PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 
675 
that only a very thin surface stratum of the sea is warmed by direct solar radiation. 
Further transference of the heat so gained, downward to the deeper strata, depends 
on other processes, discussed below (p. 678). 
Oceanographers, therefore, long have realized that the thickness of the stratum 
that receives the heat of the sun directly depends on the distribution of this energy 
along the solar spectrum and on the transparency or opacity of the water toward 
rays of different wave lengths, which, in turn, depends largely on the clarity or 
turbidity of the water. 
The altitude of the sun — i. e., the angle at which its rays strike the surface of 
the water— and the roughness of the water determine what percentage of the total 
radiation is reflected and what percentage penetrates. No attempt has yet been 
made to measure this for the Gulf of Maine; but there is no reason to suppose that 
the latter differs much in this respect from Puget Sound, where Shelford and Gail 
(1922) found about 25 per cent of the light reflected or shut out by the surface mirror 
between 10 a. m. and 2 p. m. in calm weather, with the loss increasing to 60 to 70 
per cent, or even more, when the sea was rough. On the average, then, about 50 
per cent of the solar radiation falling upon the gulf may be expected to warm the 
latter; the remainder is lost, so far as any direct effect on the temperature of the 
water is concerned. 52 
When we attempt to estimate the warming effect which the 50 per cent or so 
that does penetrate actually exerts at any given level, we must keep clearly in mind 
the distinction between the intensity of radiation and the extreme penetration of 
light. The latter has been the subject of repeated experiments, and, as might be 
expected, successive tests with more and more delicate photographic apparatus have 
revealed faint light at greater and greater depths. The mere fact, however, that 
light penetrates to depths as great as 1,000 to 1,700 meters 53 in amount sufficient to 
affect photographic plates does not imply an equal penetration of radiant heat in 
measurable amount, witness the fact that stars — even nebulae — can be photographed 
though their heat is not appreciable on the earth. On the contrary, theoretic cal- 
culation and practical experiments unite to prove that the intensity of solar radiation 
falls off very rapidly as the depth increases, especially for the longer wave lengths. 54 
Hulburt (1926) has found that sea water is slightly more opaque than fresh 
water for the shorter wave lengths but shows about the same coefficient of absorption 
as fresh water for the longer. 
The long waves below the visible end of the spectrum (the so-called “infra red” 
or “heat” rays) convey more energy than all the rest of the spectrum combined, 
bringing from 51 to 67 per cent of that part of the total energy of the sun that pene- 
trates to the earth’s surface near sea level through air of the same general order of 
humidity as prevails over the Gulf of Maine (Abbott, 1911, p. 289). The precise 
percentage conveyed by these infra red rays varies with the altitude of the sun. 
t2 This is a much greater loss by reflection than Schmidt (1915) found for fresh-water lakes, where he records only a 6 per cent 
loss with the sun 30° above the horizon. Probably the state of the surface accounts for the difference 
t! See Helland-Hansen (1912); Grein (1913). 
M For the coefficient of absorption of the visible part of the spectrum in pure water, see Krilmmel (1907), Fowle (1920), and 
Kayser (1905). 
