PHYSICAL OCBANOGEAPHY OF THE GULP OP 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 tm-n, depends largely on the clarity or 

 turbidity of the water. 



The altitude of the sun — i. e., the angle at which its rays strilce 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 GaU 

 (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. '^^ 



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 

 hght. 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 ^' 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.^* 



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. 



S2 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. 

 s' See Helland-Hansen (19121; Grein (1913). 



" For the coefficient of absorption of the visible part of the spectrum in pure water, see Kriimmel (1907), Fowle (1920), and 

 Eayser (1905). 



