1:40,000. These photographs are centered on quarters of 7.5-min quadrangles. Information 
about these photographs can be obtained from EDC. 
5. ATMOSPHERIC, GEOLOGICAL AND ASTRONOMICAL PHENOMENA 
5.1. South Florida climate 
South Florida has a tropical climate, with a summer wet season and a dry season from mid fall 
through late spring (Duever et al., 1994). Average temperatures are warm all year, but 
occasional freezes, associated with winter cold fronts, occur during the winter months. 
Thunderstorms are the major source of rainfall. During some years, tropical storms and 
hurricanes, and cold fronts can contribute significantly to the rainfall. Besides the annual cycle 
rainfall patterns are associated with a minor bimodal peak during the wet season and a 5- to 
6*yr cycle associated with global climate cycles. The long-term trend in total annual 
precipitation has been essentially constant over the past 100 yrs. Evapotranspiration is lowest 
during the cool winter months and highest in late spring, after which it declines only slightly 
during the summer months. Freezes play a large role in controlling the distribution of tropical 
elements of the fauna and flora of south Florida. In general, they are more severe farther north 
or inland from the ocean. There is no clear evidence of any change in frequency or severity of 
freezes over the last 40 yrs. Droughts can significantly alter composition and structure of 
aquatic animal communities, provide opportunities for germination of wetland vegetation, and 
set the stage for fire. Individual droughts may affect all of south Florida, but they are 
frequently restricted only to portions of it. 
The most severe drought on record in Florida as of 1961 occurred during the 3-yr period of 
1954 - 1956 (Pride and Crooks, 1962). The drought was caused by rainfall deficiencies in 
amounts ranging from 7 - 11 in during each of the three years. Minimum stream flows recorded 
at 135 continuous record stations, low-flow measurements at 190 partial record stations, and 
chemical analyses of water at 133 sites are summarized in Pride and Crooks (1962). Records 
of streamflow at 13 representative streams and records of stage for 17 representative lakes 
were compared with average flows or stages. The dissolved solids concentrations in most 
streams increased as flows declined. Less severe draughts occurred in 1916, 1920, 1926, 
1930, 1938, 1942, and 1949-1951. Annual total rainfall for the Florida lower East Coast is 
shown in Figure 3. 
Severe freezes have occurred in Florida in February 1917, January 1928, December 1934, 
January 1940, February 1947, the winter of 1957-58, December 1962, November 1970, 
January 1971, January 1977, January 1981, January 1982, December 1983, and January 
1985 (Myers, 1986). Especially cold winters were those of 1941-1942, 1957-1958, 1963- 
1964, 1969-1970, and 1989. The most severe were those of 1957-1958, 1969-1970, and 
1989. During these severe periods, the growth pattern of a Montastrea annularis specimen 
were interrupted (Emiliani et al., 1978). 
Weather may influence the bioaccumulation of chemicals as changes in temperature and salinity 
affect biochemical processes. Rainfall is one of the most important ways of mobilizing 
contaminants in soil and urban areas and may affect the amounts of contaminants that reach 
coastal and estuarine environments. The effect of global climate changes in tropical marine 
systems, such as mangroves, seagrass beds, and coral reefs, is being examined by several 
workers (e.g., Zieman et al., 1989; Cubit, 1992; Davis, 1992; Zieman et al., 1992; Twilley 
and Chen, 1994; Cubit, 1994; Davis et al., 1994; Winter et al., 1994; Zieman et al., 1994; 
and others). Seasonal variations in temperature and salinity influence abundance, productivity, 
and reproduction of macroalgae and seagrass in south Florida (Tabb et al., 1962; Josselyn, 
1977; Bach, 1979; Morrison, 1981; Zieman, 1988; Montague and Ley, 1993; and others). 
