89 



result in fairly regular and consistent seasonal variations in the 

 monthly mean sea level, even at stations not affected by the varying 

 inflow from large rivers. A study of these variations at stations on 

 the coasts of the United States, contained in Special Publication, No. 

 135, United States Coast and Geodetic Survey, shows that at North 

 Atlantic ports mean sea level is quite consistently 0.2 feet or more 

 higher dm^ing the summer months than during the winter months. 

 The annual variation in mean sea level at the South Atlantic ports of 

 Charleston and Fernandina approximates a foot, the highest eleva- 

 tions occurring in the fall. On the Gulf coast, the annual variation is 

 about three fourths of a foot; and on the Pacific coast about half a 

 foot. A comparison between the monthly changes in mean sea level 

 at Portsmouth, N. H., and at Ketchikan, Alaska, and the monthly 

 mean barometric pressure in the two regions, shows a strildng corre- 

 spondence in each case (Special Publications, No. 150 and 127, U. 

 S. Coast and Geodetic Survey). At Balboa, at the Pacific entrance 

 to the Panama Canal, an annual variation of about a foot in the eleva- 

 tion of the monthly mean sea level follows closely an annual variation 

 of about 15° F. in the monthly mean water temperature. These 

 seasonal variations are reflected in the values of the long-term com- 

 ponents Sa and Ssa derived from harmonic analysis. 



163. Variations from year to year. — Because of varying occurrence 

 of storm tides from year to year, and the varying intensities of the 

 causes of the seasonal variations in mean sea level, the mean annual 

 sea level at a station varies from year to year. At stations on the 

 coasts of the United States, where long-term observations have been 

 made, these variations are, however, not often greater than 0.1 feet. 

 The variations from year to year are quite uniform at all stations on 

 the same sea coast. 



Because of these small meteorological variations, mean sea level at 

 a tidal station cannot be expected to be identically the same during 

 any two periods, no matter how long these periods may be. A 9-year 

 average is accepted by the United States Coast and Geodetic Survey 

 as a primary determination which gives the elevation of mean sea 

 level with sufficient accuracy for all practical purposes of that 

 survey. Strictly speaking, however, it should be designated as the 

 mean sea level during the particular period from which it was derived 



HALF TIDE LEVEL 



164. This datum is the elevation midway between mean low water 

 and mean high water. Because of the distortion of the tide curves 

 by the diurnal components and the lunar and solar overtides (par. 

 156), half tide level generally does not coincide exactly with mean sea 

 level. On the Atlantic coast of the United States it usually is below 



