81 



Local air, water, and ground temperatures, which govern the 

 form in which precipitation occurs, are primarily a matter of solar 

 radiation, which becomes more intense in latitudes nearer the equator. 

 Local temperatures are, however, greatly moderated b^^ local precipita- 

 tion, cloud cover, nearby ocean conditions, and prevailing winds. Two 

 examples serve to illustrate this point : 



(1) Key West, Fla., on an island in the warm waters of the 

 Gulf of Mexico, has an average temperature of Y7° F. ; Browns- 

 ville, Tex., in about the same latitude but on the mainland, has an 

 average temperature of 74° F. At Key West annual temperatures, 

 moderated by the marine environment, range over only 49 degrees, 

 whereas the range at Brownsville is 85 degrees. 



(2) Astoria, Oreg., at the mouth of the Columbia River, and 

 Portland, Me., are both in the same latitude in zones of prevail- 

 ing westerly winds. At Astoria, where the winds are blowing off 

 the Pacific Ocean, there are Y6 inches of precipitation, including 

 4 inches of snow. At Portland the prevailing winds blow off the 

 continental land mass and there are 43 inches of precipitation, but 

 72 inches of snow. 



TIDE 



The tide stands alone as a controlling force in the estuarine envi- 

 ronment. The ebb and flow of the tide are the great facts of the 

 estuarine zone, and have determined much of man's history from the 

 time Julius Caesar lost a fleet because of the tides in the English 

 channel to the time of D-Day in 1944, which was set because of the 

 right combination of tide and moon. 



Tides are easily understood. The Sun, the Moon, and the Earth 

 mutually attract each other, according to Newton's law of gravita- 

 tion ; 2 the great masses of fluid in the ocean, being more sensitive to 

 tiny changes in gravitation force than the solid land, are pulled about 

 rather freely in a predictable fashion based on the relative positions of 

 Sun, Moon, and Earth. They are predictable to such an extent that 

 tables of accurate predictions of tidal height are published for each 

 day of each year for each major j)ort of the world. Such predictions 

 are valuable both to the captain trying to dock a large oil tanker and 

 to the fisherman who is trying to find where the big ones are biting. 



Perhaps because tides are so easily understood and predicted, and 

 are so easily observable everywhere, their importance in the estaurine 

 zone has been largely overlooked. 



Table IV.1.4 gives typical tidal characteristics in several estuaries of 

 the United States. It is immediately apparent that tides on each coast 

 of the United States are different. Along the Atlantic and Pacific 

 coasts there are semidiurnal tides, i.e., two complete tides in a little 

 over one day, but the Atlantic tides are equal and the Pacific tides 

 are unequal. In the Gulf of Mexico most places have one tide a day, 

 i.e., diurnal, but some places such as Tampa Bay exhibit both kinds 

 of tides at different times of the month. 



2 It Is Interesting to note that observations of the rhythmic rise and fall of the tide 

 led to the mathematical concepts through which the law was formulated. 



