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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



from the changes in the level of the water brought 

 about by the changing meteorological conditions. 

 In changing the level of the water from which the 

 tide rises and falls wind and weather disturb both 

 the times of occurrence and the heights of the 

 high and low waters. 



With respect to the disturbances brought about 

 by changes in barometric pressure the water in 

 any coastal body of water may be regarded as 

 constituting a huge water barometer. Wlien the 

 barometric pressure over this body of water rises 

 as compared with the pressure over the open sea 

 the water inside will fall, while with a lesser 

 pressure the water will rise. Since the specific 

 gravity of mercury is about 13 times that of 

 water a change in barometric pressure of 1 inch 

 should be accompanied by a change in the level 

 of the water of a little more than a foot. 



Actually, however, the matter is not quite so 

 simple, for it is not so much a question of difference 

 in barometric pressure over a coastal body of 

 water and the open sea as it is of barometric 

 gradient. Furthermore, pressure differences are 

 accompanied by winds, and the effects of the 

 latter on the rise and fall of the tide are much 

 more pronounced than the direct effects of baro- 

 metric pressure. 



From general considerations, it is clear that in 

 waters having equal ranges of tide the same wind 

 will have greater disturbing effects in the body 

 of water having the lesser depth. Likewise, in 

 bodies of water of equal depth, a given wind will 

 be accompanied by greater disturbing effects in 

 the body of water having the lesser range. 

 Finally, in waters having equal depths and ranges 

 of tide, wind effects are more disturbing on tides 

 of the mixed and daily types than on tides of the 

 semidaily type, and since in the Gulf the depths 

 are relatively shallow, the ranges of tide rather 

 small and the type of tide mixed and diurnal, it is 

 to be expected that the disturbing effect of the 

 wind on the tide will be quite pronounced. 



For any particular stretch of coast, too, the 

 hydrographic features enter as factors into the 

 effects of a given wind. The problem is there- 

 fore a complex one necessitating detailed studies 

 for each body of water. Such studies are almost 

 completely wanting. In connection with not- 

 able storms, the heights attained by high water 

 will be noted by the meteorologist or engineer 



studying the effects of the storm in a particular 

 harbor.' 



Needed, too, are studies correlating to the height 

 of the tide at various places to storm tracks so 

 that approximately accurate predictions of the 

 height of a storm tide to be expected may be 

 made from the characteristics of a developing 

 meteorological disturbance. A closely related 

 problem, the use of observed changes in the tide 

 to furnish information regarding the direction and 

 movement of a storm still far out in the Gulf, is 

 discussed by Cline (1920). 



In bays and harbors receiving the flow from 

 large drainage areas the tides are also subject to 

 disturbances arising from variations in run-off. 

 In general, however, these disturbing effects be- 

 come pronounced only in the upper reaches of 

 tidal streams. 



THE TIDE IN THE GULF OF MEXICO 



Thus far, the discussion of the tides at various 

 places in the Gulf of Mexico has considered the 

 tide at each place as a local phenomenon. How 

 do these tides tie together? In other words, 

 what is the dynamics of the tidal movement in 

 the Gulf as a whole? What explains the rela- 

 tively large diurnal component in the Gulf as 

 compared with the Atlantic Ocean? Only few 

 investigations of this character have appeared in 

 print. 



S. F. Grace (1932) briefly reviews the explana- 

 tions offered by various investigators. The dy- 

 namics of the tide in any large body of water is a 

 difficult problem. In the Gulf of Mexico, it is 

 further complicated by the irregular shape of the 

 basin and by its connections with the Atlantic 

 Ocean and the Caribbean Sea which pose the 

 question of co-oscillation with the tidal move- 

 ments in those bodies of water. 



Qualitatively, the simplest explanation of the 

 relatively large diurnal component in the Gulf is 

 that the length and depth of its basin are such 

 that its free period of oscillation approximates 

 24 hours; that is, it approximates the period of 

 the diurnal tide-producing forces and therefore 

 responds better to the diurnal forces than to the 

 semidiurnal forces. 



1 See for example: The Galveston Hurricane of September 8, 1900, Monthly 

 Weather Review. Sept. 1900, pp. 371-377, and The Tidal Storm at Corpus 

 Christi and Its Effect on Engineering Structures, Engineering News-Record, 

 Nov. 13-20, 1919, pp. 848-852. 



