Physics of the Earth - II The Figure of the Earth Bulletin of the National Research Council (1931) / Chapter Skim
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Chapter IV. Mean Sea-Level
Chapter IV. Mean Sea-Level
Pages 50-67
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From page 50... ...
y mean sc~-l3vcl that is employed. [or geo~phir SPED level lies at lne nose of geodetic mean se~-lc~cl, since the initi~1 se~-level in geodetic leveling must he determine]
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From page 51... ...
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 seat-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.
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From page 52... ...
From observations it is found that at certain places half-tide level lies above mean sea-level, while at other places it is below mean sea-level. Thus, as a rule, on the Atlantic coast of the United States half-tide level lies below mean sea-level by about one-tenth of a foot while on the Pacific coast it is above mean sea level by about half that amount.
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From page 53... ...
Exactly how far two points may be separated and still exhibit similar sea-level changes depends upon a number o:t' 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 :I'or points relatively near each other.
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From page 54... ...
1. Daily sea-level, Atlantic coast stations, April 1923.
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From page 55... ...
2.-Monthly sea-level, Atlantic coast stations, 1923-1924. For daily sea-level Figure 1 shows that the variations at Atlantic City are different from those at Boston and Portland.
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From page 57... ...
For comparisons with the seasonal variation of' sea-level along the Atlantic coast of' the United States illustrated in lTigure 3, there are shown in Figure 4 the curves of annual variation at several stations on the Gulf and Pacific coasts. Key West, Florida, Pensacola, Florida, and Galveston, Texas, constitute the Gulf coast stations and San Diego, California, San Francisco, California., and Seattle, Washington, the Pacific coast stations.
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From page 58... ...
FIGURE OF THE EARTH sea-level derived from a month of observations. tions continued over a number of years bear this out, as may be seen from an inspection of Inure 5 in which the yearly heights of sea-level at The results of observa Feef 2 Jan Fed tour Apr that June Join Auk Sent act //ov Dec .
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From page 59... ...
A comparison of the diagrams of Figure ~ brings to light the fact that the variations in yearly sea-level at the three places bear no apparent relation to each other. But if we compare New York with other stations on the Atlantic coast of the United States, Galveston with stations off the Gulf coast, and San lTrancisco with stations on the Pacific coast, we find, as a rule, considerable resemblance in the variations of sea-level from year to year.
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From page 60... ...
CAUSES OF VARIATION The larger variations in sea-level, whether from day to day, montn to month or year to year, are undoubtedly to be ascribed to changes in meteorological conditions, such as barometric pressure, wind, and rainfall. bVit.h regard to barometric pressure, a first approximation to the resultant changes in sea-level may be easily derived from general consiclerat.ions, as follows.
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From page 61... ...
~ review of these earlier investigations and of the subsequent work to the year 1018 is given by Sir Charles Closed Here it will be sufficient to mention only those of more recent date. In 1924 Alfred Wegener discussed observations made by him during, the winter of 1907 on the northeast coast of Greenland, from which he found the ratio of change in sea-level to cha.n>,e in barometric pressure to be from i!
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From page 62... ...
If we examine the annual variation ill atmospheric pressure at ally place along the coast it is found to have a somewhat similar character to the variation In sea-ievet, In so far as phase is concerned; but this ~l~nil~l `~nrinti~n in nro~llrn ;q so small as to be suite inadequate to ~1114~' · ~ ~ V ~ ~ ~ ~ 1 ~ account for the annual variation in sea-level. For example, the atmospheric pressure at New York is, as a rule, highest in the winter months and lowest in the summer months.
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From page 63... ...
to the variations in se~-level Irom bear to dear, it m~ be said abut these UDdoubtedly reRect cages in the yearly values of such factors as barometric pressures wiris, nontiJ~1 currents, and temperature and density of sea mater. Go quantitative studies of this pose of the quest appear to bane been makes tat the causes abut bring about the gaily and annul variations in se~-levet whim mere considered abodes may reasonably Be invoked to account for the variations from gear to gear.
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From page 64... ...
The second problem is, how can the sea-level from a short series of observations be corrected to a mean value ? It is clear that, as a general rule, the longer the period of observations the closer will the value obtained for sea-level approximate mean sealevel.
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From page 65... ...
Determinations of sea-level front observations covering periods of nine years or more may be denominated primary determinations; those based on less than nine years may be called secondary determinations. Very satisfactory secondary determinations of mean sea-level may be derived from observations coverings, periods of from one to three years, by taking advantage of the fact that the variation in sea-level from year to year is much the same over considerable areas.
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From page 66... ...
In other words, the surface of mean sea-level as determined directly from observations may not he an equipotential surface. Thus Jolly calls attention to the I'act that precise leveling in Great Britain shows that from Newlyn on the English Channel to Dunbar on the North Sea-an air line distance of about 400 nobles, but twice that distance along, the coa.st`~,eodetiG and '~eo',raphic Clean sea-level diverge 0.81 foot.1'' Avers in a recent study gives the results of precise leveling in the United States which show deviations between geodetic and geographic mean sea-level of very nearly ~ -feet as between tl~e Atlantic and Pacific coasts.
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From page 67... ...
H On the relation between wind on current and mean sea level in the Indian and the Atlantic oceans and the adjacent seas.
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