KLK.MKNTS OF CKNKKAL 



which have the same air pressure are shown on one line, and 

 the figure which represents this pressure is marked upon the 

 map close to the line. These lines are called isobars. 



The map is made in the following manner: Each morning 

 at eight o'clock (Washington time), at every weather-bureau 

 station in the United States, the barometer is read and the, 



% 



FIG. 19. Weather map for March 13, 1913 



The three maps for successive days (figs. 19, 20, and 21) should be compared, to 

 show the progress of the storm and changes in weather conditions. The unhroken 

 lines are the isobars ; the arrows fly with the wind ; the blackened circles indicate 

 cloudy weather at the time of observation ; figures near the arrows show tempera- 

 ture ; broken lines are isotherms of zero and freezing temperatures. These figures 

 are from the United States Weather Bureau 



reading is telegraphed to Washington. There the readings 

 are placed upon a map, each at the location of the city from 

 which it was telegraphed. The lines are then drawn through 

 places having the same pressures. These lines thus make it 

 possible to see very easily where the high-pressure and low- 

 pressure centers are located. Usually a map will show several 

 well-defined areas of this kind. Each pupil should examine 



