Weather Study 



875 



Snow crystal. 



Photomicrograph by 

 W. A. Bentley. 



WEATHER MAPS 



A weather map is a sort of flashlight photograph 

 of a section of the bottom of one or more of these 

 great rivers of air. It brings into view the whole 

 meteorological situation over a large territory at a 

 given instant of time; and, while a single map con- 

 veys no indication of the movements continually 

 taking place in the atmosphere, a series of maps, like 

 a moving picture, shows not only the whirling 

 eddies, the hurrying clouds and the fast-moving 

 winds, but the ceaseless on-flow of the great river of 

 air in which they float. Our present knowledge of 

 the movements of the atmosphere has been gained 

 chiefly from a study of weather maps ; they form the basis of the modern 

 system of weather forecasting, and their careful study is essential to any 

 adequate understanding of the problems presented by the atmosphere. 

 (See pages 884-88 5.) 



The Principles of Weather Forecasting 

 The forecasting of the weather has been 

 made possible by the electric telegraph. It is 

 based upon a perfectly simple, rational process 

 constantly employed in everyday affairs. We 

 go to a railway station and ask the operator 

 about a certain train. He tells us that it will 

 arrive in an hour. We accept his statement 

 without question, because we are confident that 

 he knows the speed at which the train is ap- 

 proaching, a few clicks of his telegraph instru- 

 ment has told him just where it is and the time it 

 will arrive, barring accidents, is a simple calcula- 

 tion. Information of coming weather changes 

 are obtained in a similar manner. Although 

 storms do not run on steel rails like a train, 

 nevertheless their movements may be foreseen with a reasonable degree of 

 accuracy, depending chiefly upon the size of the territory from which tele- 

 graphic reports are received and the experience and skill of the forecaster. 

 As a rule, the larger the territory brought under observation, especially in 

 its longitudinal extent (the general currents carry storms of the middle 

 latitudes eastward around the world and those of the tropics westward) , the 

 earlier advancing changes may be recognized and the more accurately their 

 movements foreseen. 



Forecasts Based on Weather Maps 



The forecasts issued by the United States Weather Bureau are based on 

 weather maps, prepared from observations taken at 8 a. m. and 8 p. m. at 

 about 200 observatories. In addition to the reports received by telegraph 

 by the Central Office at Washington, the several forecast centers and other 

 designated stations from observatories or stations in the United States, a 

 system of interchange with Canada, Mexico, the West Indies and other 

 island outposts in the Atlantic and Pacific gives to the forecaster two daily 

 photographs of the weather conditions over a territory embracing nearly 



Snow crystal. 



Photomicrograph by 

 W. A. Bentley. 



