TYPES OF CLIMATE ^^ 



4. Desert Climate. This is characterized by excessive dryness, 

 by great heat during the day, with accompanying strong convection 

 currents resulting in high winds, and by relatively cool, clear and 

 calm nights. The peculiarities of temperature and aridity are un- 

 favorable to the growth of vegetation, except certain adapted forms, 

 and the general absence of plants further intensifies the climatic 

 conditions. Rains are infrequent; parts of the Sahara are known 

 to have been without rains for 10 or 15 years, and when they occur 

 they are apt to be sudtlen downpours of great floods of water. Dust 

 storms are characteristic of the daytime, when high winds blow, and 

 it is then that deflation and the mechanical wear by drifting sands 

 are chiefly accomplished. The rivers resulting from the sudden 

 rainfalls, or fed by springs in the mountains, wither away, the water 

 sinking into the parched ground or evaporating. Brackish sinks 

 and lakes abound, and temporary playa-lakes suddenly come into 

 existence and disappear by evaporation, leaving behind a charac- 

 teristic, hard, mud-cracked surface. The plants protect themselves 

 against animals by thorns, and guard against excessive evaporation 

 by a reduction in the size of the leaves. The roots penetrate deep 

 into the dry soil and subdivide extensively to obtain all the available 

 moisture. 



5. Mountmn Climate. This is distinctive in all zones, and always 

 contrasts strongly with that of the neighboring lowlands. There is 

 a decrease in temperature, pressure and absolute humidity, and an 

 increase in intensity of insolation and radiation, and generally an in- 

 crease in the frequency and amount of precipitation. Pressure is re- 

 duced to about one-half that at sea-level, at an altitude of about 

 16,000 feet, while the zone of maximum rainfall lies at about 6,000 

 to 7,000 feet in intermediate latitudes. Inversions of temperature 

 characterize mountains at night and during the colder months, the 

 cold air flowing down the mountain sides to collect in the valleys 

 below, and being replaced by warmer air above. Thus the diurnal 

 and annual ranges of temperature are lessened, mountain summits 

 having in this respect a climate more nearly resembling the marine. 



Climatic Provinces. 



Detailed consideration of climates leads to the recognition of a 

 number of climatic types for the entire earth, the characters of these 

 types being brought about by combinations of distinct climatic ele- 

 ments. Supan has established thirty-five types arranged in thirty- 

 five provinces as follows (91 :2^2) : 



