“TIDAL DATUM PLANES 101 
distance and declination of the moon, the periods of which are approximately 29%, 
27% and 27% days respectively. Such variations are therefore largely eliminated in the 
period of a month. Within a month, too, the fluctuations in sea level from day to day 
due to changes in wind and weather tend to balance out. Hence it is to be expected 
that monthly low waters will show smaller variations than daily low waters. 
Monthly Low Water 
In Figure 46 are plotted the heights of monthly low water for each month of the 
two year period 1946-1947 at the three stations, Atlantic City, Los Angeles and Pensa- 
cola. From one month to the next, low water is seen to vary from a few hundredths of 
a foot to as much as half a foot or more. On comparing corresponding diagrams of 
monthly high and low waters in Figures 40 and 46 the variations from month to month 
are seen to be much the same. And since an annual variation in monthly high water 
was found, we may expect to find a similar annual variation in monthly low water. 
_ Figure 47 shows the average heights of monthly low water at the three stations, 
derived from the 19 year period 1930-1948. Comparison with the corresponding dia- 
grams of the annual variation in high water in Figure 41 shows close resemblance, both 
in amplitude and in phase. And as was noted in the discussion of the annual variation 
in high water, this annual variation depends on the annual variation in sea level. 
Monthly low water at any point is thus subject to variations both periodic and 
nonperiodic in character, but in both of these it follows the like variations in sea level. 
Jan. Mar. May July Sept. Dec. 
Atlantic City 
Los Angeles 
Pensacola 
Fic. 47.—Annual variation in low water, Atlantic City, Los Angeles, and Pensacola. 
