THE THUNDERSTORM— EVANS AND McEACHRON 



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UNITED STATES WEATHER BUREAU PREDICTIONS 



United States Weather Bureau stations scattered over the United 

 States make daily telegraphic exchanges of information on tempera- 

 ture, pressure, wind, rain, thunderstorms, etc. From these data, 

 weather charts are made up from which weather predictions for each 

 section of the country are made. These include the prediction of 

 thunderstorms and are the best available. It must of course be 

 realized that these stations predict for areas covering many square 

 miles. While each region is subdivided into sections for purposes of 

 prediction, it cannot be expected that all parts of each section will be 



Figure 11.— Isokeraunic map indicating the average number of thunderstorm days to be expected per 

 year throughout the United States; based on 20-year records covering the period from 1904 through 1923. 

 (After Alexander.) 



subjected to the same weather conditions. Those interested in predic- 

 tions covering a very localized territory must then supplement the 

 Weather Bureau predictions by local observations. With the Weather 

 Bureau's information on regional atmospheric conditions as a base, 

 and with an acquired knowledge of the significance of pressure, 

 temperature, humidity, wind direction, type of cloud, and degree of 

 atmospheric haze, it is frequently possible to predict the occurrence of 

 thunderstorms several hours in advance. 



DIRECTION OF TRAVEL 



Prediction of the direction in which thunderstorms will travel is 

 frequently very desirable. If general air movements at the cloud level 

 are extensive, as is usually the case for cold-front, and warm-front 

 thunderstorms, the direction of thunderstorm travel is easily pre- 



