and the clouds thicken from cirrus 

 and cirrostratus to dense cumulonim- 

 bus, attended by thunder and light- 

 ning and excessive rain. These con- 

 ditions continue for several hours, 

 spreading destruction in their 

 course. Then suddenly the eye of 

 the storm arrives, the wind and the 

 rain cease, the sky clears, or part- 

 ly so, and the pressure no longer 

 falls but remains at its lowest. 

 This phase may last thirty minutes 

 or longer, and then the storm begins 

 again in all its severity, as be- 

 fore, except that the wind is from 

 the opposite direction and the pres- 

 sure is rising rapidly. As this 

 continues, the wind gradually de- 

 creases in violence until the tem- 

 pest is passed and the tropical 

 oceans resume their normal repose. 

 The violent portion of the storm 

 may last from twelve to twenty-four 

 hours . " 



South Florida has the dubious 

 honor of being visited more often by 

 hurricanes and tropical storms than 

 any other equal-sized area of the 

 United States (Gentry 1974). The 

 Everglades/Bay/ Keys basin is exposed 

 to both Atlantic and Carribean hur- 

 ricanes. Tropical cyclones general- 

 ly strike the east coast of south 

 Florida from an ESE direction — a 

 predominant direction for Atlantic 

 hurricanes before recurvature (Jor- 

 dan 1973, Ho et al. 1975). The west 

 coast of south Florida is vulnerable 

 to late-season tropical cyclones 

 moving in a northeastward direction 

 after recurvature (Cry 1965, Bradley 

 1972). Figure 17 illustrates the 

 frequency of hurricane occurrences 

 along the Atlantic coastline for 

 five of 58 coastal segments delin- 

 eated by Simpson and Lawrence 

 (1971). Points-of-entry in south 



Florida of tropical storms and hur- 

 ricanes also appear in Figure 17. 

 Major hurricane tracks passing 

 through the Dry Tortugas appear in 

 Figure 18. The average forward 

 speed for hurricanes affecting the 



basin is 10 knots with a radius of 

 maximum winds extending an average 

 of 20 nautical miles from the center 

 (Ho et al. 1975). Detailed descrip- 

 tions of the passage of specific 

 hurricanes and tropical storms 

 through the basin exist in litera- 

 ture such as the U. S. Department of 

 Commerce's "Monthly Weather Review". 

 This journal summarizes all meteoro- 

 logical data associated with the 

 passage of tropical waves, distur- 

 bances, storms, and hurricanes for 

 each year's hurricane season. 



The three primary forces asso- 

 ciated with the passage of a hurri- 

 cane are wind, storm surge, and 

 rain. As discussed previously, sus- 

 tained winds greater than 119 km per 

 hr (74 mph) must exist prior to a 

 tropical storm's graduation to hur- 

 ricane status. Sustained winds over 

 200 km per hr (125 mph) must be 

 present for a hurricane to be clas- 

 sified a "Great Hurricane". Ball et 

 al. (1967), Pray (1966), and Perkins 

 and Enos (1968) describe the pas- 

 sages of two "Great Hurricanes", 

 Donna (Sept. 1960) and Betsy (Sept. 

 1965), through the Florida Keys. 

 Winds over 200 km per hour (125 mph) 

 have occurred in the study area on 

 several occasions during the last 

 century (Sugg et al. 1971, see Fig- 

 ure 17). The most notable was the 

 "Labor Day" hurricane in 1935 which 

 passed over Long Key with winds 

 estimated between 322 to 402 km per 

 hr (200 to 250 mph) (Bradley 1972). 



One can best appreciate the 

 ecological significance of hurri- 

 canes when it is understood that the 

 wind force increases by the square 

 of the wind speed. In other words, 

 a 150 km per hr (93 mph) wind exerts 

 four times as much force as a 75 km 

 per hr (47 mph) wind. When hurri- 

 cane winds exceed 400 km per hr 

 (249 mph, as was estimated for the 

 "Labor Day" hurricane by Gentry 

 1974) their strength becomes almost 

 inconceivable. 



40 



