seasonal insolation cjrve for the delta 



skewed to the left with 



in May, falling off 



and July because of 



coast is probably 

 peak insolation 

 somewhat in June 

 clouds . 



Temperature 



As one might expect, seasonal air 

 tenperatures follow insolation closely. 

 Mean monthly temperatures range from a 

 Decenber/January low of about 14°C to a 

 midsummer high of about 30°C. Temperature 

 at the U, S. Weather Bureau station in New 

 Orleans (Figure 7) is fairly 



representative of the coast because New 

 Orleans is surrounded by marshes and 

 water. Because of the moderating effect 

 of the water bodies and the high 

 humidities, midday temperatures seldom 

 exceed the low 30's (Celsius) despite the 

 high insolation. During winter in the 

 coastal marshes, freezes are infrequent, 

 and the average number of frost- free days 

 is about 300. In fact, the barrier 



island. Grand Isle, was chosen for the 

 site of a sugar cane breeding laboratory 

 by the Louisiana State University (LSU) 

 Agricultural Experiment Station because 

 the lack of frost allowed sugar cane fruit 

 to ripen there. Since most of the 



inshore waters are less than 1 m deep. 



90 



- 80 



?x 

 z 



lU uj 70 

 tr (T 



^ < 

 < u. 60 

 cc 



UJ w 

 0. u 



2 liJ 50 



UJ T 

 I- O 



a 40 



30 



30- 



20 



UJ 



a 

 < 

 a. 



(3 



t- 



2 

 UJ 

 O 



03 



UJ 



C 10 



o 



J F M A 



Normal 



Maximum 



Minimum 



water temperature follows air temperature 

 closely, with a lag time of a few hours at 

 most. 



Water Balance 



The water budget includes rain, 

 evapotranspiration, local runoff from 

 adjacent uplands, upstream discharge into 

 wetlands by rivers entering the region, 

 and marine water pumped in and out by 

 tidal and meteorologic forces (Figure 8). 

 Each of these varies in both time and 

 place; the resultant flooding frequency, 

 volume, and water quality on the marsh are 

 at present predictable only as average 

 trends. No present models capture the 

 details adequately. 



Precipi tation. Annual 

 averages about 160 cm spread 

 over the year (Figure 9). 

 to be the driest month and 

 test, but torrential rains 

 that any month can be either dry 

 experience precipitation of up to 50 



precipitation 

 fairly evenly 

 October tends 

 July the wet- 

 are common so 

 or 

 cm. 



Muller (Wax et al. 1978) analyzed the 

 atmospheric circulation of the Louisiana 

 coast. Typically high pressure systems 

 moving in from the north and west bring 

 cool, dry air. They dre easily recog- 

 nized during the winter as "cold fronts" 

 but occur throughout the year. They are 

 typically followed by atmospheric condi- 

 tions that bring warm gulf air in from the 

 coast, usually with heavy cloud cover and 

 rain. About two-thirds of the coastal 

 rainfall is associated with frontal activ- 

 ity of this kind. During 1971-74 about 13 

 percent of the rainfall was from infre- 

 quent, severe tropical storms and hurri- 

 canes. 



EVAPOTRANSPIRATION 



Figure 7. Mean monthly air temperature at 

 New Orleans, Louisiana (NCAA 1979). 



Figure 8. Generalized water budget for 

 the Mississippi delta marshes. 



10 



