28 THE FUTURE OF ARID LANDS 
Climate 
Arid regions have little rain; and the little they do have is often 
exceedingly irregular. It may seem a bit ironical to many that 
desert landscapes, such as those of Death Valley in the United 
States, are dominated by erosion features. With too little moisture 
for a protective plant cover, the infrequent but severe storms cause 
severe erosion. 
Rainfall and temperature have been recorded for large parts of 
the world for some time now; but these alone do not give a precise 
definition of environmental conditions. The water in the soils, 
for example, is partly a matter of total rainfall, as modified by 
slope, plant cover, and soil permeability, and partly a matter of 
evaporation, which depends upon humidity, cloudiness, and wind, 
as well as upon the temperature. 
Nor do the total or average values for these factors tell enough. 
The patterns of each factor, in relation to the others, through the 
seasons, must be defined. Many arid lands near the Mediter- 
ranean, for example, have the most rain during the coolest months 
and the least during the hottest months. Others, such as those 
near us here in Albuquerque, have the reverse pattern; both rain- 
fall and temperature are highest together. Even with the same 
totals and annual averages, these are quite different environments 
for soils and plants. 
Renewed efforts are being made to define climatic types and to 
classify them. Description is difficult because a climatic type has 
no morphology in the sense that a rock, a plant, or a soil has a 
morphology. The “morphological” definition of a climatic type 
depends upon what the climatologist decides to be relevant 
seasonal patterns of factors that he can measure and of the al- 
lowable limits of variation over a long period of years. He cannot 
“see”? what he fails to define. His notions of the relevance of 
factors and of variations within them depend, in turn, upon 
studies of the effects of the climatic environments upon soils, 
water, plants, animals, and people. 
The results of these combined studies are leading to better 
definitions and classifications of climatic types. The closer the 
network of observation stations of long record with complete data, 
