WEATHER FORECASTS. 



309 



In order to intelligently predict the weather for even a small 

 section of the country it is necessary to know the conditions that exist 

 over the whole United States, and over as much of the rest of the 

 world as possible. The Weather Bureau receives observations from 

 one hundred and fifty-four stations. There are four hundred and 

 eight-five miles of telegraph lines and submarine cables operated by 

 the Weather Bureau for connecting with such points as Cape Hat- 

 teras, jSTantucket, and islands in the Great Lakes and on the Pacific 

 coast. 



To understand the plan of work of the Weather Bureau it will 

 be necessary to enter to some extent into the laws of the weather; 

 but it will not be found difiicult to see how the forecasters of the 

 bureau, with their greater knowledge in the same directions, are able 



Fig. 12. — Showing Isobars and Wind Lines. 



to foretell the weather correctly, as they do, in over eighty-two per 

 cent of the predictions. Strange as it may seem, the weather does 

 have laws, laws that are inflexible, so that, if the conditions are cor- 

 rectly understood, the changes in the near future can be confidently 

 predicted. All of these laws have not, however, been discovered, 

 and some that are well known have yet to be satisfactorily explained. 

 Primarily, the winds result from the sun heating the tropics to a 

 much greater extent than it does the polar regoins, causing the air to 

 rise in the tropics and flow toward the poles at a high altitude; from 

 which regions it returns toward the equator along the surface of the 

 earth. Owing to the rapidly lessening circumferences of the parallels 

 of latitude toward the poles and other causes, there is an ascending 

 belt of air near the parallel 64, toward which the surface wind 



