HURRICANES GENTRY AND SIMPSON 311 



240 nautical miles about the center of the hurricane at all levels up 

 to 30,000 feet. Defining the pressure weighted mean of the winds in 

 this ring as the steering current, she found that it was 9.7 knots in 

 the direction in which the average hurricane was moving. The average 

 speed of motion of the storms included in this study was 11 knots. 

 This difference is fully within the limitations of the data. Mrs. Jordan 

 also computed the component of the basic current perpendicular to 

 the main movement of the hurricanes and found it to be less than 1 

 knot. It follows then, in the mean, that tropical storms move in the 

 direction and with the approximate speed of the steering current 

 when the steering current is denned as the pressure weighted mean 

 flow from the surface to 300 mb. 



The internal forces result from interaction of the other two circu- 

 lations, namely, the tangential and radial components of the winds, 

 with each other and with the basic current. Asymmetries of the 

 circulation about a hurricane and small-scale circulations imbedded 

 within the hurricane wind field also contribute to the movement of 

 the hurricane as a part of the internal forces. Since the tangential 

 and radial components of the wind vary from time to time and differ 

 in various sectors of the storm, their interactions build up forces which 

 could reasonably be expected to cause changes in the direction and 

 speed of movement of a hurricane. Low-level wind data and radar 

 observations of hurricanes indicate that locations of areas of maximum 

 mass convergence in the lower levels vary considerably with respect to 

 the hurricane center from storm to storm and from time to time in the 

 same storm. It has been calculated that the hurricane that passed 

 over southern Florida and New Orleans, La., in 1947 drew 2 to 3 

 billion tons of air per minute into the hurricane circulation through 

 the lower levels of the storm and presumably returned the same 

 amount of mass to the surrounding atmosphere after expelling the 

 air from the storm at high levels. It should not, therefore, be assumed 

 a priori that the internal dynamics of the storm itself play an insignifi- 

 cant role in the over-all physics of hurricane movement. 



Let us discuss two of the internal forces that have been investigated. 

 Even a casual inspection of hurricane tracks will reveal that most of 

 the storms have some northward component. This suggests some in- 

 ternal force. Dr. C. G. Eossby [17] argues that the northward accele- 

 ration can arise from the variation of the coriolis forces across the 

 width of the storm. (The coriolis is an apparent force caused by the 

 movement of air across a rotating spheroid and varies directly with 

 the sine of the latitude.) In general, the northward acceleration ac- 

 cording to the formula developed by Eossby will be greater, the more 

 intense and the larger the hurricane. Dr. George P. Cressman [4] 

 estimated that the magnitude of this northward force would probably 



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