SECT. 2] LARGE-SCALE INTERACTIONS 231 



perturbations. In the stronger disturbances, or true tropical storms, where 

 central wind speeds exceed the average (see example in Fig. 56d), a still further 

 amplification of the exchange might be expected. We shall next look at the 

 extreme case of transfer in the tropical hurricane. 



c. Exchange and the mature hurricane 



Ocean-atmosphere interaction reaches its acme in the full hurricane. The 

 only vicious offspring of the normally benign tropics, it draws both its life-blood 

 and its ability to deal death from the sea. Feeding on evaporated sea-water, it 

 pays back for its keep by boiling up the oceans into chaotic mountains and 

 often driving their unleashed fury on shore. Wave heights exceeding 50 ft are 

 not uncommon in mid-ocean, hurled by 100-200 knot winds, against which 

 even huge ocean liners are helpless. At coast lines, surf damage is augmented 

 by abnormal tides and the terrifying "storm surge" of embay ments, which may 

 wipe out entire towns in a few hours (Dunn and Miller, 1960, pp. 206-230; 

 Tannehill, 1938; also see Chapter 17 of this volume). 



Unnecessary in the general circulation, the only good blown by these ill 

 winds is the impetus their dire necessity has given research. Through the 

 establishment, in 1955, of the National Hurricane Research Project of the U.S. 

 Weather Bureau and its instrumented aircraft program, more observational 

 material is available on the interior structure of hurricanes than for any other 

 atmospheric phenomenon. As a consequence, a physical model of this type of 

 thermal engine is in the process of evolution, which we shall draw upon to build 

 our description. Some of its analytical phases will be introduced subsequently 

 to examine sea-air interaction at its terrestrial extreme. 



(^ ) Operation of the hurricane engine 



The hurricane is a thermally driven circulation whose primary energy source 

 is the latent heat of condensation. The heating acts to estabhsh the pressure 

 gradients which produce and maintain the furious winds. Radar and photo- 

 graphic maps (Jordan, Hurt and Lowrey, 1960; Malkus, Ronne and Chaffee, 

 1961) demonstrate abundantly that latent heat is not released uniformly 

 throughout the rain area, but that it is concentrated in spiral convective bands 

 of narrow width oriented along the low-level inflowing air trajectories and 

 particularly concentrated in a vicious ring of penetrative hot towers encircling 

 the calm eye at the center. The storms vary widely in size, from the midgets of 

 50-100 km radius to the giant Pacific typhoons which dominate whole ocean 

 regions more than 2000 km across. As schematically illustrated in Figs. 66 and 

 67, the typical hurricane consists of the following radial subdivisions : 



1. An external ring of weak subsidence and divergence, 



2. An "outer rain area" with pressures above or near 1000 mb and winds 

 below or marginally at hurricane force (65 knots). 



