204 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



storm frequency sweeping in a great arc from northwestern Canada down across the 

 Rocky Mountain region to Denver and Texas, thence eastward through Louisiana, and 

 up the Atlantic coast to Maine. This, it will be seen, is roughly parallel to the mean 



105 100 95 90 85 80 75 TO 65 



Fig. 69. — Changes in Storm Frequency by Months according 

 to Longitude in Latitude 30-35°. 



Fig. 70. — Changes in Storm Frequency by 

 Months according to Longitude in Lati- 

 tude 2.5-30". 



storm track as defined above, although much more curved than that, and lies in exactly the 

 place where we should expect an increase if the track had been shoved southward. 



In addition to the points already mentioned, there is another and wholly different 

 aspect of the shift of the storm track to which attention should be called. The general 



Fig. 71.— Summary of Differences in Storm Frequency, 1878-1887 and 1899-1908. 



conclusion drawn from the maps seems to be that there has been a slight southerly and 

 westerly shift of the storm track in the United States in the interval of twenty-one years. 

 In this same interval the magnetic field in the United States has shown a similar shift. 

 In discussing the mysterious changes in the secular variation during this period, Dr. L. A. 

 Bauer says (Congressional Documents of the United States, vol. 5139, p. 217): 



