TEMPERATURE OF THE AIR 241 



Fig. 216 shows annual isotherms for each 20 F. At the ex- 

 treme north there is the isotherm of F., which lies north of 

 Europe and Asia, and barely touches North America. The average 

 annual temperature of places on this line is F. The isotherm 

 of 10 F. lies south of the isotherm of F. The average temper- 

 ature of places between these two lines is more than 0, and less 

 than 10. South of the isotherm of 10 follow in order the isotherms 

 of 30, 50, 60, and 70, the last being everywhere below the lati- 

 tude of 40 N. 



The coldest isotherm shown on the chart in the southern hem- 

 isphere is that of 30, lying south of all lands except Antarctica. 

 The latitude of this isotherm corresponds nearly to the latitude of 

 the isotherm of 30 in the northern hemisphere. Next north 

 (toward the equator) of the southern isotherm of 30 is the isotherm 

 of 50, followed by those of 60, and 70, the last being everywhere 

 north of latitude 40 S. 



The line of highest temperature about the earth is the thermal 

 equator (the broken line, Fig. 215). The thermal equator does 

 not follow a straight course around the earth, and it lies a little 

 north of the geographic equator most of the year. 



Fig. 216 shows that the annual temperature is highest near 

 the equator, and that it becomes lower toward the poles. This 

 shows that there is some relation between isotherms and latitude. 

 The explanation of this relation has been given. 



Fig. 217 shows the isotherms for the month of January. On 

 this chart all isotherms are farther south than on the chart show- 

 ing the annual isotherms. Thus the isotherm of F. (-17.78 C.) 

 in the northern hemisphere runs through central Asia, instead of 

 lying north of it, and the isotherm of 60 is everywhere south of 

 latitude 40, instead of being partly north of it, as in Fig. 216. 

 At this time of the year the sun is shining vertically south of the 

 equator, and this seems to be a sufficient reason for the change. 



Fig. 218 shows the isotherms for July. All isotherms are 

 farther north than the corresponding ones on either of the other 

 charts. Thus the isotherm of 50 in the northern hemisphere is 

 about where the isotherm of 20 was in January (Fig. 217). 



Comparing Figs. 217 and 218, it is seen that the difference 



