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 Hatteras, Nantucket, 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 difficult 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 correctly 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
