RECORDS OF WEATHER CONDITIONS 



213 



heat is almost unbearable, but at night it is so cold that 

 heavy blankets are needed to keep the traveler warm. 



The nearness to the sea and the direction of the wind 

 also greatly affect the temperature of a place. In some 

 parts of the earth these are the principal causes in deter- 

 mining the temperature. Thus the temperature of the 

 atmosphere at any place is not due to a single cause, but 

 is the result of many and complex causes such as latitude, 

 height, direction of prevailing winds, ocean currents, near- 

 ness to the sea, and kind of soil. 



Graphic Method of Showing the Temperature of a Region. 

 It is often quite essential that the temperature over a 

 considerable region should be 

 known and a record of it made 

 and preserved. This might be 

 done by taking a map and writ- 

 ing their temperatures above the 

 different places marked on the 

 map. This would make a map 

 full of small figures and very 

 difficult to read. 



A much better method has 

 been developed and is now almost 



universally used. In making this map the temperatures 

 are first written on the map and then lines are drawn 

 through places which have the same temperature. These 

 lines are called isotherms and the map is called an isothermal 

 map. By the use of such a map it is possible at a glance 

 to determine the temperature prevailing at any place 

 and to see the relation which this has to the tempera- 

 ture of other places on the map. As a rule the isotherms 



\1> IS- . 



FIGURE 84 



