ix Climates of the World 133 



respond to the average temperatures of January, July, 

 and of the whole year. The curve of average tempera- 

 ture for the year shows that the warmth of the air at 

 80 N. is only 5 F., that in 4 N. there is the maximum 

 temperature of 81 F., and that in 60 S. it is 35 F., the 

 northern hemisphere being as a whole a little colder than 

 the southern. The mean surface air temperature of the 

 whole Earth is about 59-5. The curves for January and 

 July show a great annual range of temperature in the 

 northern hemisphere, increasing toward the north where 

 the land preponderates, and a slight annual range in the 

 southern hemisphere, decreasing toward the south where 

 the sea influence prevails. The student should study this 

 diagram, comparing the temperature at each season at 

 various latitudes in the two hemispheres. To do this follow 

 the vertical line of latitude until it cuts the curve ; the point 

 on the thermometer scale in a horizontal line with this inter- 

 section is the temperature at that particular latitude. 



1 88. Isotherms. If the temperature of every place in 

 the world at some one instant were marked in figures on a 

 large map the result would be very confusing to look at. 

 But if all the figures except those showing a difference of 

 i o degrees were blotted out the map would be much simpler. 

 Near the equator the number 80 would occur frequently, 

 farther north and south there would be rows of 70, still 

 farther strings of 50, and so on. A line might be drawn 

 through all figures 80, and the figures themselves might 

 then be blotted out, except one left to mark the line, and 

 the same might be done for 70, 60, and the rest, greatly 

 simplifying the map. Such lines are termed isotherms, or 

 lines of equal warmth, as they pass through all the places 

 where the air temperature is the same. In interpreting 

 the maps (Plates III. and IV.) it is usually assumed that 

 the temperature between two isotherms is proportional to 

 the distance. For example, in the January map (Plate III.) 

 the line of 70 temperature in Central America is one inch 

 from the line of 80 in South America, so that between 

 them one-tenth of an inch on the map corresponds to a 

 change of i of temperature. The lines of 40 and 50 in 



