248 PHYSIOGRAPHY 



put down on the chart as 70 (60 + 10). Isothermal charts, 

 therefore, are intended to show the temperature as it would be if the 

 land were at sea-level. 



Daily range of temperature. The temperature of a day when the 

 sun shines is generally warmer than the temperature of the night. 

 The difference is often as much as 40 or 50 F. in dry, interior 

 regions, and in the Sahara it is sometimes 70. The daily range is 

 greater when the air is dry than when it is moist, and it is greater 

 far from the sea than near it. Other things being equal, the daily 

 range is greatest when days and nights are nearly equal. 



Seasonal range of temperature. The seasonal range of tem- 

 perature is affected by (1) latitude, (2) position with reference to 

 land and sea, (3) prevailing winds, and (4) the presence of snow 

 during the warm season. 



1. The seasonal range of temperature increases with the lati- 

 tude (compare Figs. 217 and 218, pp. 243, 245), because the yearly 

 variation in insolation increases with the latitude. This range is 

 greatest at the poles, where there is six months of insolation and 

 six months without it. The great range of seasonal temperature 

 to which the poles would be entitled by their latitude is greatly 

 modified by the conditions mentioned under (2) and (4) above. 



2. Islands have a lesser range of temperature than continental 

 lands in the same latitude, and coasts have a lesser range than 

 interiors, because the range of sea temperature is less than the 

 range of land temperature (Figs. 217 and 218). 



3. A coast to which the prevailing winds blow from the ocean, 

 has a less range of temperature than a coast to which the prevailing 

 winds blow from the land. Thus the range of temperature is less 

 on the Pacific coast of the United States than on the Atlantic in 

 the same latitude (Figs. 217 and 218), the winds being chiefly 

 from the west in both cases. 



4. The presence of snow during the warm season, as in high 

 latitudes and high mountains, prevents a high temperature in 

 summer, even though insolation is strong (p. 239). 



The annual range of temperature has some effect on vegetation, 

 and so on all industries connected with the soil. The range of 

 temperature, or more exactly the temperature of winter, has some 



