2g4 The National Geographic Magazine 



tion and amount of movement of a storm 

 is the resultant of these two forces. 

 Thus, for instance, a December storm 

 charted in Colorado, subject to a press- 

 ure that tends to force it southward 400 

 miles in 24 hours, is during the same 

 period being carried eastward 450 miles 

 by the flow of the upper currents. It 

 is evident that the storm's actual path 

 will lie between the two lines represent- 

 ing the eastward drift and the pressure 

 that forces the storm to the south, the 

 resulting movement being almost due 

 southeast and a distance of approxi- 

 mately 600 miles. 



' ' From a study of storm movement 

 along the lines outlined above it is ap- 

 parent that the rate and direction of 

 movement of a storm in relation to its 

 normal movement is governed by this 

 variable component, representing the 

 deflective force, or the resultant of the 

 pressure exerted on the storm from all 

 directions ; hence it follows that when 

 this deflective force is toward the left 

 (when facing the direction of normal 

 progression) the storm will move to that 

 side of the normal direction of advance, 

 and when toward the right the converse 

 will be true. When this deflective force 

 is acting in conjunction with the east- 

 ward drift the storm's rate of movement 

 will be accelerated, and when in oppo- 

 sition the storm's progress will be re- 

 tarded. It appears that in nearly all in- 

 stances the storm increases in intensity 

 when this component, representing the 

 pressure of the air toward the storm 

 center, is acting to the left of the normal 

 direction of advance, but when toward 

 the right the storm, as a rule, will de- 

 crease in intensity. 



1 ' Naturally exceptions are to be found 

 in applying the method outlined above, 

 but in practically all instances the ex- 

 ceptions have been the result of an un- 

 foreseen increase or decrease in the press- 

 ure toward the storm center from some 

 one of the several directions, which, in 

 addition to offering an explanation of 



the exceptions, tends to prove the cor- 

 rectness of the principle. Of course the 

 application of the method is limited 

 when the storm center is near a region 

 from which no pressure observations are 

 available — as, for instance, the storms 

 that move along the Canadian border. 

 In cases where there are a number of 

 ill- defined storm centers it is not always 

 possible to determine which center will 

 become the primary one and which cen- 

 ters will be dissipated, and therefore 

 there is more or less doubt whether the 

 deductions will be borne out by subse- 

 quent events. In nearly all instances 

 involving exceptions the error in prede- 

 termining the movement of the center is 

 apparently due to inability to determine 

 the exact values that should be used to 

 represent the pressure toward the storm 

 center from the several directions. 



" The values determined by the meth- 

 ods used in the research along the lines 

 indicated above are necessarily approx- 

 imations only, and therefore tentative ; 

 but it is believed that by refined meth- 

 ods of computation values representing 

 the pressure exerted on the storm center 

 as well as the normal direction and ve- 

 locity of the eastward drift can be found 

 that will show the exact conditions, and 

 thus lead to a higher degree of accuracy 

 in charting the direction and movement 

 of storms. 



' ' The accompanying charts illustrate 

 the method followed in developing the 

 ' normal storm tracks ' and the appli- 

 cation of the system in practical fore- 

 casting to determine the direction and 

 rate of movement of storm centers dur- 

 ing 24-hour periods. 



' ' Chart XVI shows the method fol- 

 lowed to determine the correct value for 

 each tenth of an inch increase in the 

 barometric readings along lines radiat- 

 ing from the storm center to the north, 

 northeast, east, etc., to represent the 

 influence of the pressure exerted on the 

 storm center from the several directions ; 

 it also illustrates the method followed in 



