240 



PHYSIOGRAPHY 



evaporating snow, not in raising the temperature of its surface. 

 It is to be especially noted that the air over the heated rock of high 

 altitudes, whether of mountain or plateau, does not get so warm 

 as the rock itself. 



Representation of Temperature on Maps 

 It is desirable to have some method of representing the tem- 

 perature of all parts of the earth on maps. Maps showing the 

 distribution of temperature are thermal maps. 



Lines may be drawn on the surface of the earth, connecting 

 points having the same temperature. Such lines are isotherms. 



I2QC 140* 160* ISO' 160* I4Q" 120' 100' 80* 60* 40* 20* 0* 20* 40* 60* 60* 100* I 



20* 40 60* 80* 100' 120' 140' 



Fig. 215. Chart showing a few isothermal lines. The dotted line near 

 the equator represents the position of the heat equator, at the time 

 represented by the map. 



An isotherm connecting places having the same average tempera- 

 ture for the year is an annual isotherm. Fig. 215 shows a very 

 simple isothermal chart, on which the isotherms of -30 F., 0, 

 30, 50, and 70 are represented . An isotherm connecting places 

 which have the same summer or the same winter temperature is 

 a seasonal isotherm. Isotherms may be drawn for a month or for 

 any other specified period of time. A map showing isotherms 

 is an isothermal map, and an isothermal map should always tell 

 whether the isotherms are annual, seasonal, or monthly. 



