease thereafter 
has been accounted for by a decrease in rainfall of both summer 
and fall. 
It is likely that the considerable increase of summer rainfall 
trom the turn of the century until the middle of the 30’s must 
have been associated with a strengthening of the summer easterlies 
over Mexico. This in turn was caused by a general shifting north- 
ward of the subtropical high-pressure cells and particularly by a 
transition to the west of the summer cell in the Caribbean Sea. 
Such a northward shift goes very well with the well-known 
warming up of the winters of northern latitudes during the same 
period. 
The decrease of annual precipitation since the middle of the 
30’s suggests that a gradual regression of the subtropical high- 
pressure cells has taken place. This has also been suggested by 
evidences in northern latitudes of a cooling off in recent years 
and particularly since the end of the 30’s (5). 
Figure 3 shows the fluctuations of annual rainfall since 1920 in 
different areas of Mexico. There is an overall tendency in all 
regions, except for the southernmost one, to show first an upward 
trend and later a decrease of rainfall. The turning points of the 
curves from an upward to a downward trend occur at different 
times so that they generally appear earlier in the south than in 
the north. This is even more obvious on the western side than on 
the eastern side. This gradual displacement of the maximum, as 
we proceed northward, fits in very well with the idea of a gradual 
shifting northward of the subtropical high-pressure cells and a 
gradual moving back. 
Variability of Rainfall in Mexico 
In our study of the variability of rainfall in Mexico we have 
used several measures. Of most interest were the studies of the 
