AS U. S. COAST AND GEODETIC SURVEY 
The lower diagram of Figure 22 shows that during July, when weather conditions 
were relatively more uniform, the changes in sea level from day to day were less than 
in January. Nevertheless, changes up to half a foot occur from day to day, and 
during the month sea level on the 23d was about a foot higher than on the 8th. 
The change in sea level from day to day depends primarily on variations in 
meteorological conditions, hence such changes are not periodic; that is, from one day 
to the next, sea level may be either higher or lower, depending on the weather. But, 
as will be shown in the discussion of monthly sea level, there is a seasonal variation in 
sea level, or more precisely, an annual variation. Thus, at Atlantic City sea level is, 
on the average, lowest during the early months of the year and highest in the late 
summer or early fall months. Since within a single month daily sea level may differ 
by as much as 23; feet as shown in Figure 22, it follows that within a year the differences 
between two daily sea levels may be greater. During 1939 sea level at Atlantic City 
for August 29 was 1.9 feet above the average sea level for that year, and for December 
8 it was 1.8 feet below. For these two days, therefore, sea level differed by 3.7 feet. 
Regions subject to storms of great intensity, especially those fronting shallow 
bodies of water, exhibit much greater variation in daily sea level than found at Atlantic 
City. Likewise, in tidal streams subject to considerable fluctuation in drainage 
waters, there is greater variation in daily sea level than on the open coast, and this is 
especially marked in the upper reaches of the streams. 
It is obvious that changes in sea level from day to day must, in general, be much 
the same at points near each other and which are subject to similar meteorological 
conditions. As will be seen later, advantage is taken of this fact in determining the 
plane of mean sea level from short series of observations by correcting the sea level 
derived from these observations to a mean value. Just how far two points may be 
separated and still exhibit similar sea level changes depends on a number of factors 
Within a long tidal river subject to considerable variation in fresh-water run-off the 
changes in daily river level may be quite different for points relatively near each other. 
But on the open coast and in tidal waters not subject to large variations in fresh-water 
discharge the changes in daily sea level resemble each other closely over areas of 
considerable extent. 
In Figure 23 are plotted the heights of daily sea level for the month of October 
1947 at six stations on the Atlantic coast from Portland, Maine, to Mayport, Fla. A 
elance shows that in regard to changes from day to day, Portland and Boston follow 
each other closely; likewise, New York and Atlantic City and to a somewhat lesser 
extent, Charleston and Mayport. There is some resemblance in the changes between 
the first pair and the second pair of stations, but none between the first pair and the 
third pair. 
Examining the locations of these stations it is found that Portland and Boston 
are about 100 miles apart but lie in the same general embayment of the coast, and 
therefore the changes in sea level from day to day at the two stations are much the 
same. New York and Atlantic City likewise are about 100 miles apart and lie in the 
same embayment of the coast, which is different from the embayment on which Port- 
land and Boston lie. Charleston is about 550 miles south of Atlantic City and in a 
different embayment of the coast, the distance and different exposure making the sea 
level changes different. Mayport and Charleston lie in the same embayment and 
although nearly 200 miles apart, show similar changes in sea level from day to day. 
