ABSTRACT
The climates of the Mexican Republic vary from very humid to desert, and from very hot to that of perpetual snow. The practical result of the modified system was the finding of an acceptable correlation, between the sub-types of climate and the distributions of the climatic conditions which were deduced from the meteorological data and from the distribution of diverse vegetal associations. The climatic study of Mexico's arid zones presents two basic problems: one is delimitation of the geographical confines of these arid zones and the other is characterization these zones with the object of providing the policies for their better management. In order to delimit the arid zones of Mexico, diverse formulas or indices, some more complicated than others, have been employed which attempt to include the variable and complex conditions of the environment. The necessity arose of adapting and modifying the Koppen system to the particular conditions of the Mexican Republic, resulting in the Modified System.
