TIDES AND SEA LEVEL IN THE GULF OF MEXICO 



By H. A. Marmer, United States Coast and Geodetic Survey 



The feature of the tide that makes the most 

 marked impression on the observer along the shores 

 of tidal waters is that relating to the magnitude of 

 rise and fall, or the range of the tide, and where 

 the range is large the tide is indeed an impressive 

 phenomenon, visualizing in striking fashion the 

 ceaseless warfare between land and sea. Wide 

 stretches of foreshore constitute the field of battle. 

 At low water these are seemingly part of the land, 

 but a few hours later, at high water, the invading 

 tide claims them again for the sea. 



In the Gulf of Mexico the tide exhibits no such 

 impressive sights, for the range of tide here is 

 small, being at most places not much more than 

 a foot or two on the average; but what the tide 

 lacks in impressiveness for the casual observer, it 

 more than makes up in the variety and complexity 

 of the phenomena it oflFers the investigator. 



Since the Gulf of Mexico is an arm of the 

 Atlantic it is natural to attempt to correlate the 

 tides in the two bodies of water. Figure 23 shows 

 the tide curves for three stations on the coast of 

 Florida for the last 8 days of June 1948. Miami 

 Beach is on the Atlantic coast, and Cedar Keys 

 and Pensacola are on the Gulf coast. The 

 vertical lines at the top of the figure indicate noon 

 of each day. The horizontal line associated with 

 each set of curves represents the average or mean 

 level of the sea at each station for the 8-day period. 

 The height of the tide at each station is shown, in 

 feet, by the scale at the left and is reckoned from 

 sea level. 



Examining the curve for Miami Beach it is 

 seen that high water and low water succeeded each 

 other at intervals of about 6)4 hours so that in each 

 tidal day (which has an average length of 24 hours 

 and 50 minutes) there are two high waters and 

 two low waters. The high waters rose approxi- 

 mately the same height above sea level as the low 



Note.—H. A. Marmer died before this article was set in print. The 

 proofs were corrected in the Division of Tides and Currents, U. S. Coast and 

 Geodetic Survey. 



waters fell below it, and while the two high waters 

 of a day, as also the two low waters, differed some- 

 what in height, the difference was relatively small 

 as compared with the range of the tide which for 

 the 8-day period averaged 2.0 feet. 



At Cedar Keys, as the middle diagram shows, 

 there were likewise two high and two low waters 

 a day, but consecutive cycles of rise and fall 

 differed considerably more than at Miami Beach. 

 For example, from the first high water on the 23d 

 to the first high water on the 24th the consecutive 

 ranges, in feet, were as follows: 1.2, 2.3, 4.5, and 

 3.4. For that tidal day, therefore, the average 

 range was 2.85 feet, but individual ranges varied 

 from 58 percent above to 58 percent below that 

 average value. The durations of rise and fall 

 likewise varied considerably. On the first day 

 they were, in hours, 5.0, 5.5, 7.7, and 6.4. For 

 the succeeding days the differences in range and 

 in duration diminish more or less regularly until 

 on the last 2 days the characteristics of rise and 

 fall at Cedar Keys are much the same as at Miami 

 Beach. 



Now it must be noted that the differences found 

 in character of tide at the two places are in no 

 way due to the disturbing effects of wind or 

 weather. As will be seen later such disturbing 

 effects do occur and result in modifying pro- 

 foundly the normal features of rise and fall. The 

 last 8 days of June 1948 were chosen for illustra- 

 tion precisely because of the freedom during this 

 period of disturbing effects of wind and weather. 

 The differences in the features of the tide at the 

 two places for the 8-day period are due to differ- 

 ences in tidal character. 



Examining now the tide curve for Pensacola, 

 the difference from the other two tide curves that 

 strikes one immediately is that at Pensacola there 

 were but one high and one low water a day for 7 

 of the 8 days. In other words, the periods of rise 

 or fall here are approximately 12 hours against 6 

 hours at the other two places. 



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