70 U. S. COAST AND GEODETIC SURVEY 
Monthly half-tide level is subject to variations of both periodic and nonperiodic 
character, so that within a year half-tide level for two different months may differ 
by as much as a foot. Yearly values of half-tide level may show differences of a 
quarter of a foot or even more. 
Relation to Mean Sea Level 
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 half-tide level would comcide. But the rise 
and fall of the tide does not take place in accordance with the ordinates of a simple 
sine curve. The movement of the tide is compounded of the movements of a number 
of simple sine curves, some of which have fixed phase relations with respect to each 
other. The rise of high water above sea level is therefore generally not exactly the 
same as the fall of low water below sea level, and hence mean sea level and half-tide 
level generally differ. 
Obviously, any cause that tends to disturb the regularity of the tide-curve tends 
to change the relation between sea level and half-tide level. Decided changes in wind 
and weather may therefore change that relationship somewhat. In general, however, | 
the relation is very nearly constant. Figure 34 shows in diagrammatic form for each 
day of the month of October 1947 the relation of sea level to half-tide level at Boston. 
Half-tide level for each day was derived as the average of the four high and low waters 
of the day. On days when but one high or one low water occurred, the other one 
occurring nearest to the day in question was used to make up the group of four high 
and low waters. Sea level for each day was, as heretofore, derived as the average of 
the 24 hourly heights of the tide. 
Sea fevel 
Half-tide level 
Fic. 34.—Daily sea level and half-tide level, Boston, October 1947. 
