262 THE SMALL GRAINS 



annual relative humidity for all Great Plains points south 

 of Bismarck is under 70 per cent, while that for the 

 summer months for points in the Chernozem also range 

 below 70 per cent. On account of the close proximity of 

 the Russian point Genichisk to the Azov Sea, its summer 

 humidity is much greater than it would be otherwise, the 

 rainfall being very light. 



The most important feature of these humidity relations 

 is that, while the rainfall of the growing season may be 

 actually greater in the semi-arid prairies than in the 

 forested region a fact of much significance as already 

 pointed out the relative humidity is, as a rule, less ; 

 that is the rain of the prairie regions falls in quick storms, 

 which alternate with clear hot days, ideal conditions for 

 the perfect development of cereal kernels. 



277. The Chaco-Pampean plains region of Argentina 

 has climatic conditions similar to those above described, 

 but of course solar temperatures decrease in the reverse 

 direction of north to south. The severity of climatic 

 extremes is not so great, but the same kind of extremes of 

 both temperature and moisture occur. For example, in 

 the latitude 30 to 35 going west from Rosario, even 

 the isotherm of mean annual maximum temperature 

 turns southward in the central plains area, while the 

 absolute maximum isotherm of 42 C., after leaving 

 Rosario, dips sharply far south to northern Chubut, 

 then back north again, on reaching the Andes, to the 

 northern Argentine boundary. The mean annual tem- 

 perature of the Chubut locality for this isotherm is, 

 however, 11 (19.8 F.) lower than those of the north- 

 ern points through which the same isotherm passes. 

 Quoting from Davis (1910, p. 604) : " These condi- 

 tions recur nearly every year, it being an exceptional 



