VIII. MEAN LOW WATER 
Variations in Fall of Low Water 
The variations in the fall of low water resemble closely those in the rise of high 
water, especially in regard to those depending on the moon’s position. Not only does 
the tide rise higher than usual at the times of full and new moon, but it also falls lower, 
while at the times of the moon’s first and third quarters the less-than-average rise of 
high water is accompanied by a fall in low water also less than average. Similarly, 
when the moon is in perigee the fall, like the rise, is greater than usual, while at the time 
of the moon’s apogee the rise and also the fall are less than usual. 
The periodic semimonthly change in the declination of the moon brings about 
variations in the fall of low water, causing consecutive low waters to differ. This 
diurnal inequality in the low waters necessitates the distinction between higher low 
water and lower low water. When the moon is near its semimonthly maximum declina- 
tion, the two low waters of a day show the greatest difference in fall, and when the 
edna of the moon is small—that is, when the moon is near the Hiquetor— the 
difference between the two low waters is leacia 
In Figure 44 are plotted the heights of the successive low waters for the month of 
October 1947 at Atlantic City, Los Angeles, and Pensacola. These ilustrate the 
typical variations from day to day in the semidaily, mixed, and daily types of tide, 
respectively. , 
At Atlantic City there were two low waters each tidal day and in general the heights 
of successive low waters do not differ much. On comparing this curve of low waters 
with the corresponding curve of high waters in Figure 38 it is seen that successive 
high waters at Atlantic City exhibit greater differences than do successive low waters. 
This is typical for the Atlantic coast of the United States, the diurnal inequality along 
that coast being greater in the high waters than in the low waters. 
At Los Angeles there are seen to have occurred, likewise, two low waters each 
tidal day, but here the differences between successive low waters are considerably 
greater than at Atlantic City. On the 8th, for example, morning and afternoon low 
waters differed by 3.3 feet. On comparing this curve with the corresponding curve 
for the high waters at Los Angeles in Figure 38, it is seen that the inequality in the low 
waters is greater than in the high waters. For that particular month the difference 
between the two high waters of a day at Los Angeles averaged 1.1 feet while the dif- 
ference between the low waters averaged 1.8 feet. This difference in inequality as 
between the high and low water is a characteristic feature of the tide at Los Angeles, 
but as we shall see later these inequalities are subject to cyclic variations. 
It is of interest to note also that no high water at Los Angeles during the month of 
October 1947 fell below the average half-tide level for the month. In Figure 44 seven 
low waters rose above half-tide level. For that month the difference in height between 
the highest and lowest low waters—on the 21st and 31st, respectively—was 4.2 feet. 
The average difference between the high and low waters that month was.3.8 feet, so 
that the difference between two low waters that month exceeded the average difference 
between the high and low waters. 
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