310 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



the movement to external forces. In figure 5 there is shown sche- 

 matically the winds of a hurricane broken into three components: 



(a) Those tangent to a circle with a speed ranging from 75 to 150 m.p.h. 



(b) Those inward or outward from the center (in the lower levels the 



winds usually have an inward component and at high levels the winds 

 have an outward component). The speed ranges up to 35 m.p.h. or 

 more in lower levels. 



(c) The basic current in which the hurricane is imbedded and in which 



the speed varies from zero up to about 60 m.p.h. with the average 

 being near 15 m.p.h. 



\ 



/ 



Circular Inflow 



(usually 75-150 mph) (usually 5-35 mph) 



Steering 

 (usually 5-25 mph) 



Figure 5. — Schematic presentation of the winds of a hurricane separated into three compo- 

 nents. (In actual cases the strength of the various components is usually different in 

 the various quadrants of the storm and for different radii.) 



The external forces are usually attributed to this basic or "steer- 

 ing" current. Since in actual cases it is very difficult to separate the 

 basic current from the other two components, forecasters have devised 

 many methods for estimating the contribution of "steering" to hurri- 

 cane movement. It is now generally agreed that most of the rapid 

 forward progress of hurricanes can be accounted for by the effects of 

 the basic current, and that a hurricane moves in approximately the 

 same direction the basic current is flowing. 



To determine if the steering current caused a hurricane to move, 

 Mrs. Elizabeth Jordan [11], while working at the University of 

 Chicago, made calculations from mean data. She computed the steer- 

 ing current based on mean winds from an area ranging from 120 to 



