IX. LOWER LOW WATER 
Definitions 
The apparent daily movements of sun and moon about the earth take place in 
planes inclined to that of the Equator, and this gives rise to two different constituents 
in the tide, one having a period of half a day and the other a period of a day. The 
actual tide, resulting from the interaction of the semidaily and daily constituents, is 
therefore characterized by differences as between morning and afternoon tides, or, 
more precisely, by diurnal inequality. In general, consecutive low waters differ in 
height, necessitating the distinction between lower low water and higher low water. 
Of the two low waters of a day the lower is designated as the lower low water and the 
higher as the higher low water. 
Since the length of the tidal day is 24 hours and 50 minutes, there will be calendar 
days when but one low water occurs, the second one coming after midnight of that 
day and therefore on the next day. In such cases the question arises as to the desig- 
nation to be applied to that single low water. Various rules may be formulated for 
this purpose, but for the practical purposes of datum plane determination a satisfactory 
rule is to give such a single low water the opposite name from the immediately preceding 
low water: that is, if the immediately preceding low water was the lower low water for 
the day, then the single low water in question will be designated as a higher low water, 
and vice versa. Thus, as shown in the column of low waters in the tabulation of 
Figure 16, only one low water occurred at Boston on June 28, 1944. The immediately 
preceding low water which occurred at 23.3 hours on the 27th was a higher low water; 
hence the single low water of the 28th would be designated as a lower low water. 
At some places, however, only one low water may occur during a day because the 
tide is of the diurnal or daily type. In such cases the single low water of the day is 
designated as the lower low water. There is usually no difficulty in deciding whether 
the single low water of a day is due to the tide becoming diurnal or to the failure of the 
second low water to occur before midnight of that day. Diurnal tides occur only in 
regions having very considerable diurnal inequality, so that the characteristics of the 
tide for the day in question readily determine whether or not the single low water is 
due to the occurrence of a diurnal tide. 
Variations 
The depth to which lower low water falls varies from day to day. For Los Angeles 
for the month of October 1947 this variation is brought out graphically in the middle 
diagrams of Figures 44 and 45. 
A detailed study of the lower low waters at any place brings out the fact that the 
variation from day to day is partly of a periodic nature, due to the change in position 
of the moon relative to earth and sun, and partly nonperiodic, due to changes in sea 
level. Referring to Figure 45, it is seen that with regard to the moon’s declination, 
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