A6 U. §. COAST AND GEODETIC SURVEY 
It is to be observed that except in regions of large range of tide, there is very little 
difference in the values of daily sea level determined by the three formulae. Thus taking 
the first day shown in Figure 15 (June 24, 1944), when the range of tide averaged a little 
over 9 feet, the first formula gives as the value of sea level on the tide staff at Boston as 
9.37, the second 9.42 and the third 9.39. As will be seen later, sea level derived from 
one day of observations may differ from mean sea level by a foot or more. Insuch cases 
a difference of a few hundredths of a foot is negligible, and hence for most purposes the 
third formula is the most convenient one. 
Half-Tide Level 
Mean sea level must be carefully distinguished from half-tide level or, as it is 
frequently called, mean tide level. Half-tide level is the plane that lies exactly mid- 
way between the planes of mean high water and mean low water and is determined by 
averaging the heights of the high and low waters. 
If the curve representing the rise and fall of the tide were that of a simple sine 
curve, the planes of mean sea level and of haltf-tide level would coincide. But the tide 
curve is not a simple sine curve; it is compounded of a number of simple sine curves, 
some of which have fixed phase relations with respect to each other. The average 
rise of high water above mean sea level is, therefore, generally not exactly the same as 
the average fall of low water below mean sea level, and hence mean sea level and half- 
tide level generally differ. 
It will be more convenient to take up in detail the plane of half-tide level after 
the discussion of the plane of mean sea level. Here it will be sufficient to call attention 
to the fact that at any point on the open coast the planes of mean sea level and of half- 
tide level generally differ only by small quantities, and that over periods of a year or 
more the differences between these two planes are very nearly constant. 
Variations in Sea Level 
If the level of the sea were to fluctuate only in response to daily and semidaily 
tide-producing forces of unvarying periods, then mean sea level could. be determined 
from one day of tidal observations. Averaging the hourly heights of the tide through 
one day would eliminate the effect of the tide, the resulting average height being the 
height of mean sea level. But the tide-producing forces to which the sea responds 
include, besides those of daily and semidaily periods, also those with periods of half a 
month or more. Daily sea level therefore varies from one day to another in con- 
sequence of these so-called long-period tides. 
Tt can be shown that the variations in sea level from day to day resulting from the 
long-period tidal forces are relatively small. Far greater variations are brought about 
by the response of the waters to changes in wind and weather. It is a matter of common 
knowledge that a wind blowing toward the shore tends to raise the level of the sea along - 
the shore, while a wind blowing from the shore tends to lower it. 
Variations in barometric pressure likewise bring about fluctuations in sea level. 
