CLIMATE 319 



lower latitudes, the annual range of temperature is often less than 

 in some other places. This is because a large part of the surface is 

 covered with snow or ice, and the heat received from the sun can- 

 not bring the temperature of the surface above 32 F., so long as 

 the snow and ice remain. Where these conditions exist, the sum- 

 mer temperature of the air is raised but little above the freezing- 

 point. 



Precipitation in the polar zones is not usually heavy, and much 

 of it falls as snow. Where the surface is continually covered with 

 snow or ice, the precipitation is generally heaviest in summer. 

 The winds are then more heavily laden with moisture, and blowing 

 over the surface of snow and ice, the air is cooled to the dew-point 

 (p. 257) or below. Because of the low temperature of winter, the 

 air of that season contains but little water vapor, and so gives but 

 little snow. 



Rainfall and Agriculture 



The amount of rain which is necessary for agriculture varies 

 (1) with the crops to be raised, (2) with the temperature of the 

 regions, and (3) with the distribution of the precipitation through 

 the year. The higher the temperature the more the rainfall nec- 

 essary for growing plants. A few inches of rain in temperate lati- 

 tudes would be enough for crops, if it fell just when the growing 

 crops needed it. With the existing irregularity of rainfall, the 

 amount should not be less than 20 inches per year in middle lati- 

 tudes to make crops at all sure. Even more than this is necessary 

 in the lower latitudes of the intermediate zone. Of this amount, 

 much should fall in the season when crops are growing. 



In the cultivation of semi-arid land, care should be taken in 

 the selection of the crops to be raised. 



Hann calls attention to the fact that in Jamaica and the Barba- 

 does the sugar crop can be calculated with approximate accuracy 

 from the amount of precipitation. In South Australia, land which 

 has 8 to 10 inches of rain will support 8 or 9 sheep to the square 

 mile. In New South Wales, 4 inches more of rainfall will allow 

 the land to support 96 sheep per square mile; an increase of 7 

 inches more (20 inches in all) will allow an equal area of land to 

 support 640 sheep. In Argentina, with 34 inches of precipitation, 



