GULF OF MEXICO 



111 



utes. In the daily t^vpo tlu- average retardation 

 in time likewise is 50 minutes, but from one day 

 to the next it may vary from this averap;e by 

 several hours either way. This seeniino; irrej^u- 

 larity in time is not due to ehanee effects, the 

 tides of the daily type can be predicted quite 

 accurately, but to the interaction of the com- 

 ponent tides. 



The most prominent variation in range of tide 

 in the daily type is the fortnightly change asso- 

 ciated with the moon's declination. Wlien the 

 moon is near its maximum semimonthly declina- 

 tion the range of tide in the daily type is greatest, 

 the tides then being called tropic tides, and when 

 the moon is over or close to the equator the tide 

 has its least range, the tides then being called 

 equatorial tides. This is exemplified for the tide 

 at Pensacola by the lower curve of figure 23. For 

 the latter part of June 1948 the moon was at its 

 maximum southern declination on the 21st and 

 over the equator on the 29th. 



In general, it may be expected that in the 

 diurnal type of tide, there will be several days 

 around the time the moon is over the equator 

 when two high waters and two low waters will 

 occur, the larger the ratio of Ki-f-Oi to M2 + S2 

 the smaller the number of these days. These 

 secondary tides pose troublesome questions in 

 connection with the determination of the mean 

 range of tide in tides of the diurnal type. Such 

 secondary tides do not enter suddenly nor do they 

 fade out suddenly. Practically, it becomes diffi- 

 cult to determine on a tide record whether the 

 fluctuations are true secondaiy tides or fluctua- 

 tions due to wind and weather. Furthermore, 

 there is the question at what stage of development 

 of these secondary tides shall they be included; 

 when the range is 0.01 foot, 0.1 foot, or what? 



To obviate these difficulties, it is customary to 

 consider the diurnal tide as if it were diurnal at 

 all times and in deriving the mean range to disre- 

 gard the secondary tides. Thus, in figure 23 in 

 the tide curve for Pensacola on the 29th, the sec- 

 ondary low water immediately after noon and 

 the following secondary high water would be dis- 

 regardeti, and but one high water and one low 

 water in the day would be considered. In deter- 

 mining mean high water and mean low water in 

 tides of the diurnal type only one high water and 

 one low water a day, likewise, are considered. 



in figure 23, on June 23 when the moon was 2 

 days past its semimontlily maximum declination, 

 the range of tide at Pensacola was 1.7 feet; on the 

 21st, the range was 1 .9 feet. On the 29th, when the 

 moon was over the equator the range (lea.ving the 

 secondary tides out of consideration) was 0.5 foot. 

 In the 8-day period, therefore, the range de- 

 creased 74 percent. This relatively large per- 

 centage change in range in response to the moon's 

 declination is typical of daily tides. 



At times, the terms "spring tides" and "neap 

 tides" are used in connection with the fortnightly 

 variation in range in the daily type discussed 

 above, but this usage is confusing, the correct 

 terms being tropic tides and equatorial tides. 

 In the daily type of tide the variation in range 

 with the moon's phases is so small that it may be 

 disregarded for most purposes. The same remark 

 applies to the variation in response to the moon's 

 parallax, that is, apogean and perigean tides. 



A variation in range of tide that is prominent 

 in the daily type but not in the semidaily is of an 

 annual period in response to the annual variation 

 in the declination of the sun. This is illustrated 

 in figure 30 bv the results of the observations at 



FiGiRE 30. — Annual variation in range of tide, 

 Pensacola. 



Pensacola from the 19-year series 1931-49. The 

 horizontal line, corresponding to the value of 1.26 

 feet, represents the mean value of the range at 

 Pensacola, while the circles represent the average 

 ranges for the different months of the year. 

 During the year, the range is least in March 

 and September, corresponding with the sun's 

 equinox and greatest in June and December, cor- 

 responding to the sun's solstices, the difference 

 between September and December being on tlie 

 average 0.26 foot for Pensacola. 



