50 U. S. COAST AND GEODETIC SURVEY 
Monthly Sea Level 
It is obvious that sea level determined for periods of a week will show smaller 
variations than daily sea level. There is, however, no need of discussing such varia- 
tions in detail, since they will lie between the daily variations and the monthly varia- 
tions. Within a month the larger fluctuations exhibited by daily sea level will tend 
to balance out, so that monthly sea level shows much less variation than does daily 
sea level. For example, as noted in the discussion of daily sea level at Atlantic City, 
two daily sea levels in 1947, one in August and the other in December differed by 3.7 
feet; during that same year the difference between the highest and lowest monthly 
values of sea level at Atlantic City was 0.9 foot. 
In Figure 24 the monthly heights of sea level at the same six stations used in 
Figure 23 are shown for the 2 year period 1946-1947. The changes from month to 
month for each pair of stations—Portland and Boston, New York and Atlantic City, 
Charleston and Mayport—are closely similar, and there is a general resemblance for 
all six stations in that sea level is low in the later winter months and high in the autumn 
months. 
In discussing the changes in sea level from day to day, attention was directed to 
the fact that such changes are in no way related to the range of tide. Figure 24 empha- 
sizes this fact with regard to monthly sea level. New York with a mean range of 4.4 
feet and Atlantic City with a range of 4.1 feet show much greater changes in monthly 
sea level than do Portland and Boston with ranges of 8.9 feet and 9.5 feet, respectively 
Still greater changes are shown by Charleston and Mayport, the ranges of which are, 
respectively 5.1 feet and 4.5 feet. 
All six stations, but especially the four most southerly, give evidence of a seasonal 
or, more accurately, of an annual variation in sea level, reflecting the periodic seasonal 
changes in wind and weather. Since wind and weather do not repeat themselves 
exactly from year to year, the periodic annual variation in sea level in any one year 
may be somewhat masked; but if monthly heights of sea level for corresponding months 
are averaged over a number of years, the irregularities tend to balance out. 
Annual Variation 
The six curves of Figure 25 represent, as indicated, the annual variation in sea 
level at Atlantic City as derived from the monthly heights of sea level. The five upper 
curves give the monthly heights of sea level for each of the five consecutive years 1925— 
1929, while the lowest curve is the mean curve of annual variation derived by averaging 
the corresponding monthly heights of the five-year period. The horizontal line associ- 
ated with each diagram represents the average value of sea level for the period in 
question. 
For any one individual year shown in Figure 25 there are seen to be irregularities 
in the change of sea level from month to month. At the same time, however, there is 
a large element of periodicity in this change. The lowest curve shows that sea level is 
low in the winter and early sprmg months and high in the late summer and early fall 
months. And to a large extent this is seen to be the case for each of the 5 years. 
The annual variation in sea level at any place is characteristic for a considerable 
area in its vicinity, but from this statement must be excluded the upper reaches of tidal 
streams subject to large fluctuations in fresh-water flow. Thus the annual variation 
