HURRICANES GENTRY AND SIMPSON 307 



of 1956, however, indicated that on particular days in those two storms 

 the maximum wind speeds were about the same even at 20,000-30,000 

 feet as at the surface. 



Data collected by the reconnaissance crews and by radar have dem- 

 onstrated that hurricanes are much more complicated than was thought 

 a century ago. The spiral rainband was one of the earlier discoveries 

 of the radar meteorologists (pi. 1). Most of the heavy rain in hur- 

 ricanes occurs in ra.inbands that spiral in toward the center of the 

 storm. Between the bands, rainfall is relatively light, and near the 

 outer edges of the storm there frequently is no rain at all between the 

 bands. To some extent the highest winds are also associated with 

 these spiral bands. Reconnaissance has also brought out the fact that 

 in tracking the hurricane several different kinds of centers may be 

 specified, and these centers do not always coincide. These various 

 centers are: (1) The center of the wind circulation, (2) the point of 

 lowest air pressure, and (3) the point around which the spiral rain- 

 bands or cloud streets rotate. In general, the location of these various 

 centers may differ by as much as 20 miles, depending upon the rate 

 of storm travel. Many of the earlier writers described hurricanes 

 as symmetrical circular storms. We know now that most hurricanes, 

 particularly moving ones, are not symmetrical — the winds are much 

 stronger in some quadrants, the rain area extends out farther in some 

 quadrants, there are asymmetries in the cloud structure, and all three 

 elements tend to be concentrated along spiral bands. 



Hurricanes frequently move along an irregular path that oscillates 

 about the relatively straight or smooth curved path that the storms 

 were depicted as following in track charts prepared before the 1940's. 

 The more frequent fixes obtained in recent years confirmed these 

 oscillations, some of which have a relatively short period of 3 to 6 

 hours duration, and some with a period of 12 to 36 hours. In recent 

 years these oscillations have been attributed to forces within the hur- 

 ricane (Yeh [25]). 



It has been well established that the primary energy source for 

 hurricanes is the warm moist tropical air that is found in the areas 

 where hurricanes form. As this warm moist air converges in toward 

 the storm center, it accumulates additional latent and sensible heat 

 from the warm ocean surface. Near the center the air rises rapidly 

 (fig. 4) and most of its water vapor is condensed to liquid water, 

 thus releasing great quantities of latent heat. Only a small propor- 

 tion of the heat energy thus released is converted into kinetic energy 

 for driving the hurricane winds, and the mechanism for transforming 

 the energy still remains pretty much a mystery. 



