from the weather cycle through pav- 

 ing and other urban development 

 further amplifies the shift toward 

 higher temperatures. 



The insidious implications of 

 this situation for fish and wild- 

 life, as well as for the human 

 population of south Florida, have 

 recently been noted by Arthur Mar- 

 shall (Boyle and Mechum 1982). 

 His hypothesis is that development 

 and drainage have slowly replaced 

 Florida's wet season "rain machine" 

 with a relatively drier "heat ma- 

 chine" during summer months. Thus 

 wet season rains which are so vital 

 to south Florida's ecosystems occur 

 less frequently due to massive 

 changes in the daily heat budget. 



3.3 WINDS 



Wind patterns in south Florida 

 are determined by the interaction of 

 prevailing easterly tradewinds and 

 localized diurnal factors produced 

 by land-sea convection patterns 

 (during the wet season), or synoptic 

 scale cold fronts (during the dry 

 season) (Echternacht 1975). In a 

 comprehensive examination of season- 

 al differences in the large scale 

 wind fields for the Florida penin- 

 sula, Gruber (1969) described the 

 seasonal streamlines at three ver- 

 tical levels: 950 millibars (mb) 

 occurring at to 610 m (0 to 2000 

 ft); 500 mb, occurring at 5,486 to 

 6,096 m (18,000 to 20,000 ft); and 

 200 mb, occurring at approximately 

 12,192 m (40,000 ft). His work was 

 summarized by Echternacht (1975) in 

 an attempt to apply the wind field 

 patterns to potential air pollution 

 problems affecting south Florida. 

 Figure 13 illustrates the four sea- 

 sonal wind field patterns adapted by 

 Echternacht (1975) at the 950 mb 

 level (i.e., for low-level winds). 

 For the Everglades/Bay/Keys basin 

 Figure 13 shows a dominant easterly 



influence varying from due east in 

 fall and winter seasons, to east 

 southeast in spring and summer. 



This prevailing easterly flow 

 interacts with the two seasonal wind 

 patterns described previously. Dur- 

 ing the wet season (May to October), 

 convective scale winds initiated by 

 thermal gradients at the land-sea 

 interface find support from the 

 prevailing southeasterly winds 



(Pielke 1973). The heating of the 

 land surface promotes Seabreeze cir- 

 culation during the day resulting in 

 the convergence of warm moist air 

 over the peninsula (Dames and Moore 

 1978, Gannon 1978). At night the 

 process reverses, the land cools 

 faster than the ocean, and air tends 

 to diverge away from the peninsula. 

 The recurrent wind cycle and mari- 

 time influence is significant to the 

 area's wet season climate due to the 

 flat terrain and proximity to the 

 water ( < 40 m or 25 mi) (Bradley 

 1972, Echternacht 1975). Frank 



et al. (1967) monitored the daily 

 changes in divergence over the 

 Florida peninsula for the summer 

 months of June through August. As 

 illustrated in Figure 14, a pro- 

 nounced diurnal pattern was recorded 

 showing very strong convergence 

 (i.e., negative divergence) during 

 the day (peaking around 12:00 to 

 2:00 E.S.T.). Therefore, the con- 

 vective scale is the fundamental 

 scale of motion during the basin's 

 wet season (Echternacht 1975). 



In the dry season (November to 

 April) the convective influence 

 diminishes as the sun's angle of 

 incidence decreases, reducing the 

 radiant heating of the land's sur- 

 face during the day and thus mini- 

 mizing the thermal gradient between 

 the land-sea surfaces (Blair and 

 Fite 1965). During this time the 

 wind patterns are influenced by 

 synoptic scale systems or winter 



31 



