WEATHER 159 



the position of isotherms from day to day, as will be seen 

 by consulting the daily weather maps. If these lines are 

 averaged for a certain number of days, the result will be 

 the mean temperature for that period of time. Figure 

 135 shows an isothermal map for July, Figure 136 one for 

 January, and Figure 137 shows a chart of the mean annual 

 temperatures for the year. On consulting these charts 

 it will be noticed that the isotherms of 70 degrees lie at 

 some distance on either side of the equator, but that the 

 distances for July and January are not the same. These 

 isotherms form the boundaries of the hot zone through 

 which runs the heat equator or the line of highest tempera- 

 ture. It lies north of the true equator in July and south 

 of it in January. The temperate zone is the belt inclosed 

 between the isotherms of 70 degrees and 30 degrees. 

 Beyond 30 degrees lie the frigid zones. These zones 

 follow the movements of the heat equator and are not 

 fixed belts. 



Change of Temperature with the Seasons. As the 

 earth moves in its orbit around the sun, there are six 

 months in each year in which the northern hemisphere is 

 inclined toward the sun and six months in which it is 

 inclined away from the sun. In the months in which 

 it is inclined toward the sun it gains more heat by absorp- 

 tion during the long days than it loses by radiation during 

 the short nights, and the temperature rises above the 

 average temperature for the year. The opposite is true 

 of the southern hemisphere during this period. During the 

 other six months of the year the northern hemisphere 

 is inclined away from the sun and the southern hemisphere 

 toward the sun, with the result that the southern hemi- 

 sphere is then the summer hemisphere and the northern 

 the winter hemisphere. 



