CLIMATIC RELATIONS 265 



July and January isotherms cross each other almost at 

 right angles. Isohyetal lines run, usually, north and 

 south, but two of these near Poltava and Kherson pass 

 northeastward to the Volga River and Ural mountains, 

 then bend to the northwest and extend into Finland. 

 Similar peculiar relations exist in the Great Plains, but 

 are not so marked. 



279. Directions of climatic resistance. As heat is 

 an important requirement in the maturity of good kernels, 

 summer temperatures, even in plains regions, are rarely 

 too high for the small-grain crops. The summer rainfall, 

 however, is usually more or less deficient, although greater 

 in proportion to the total for the year than in forest regions. 

 In summer, therefore, the limiting climatic factor is dry- 

 ness. In winter, precipitation is less important, though 

 it may be very helpful, but low temperature often aided 

 by dryness causes winter-killing in winter grain districts, 

 and is, therefore, the limiting climatic factor in winter. 

 These two factors act greatest as a hindrance in certain 

 directions, #nd the resultant direction of increase in the 

 combined effects of the two is the direction of climatic 

 resistance to the cereal crop. For example, winter tem- 

 perature decreases usually northward in the northern 

 hemisphere, while the rainfall in South Dakota decreases 

 westward. In South Dakota, therefore, the direction 

 of greatest climatic resistance to cereal crops lies midway 

 between these two, or to the northwest, supposing the 

 two factors to be equally severe. Sometimes the direction 

 of decrease of temperature is much affected by ocean 

 currents or continental features, so that in Saratov, Rus- 

 sia, for example, temperature decreases practically east- 

 ward. Here then, as precipitation decreases to the 

 southeast, the direction of greatest climatic resistance 



