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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



where the daily constituent is less than twice that 

 of the semidaily, the resultant tide will exhibit 

 two high and two low waters a day with the in- 

 equality in the tides depending on the phase rela- 

 tions between the constituents. When the ratio 

 is between 2 and 4 the resultant tide will have 

 either two high and two low waters a day or only 

 one high and one low water depending on the 

 phase relations between the two constituents. 

 When the ratio is 4 to 1 or greater, there will be 

 only one high water and one low water a day. 



With this brief discussion of harmonic constants 

 and the combination of constituent tides, ad- 

 mittedly sketchy, we are in a position to discuss 

 types of tide. 



TYPES OF TIDES 



The great variety of tides found throughout the 

 world can be grouped under three large classes 

 or types: semidaily, daily, and mLxed. The 

 idealized forms for these different types are fur- 

 nished by the component tides and their com- 

 binations. 



The semidaily type of tide includes all those in 

 which the tidal cycle is completed in half a day; 

 that is, there are two high and two low waters a 

 day with only little difference between correspond- 

 ing morning and afternoon tides. 



The daily type of tide includes tides in which only 

 one high and one low water occur in a day. The 

 mixed type of tide includes those tides which 

 feature two high and two low waters a day but 

 with considerable difference between the two 

 high waters and/or between the two low waters 

 of the day. 



It is to be noted that when the tide at any 

 place is classed with a particular type in accord- 

 ance with the above definitions, it is the generally 

 prevailing or predominating features that are 

 considered. For example, in figure 23 the tide at 

 Miami would be designated as the semidaily type, 

 although for a few days each fortnight, morning 

 and afternoon tides exhibit some inequality. In 

 the same way the tide at Cedar Keys would be 

 classed with the mixed tides, although as the last 

 2 days of figure 23 show, but little inequality was 

 featured by the tide. Pensacola would be classed 

 with the daily tide because for the greater part 

 of the time but one high and one low water a day 

 occur. 



For general purposes, the definitions of the three 

 types of tide given above are sufficient, but it is 

 clear that these definitions do not provide sharp 

 distinctions between the three types. For tech- 

 nical purposes, definite criteria may be determined 

 through the use of harmonic constants. 



Of the five principal harmonic constituents 

 listed in table 1, K, and Oi represent the principal 

 daily components, while M2 and S2 represent the 

 principal semidaily components. The ratio of 

 Ki-l-Oi to M2-I-S2 at any place therefore defines 

 the relative magnitudes of the principal daily 

 and semidaily components at that place. Van 

 der Stok (1897) suggested the use of these ratios 

 for classifying types of tides. Where (Ki + 0i)-4- 

 (M2+S2) is less than 0.25 the tide is classed as 

 semidaily; where it is between 0.25 and 1.50 it is 

 classed as the mLxed type; and where it is greater 

 than 1.50 it is classed with the daily type. 



The diurnal type of tide under the above classifi- 

 cation is found to cover a great variety of tides, 

 from those only infrequently diurnal to those 

 predominantly diurnal. In recent years it has 

 been suggested (Courtier 1938) that the diurnal 

 type be separated into two types: (1) mixed 

 diurnal, where the ratio is between 1.5 and 3; 

 (2) diurnal, where the ratio is greater than 3. 

 The term "mixed diurnal," in turn, suggests mixed 

 semidiurnal as a better designation than mixed. 



For convenience, the four types of tide and the 

 corresponding ratios of K,-fOi to M2 + S2 are 

 given in tabular form in tabic 2. It should be 



Table 2. — Types of tide 



noted that the terms semidaily and semidiurnal 

 are used as synonymous and likewise the terms 

 daily and diurnal. 



The harmonic constants of the tide at any place 

 are derived from tide observations at that place by 

 means of the harmonic analysis. For a precise 

 determination of these constants, a year of ob- 

 servations is desirable, but shorter series of 

 observations will furnish dependable results. The 

 harmonic analysis is a highly specialized process, 

 which need not be considered here. For a de- 

 tailed discussion see Manual of Harmonic Analysis 



