Tropical storms and hurricanes, 

 because of their infrequent occui — 

 rence, significance as an ecological 

 force, and unique climatic charac- 

 teristics, are treated here as a 

 separate climatic element. 



In summer and fall, occasional 

 low-pressure areas are observed 

 which originate in the warm, moist 

 air of the equatorial trough. The 

 winds are light and usually drift 

 from east to west. Then an atmo- 

 spheric wave appears in the easterly 

 flow and proceeds westward at 16 to 

 24 km per hr (10 to 15 mph) (Blair 

 and Fite 1965). These easterly 



waves usually form between 5° and 

 20° north of the equator. From this 

 point the easterly wave development 

 may go through one or all four 

 stages of a tropical cyclone as 

 described by Riehl (1954): 



(1) Formative stage. Winds usu- 

 ally remain below hurricane 

 force with the strongest 

 winds generally occurring in 

 one quadrant, poleward and 

 east of the center of a 

 deepening of the barometric 

 trough. Areas of weak wind 

 circulation (less than 61 km 

 per hr or 38 mph) are refer- 

 red to as "tropical depres- 

 sions" or "tropical distur- 

 bances". These disturbances 

 move in a very rough coun- 

 terclockwise direction and 

 may travel great distances 

 organized as such (Gentry 

 1974). 



(2) Immature stage. If the 

 shallow depressions inten- 

 sify with winds exceeding 

 61 km per hour (38 mph) the 

 "tropical depression" has 

 become a "tropical storm" 

 characterized by barometric 

 pressures dropping to 1000 

 mb and below, and winds 

 forming tighter concentric 

 bands around the center or 

 eye. The cloud and rain 



patterns also change from 

 disorganized squalls to nar- 

 row organized bands spiral- 

 ing inward (Riehl 1954). 

 If the winds intensify to 

 119 km per hr (74 mph) or 

 more, a tropical cyclone or 

 hurricane is born (Gentry 

 1974). Still only a rela- 

 tively small area is invol- 

 ved, i.e., hurricane force 

 wind radius of 32 to 48 km 

 (20 to 30 mi) (Riehl 1954). 



(3) Mature stage. The surface 

 pressure at the center is no 

 longer falling and the maxi- 

 mum wind speed no longer 

 increases (Riehl 1954). In- 

 stead, the circulation ex- 

 pands, extending the radius 

 of hurricane force winds. 



(4) Decaying stage. Tropical 

 cyclones, both mature and 

 immature, generally move 

 westward in the prevailing 

 westward drift of the east- 

 erlies. They enter the de- 

 caying stage as they recurve 

 from the tropics and enter 

 the belt of westerlies, 

 usually decreasing in size 

 (Riehl 1954, Blair and Fite 

 1965). 



During the immature and mature 

 stages the general westward movement 

 ranges from 16 to 48 km per hr (10 

 to 30 mph). The typical path is 

 parabolic, although the actual path 

 of any given storm is governed by 

 the winds existing above it, result- 

 ing in a multitude of speed and 

 directional changes (Blair and Fite 

 1965). Blair and Fite (1965) pro- 

 vide a concise description of the 

 passage of a hurricane over the 

 Everglades/Bay/Keys basin: 



"As such a storm approaches, the 

 barometer beings falling, slowly at 

 first and then more and more rapid- 

 ly, while the wind increases from a 

 gentle breeze to hurricane force. 



39 



