Fluctuations and Variability 
in Mexican Rainfall 
C. C. WALLEN 
Swedish Meteorological and Hydrological Insti- 
tute, Stockholm, Sweden. 
The need for climatological research in arid and semi-arid 
regions to the benefit of agriculture has been stressed very clearly 
by Thornthwaite (pp. 67-84), especially as regards the study 
of the water balance. There are other meteorological branches, 
however, the study of which seem to be of similar importance to 
the arid regions as practically all problems of such areas emanate 
from atmospheric conditions. 
In Table 1 are summarized the most essential meteorological 
research problems to be studied in the arid zones. These might 
be separated into two principal groups: general meteorological 
and synoptic climatological studies related to the circulation 
conditions on the one side, and studies of the water balance on 
the other. 
It is obvious that in applying to agriculture in arid and semi- 
arid regions both direct precipitation and ground water it is 
necessary to know in detail, ‘‘Why it rains, when it rains, where 
it rains,” which therefore always must involve the basic problem 
of all climatic research in such areas. In investigating the direct 
water supply for vegetation and agriculture as well as in trying 
to discover ground water resources, it is necessary to have a basic 
knowledge of the precipitation mechanisms and _ circulation 
conditions that give or prevent rainfall and also to know the 
actual amount of precipitation, its frequency of occurrence, its 
variability, and long-term fluctuations. It is therefore as im- 
portant to study, for example, the classification of circulation 
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