MEXICAN RAINFALL 143 
rainfall and of which circulation conditions create convection and 
heavy showers are extraordinarily important in relation to soil 
erosion, which is a serious danger in these regions. It should 
finally be kept in mind that inducing rainfall by artificial means 
never could be successful unless one has a thorough knowledge 
of the circulation mechanism that creates the necessary cloud 
types in the areas concerned. 
An ultimate goal of the statistical and synoptic climatological 
studies mentioned in the first group in Table 1 should also be to 
give methods for long-range forecasting (monthly, seasonal, or 
longer) of rainfall. This is of particular importance to agriculture 
and water supply in arid and semi-arid lands. 
The suggested studies of the water balance (group //) in itself 
may be separated into a macroclimatological and a microclimato- 
logical type. The former type often serves as a bridge between the 
investigations in general meteorology and synoptic climatology 
of the first-mentioned group (I) and the detailed studies included 
in the microclimatic type. These investigations, in other words, 
should give more detailed information of rainfall mechanisms and 
cloud physics as well as of temperature conditions than might be 
obtained by the general investigations of the first group. 
The aim of the microclimatic studies of heat and water balance 
discussed by Thornthwaite is to understand how economically to 
apply irrigation for the reduction of evaporation, soil conservation, 
reforestation, etc., with due regard for results given by the 
general investigations. It is not advisable to take up such micro- 
climatic studies of the water balance without having laid a solid 
foundation by investigating the first-mentioned general problems. 
As an example of a basic statistical and synoptic-climatic study 
of a partly semi-arid or arid country I shall discuss briefly a 
study of fluctuations and variability of Mexican rainfall which 
was performed during my work as UNESCO advisor in economic 
climatology to the Mexican Government in 1954 (6). 
Precipitation in Mexico 
Detailed studies of more delicate characteristics of precipitation 
in Mexico are still lacking, mainly because the scattered network 
