110 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



For comparison with the tides in the Gulf it 

 will be of interest to have the ratio of Ki + d 

 to M2 + S2 for Miami Beach, on the Atlantic 

 coast of Florida, for which place the tide curve 

 is shown in figure 23. For Miami Beach this ratio 

 is 0.17, and by the criteria of table 2 the tide there 

 is of the semidaily type. From the last column 

 of table 3 it is seen that at no place in the Gulf 

 does this ratio fall below 0.5 so that in the Gulf the 

 semidaily type is not represented. 



From Key West to St. Marks Light the ratio 

 is seen to be greater than 0.25 but less than 1.50 

 except for South Boca Grande. Hence, the tide 

 in this stretch is of the mLxed semidaily type. 

 From Warrington to Bayou Rigaud the ratio is 

 greater than 3, and therefore in this stretch the 

 tide is of the daily type. From Weeks Bay around 

 the remainder of the periphery of the Gulf to 

 Habana the ratio fluctuates between 1.5 and 6 

 except for Calcasieu Light where it is 1.26 and for 

 Habana where it is 1.14 so that the tide in this 

 stretch varies from mixed diurnal to diurnal with 

 only few exceptions. 



The classification of the tide into types is of 

 great convenience in tidal investigation, for as 

 soon as the tide at any place is particularized as 

 to type we are in possession of guiding principles 

 for the discussion of the tide at that place. It 

 will therefore be of advantage to summarize 

 briefly the characteristic features of the different 

 types of tide. 



SEMIDAILY TYPE 



The distinguishing feature of the semidaily 

 type of tide is the occurrence of two high waters 

 and two low waters in the tidal day (which has 

 an average length of 24'' 50™) with relatively 

 little difference between corresponding morning 

 and afternoon tides. High waters succeed each 

 other at intervals of about 12K hours, and likewise 

 the low waters. 



The predominant change in range in the semi- 

 daily type of tide is that in response to the moon's 

 phase, the greatest range, the so-called spring tides 

 coming near the times of new and full moon, and 

 the least range, the neap tides coming near the 

 times of the moon's first and third quarters. 

 The average interval between spring and neap 

 tide is one-quarter the length of the month of the 

 moon's phase (the synodic month), or approxi- 

 mately 7% days. During this interval from springs 



to neaps the range of tide in the semidaily type of 

 tide decreases from about 20 percent above the 

 average to about 20 percent below the average; 

 and in the next period from neaps to springs 

 returns to 20 percent above the average or mean 

 range. 



Another prominent variation in the range of 

 tide of the semidaily type is that in response to the 

 moon's parallax which has a period averaging 

 approximately 27 }2 days (the anomalistic month). 

 Near the times of the moon's perigee the range of 

 tide of the semidaily type is about 20 percent 

 greater than average, while near the times of the 

 moon's apogee the range is about 20 percent less 

 than average. Obviously, therefore, at such 

 times when the moon's perigee occurs near the 

 times of new or full moon, the range of tide will 

 be about 40 percent above the mean range, while 

 at the times when the moon's apogee occurs near 

 the times of the moon's quadratures the range 

 will be about 40 percent less than the mean. 



The third variation in range to which tides are 

 subject is that in response to the moon's declina- 

 tion, which manifests itself in a difference between 

 morning and afternoon tides, but is relatively small 

 in the semidaily type of tide. In the moon's fort- 

 nightly change from maximum northerly to maxi- 

 mum southerly declination, the differences be- 

 tween morning and afternoon tides are greatest 

 near the times of maximum declination and least 

 about the times the moon is over the equator. 



The range of tide is also subject to a slow change 

 over a period of 18.6 years due to changes in the 

 inclination of the moon's orbit to the equator. 

 When the inclination of the orbit to the equator 

 is at a maximum the semidiurnal range is less and 

 when the inclination is at a minimum the semi- 

 diurnal range is greater than the 18.6 year- 

 average. The change varies from about 3 percent 

 below to 3 percent above average. The year 

 1950 corresponds to the year of minimum range, 

 the year of the next maximum of this variation 

 being 1959. 



DAILY TYPE 



The dailj- type of tide is characterized by the 

 occurrence of only one high water and one low 

 water the greater part of the time. The change 

 from day to day in time of tide of this type is not 

 nearly so regular as in the semidaily type in which 

 the tide becomes later each day by about 50 min- 



