42 



BJ0RN HELLAND-HANSEN 



|REP. OF THE "MICHAEL SARS" NORTH 



f varies greatly with the wave length of the rays. 

 When we have pure distilled water and L in the formula 

 above is reckoned in metres, f is 001 — 002 for blue 

 and green rays at wave lengths between 045 and 0-54 m 

 0-3 for red rays at about 0-65 n and 2-0 for dark rays at 

 ;. =:0-8/(, increasing very much with greater wave lengths. 



The conditions in sea-water are little known yet, but 

 the coefficients of absorption are not very different from 

 those found for distilled water. The greater part of the 

 heat rays are absorbed in the uppermost layer of water. 

 Only a very small fraction of the dark heat rays reaches 

 as far down as one metre below the surface without being 

 absorbed. The intensity of red rays of a wave length of 

 about 0-65 /( is reduced to 1 per cent after having passed 

 through some 15 metres of water, and to 1 per mille after 

 some 30 metres. The intensity of the green rays is less 

 than that of the red rays at the surface, but already at a 

 depth of some metres the ratio is, in most cases, probably 

 reversed. 



By some preliminary investigations with a photo- 

 meter constructed by the present writer it was found, 

 during the "Michael Sars" expedition, that the transparent 

 waters between the Canary Islands and the Sargasso Sea 

 contained many of the visible rays at 100 metres, the 

 intensity being greater in the blue and green part of the 

 spectrum than in the red. At 500 metres below the sur- 

 face the intensity of the red rays seemed to be very small 

 while that of the blue rays was quite appreciable (exposure 

 with Wratten and Wainwright gelatine colour filters for 

 40 minutes in the middle of the day). Even at 1000 metres 

 the photographic plate (without colour filters) was black- 

 ened after having been exposed for 80 minutes. Some 

 radiation reaches even the greatest depths of the ocean, 

 but here the intensity is so minute that it escapes obser- 

 vation even if the effect be accumulated (as in the case 

 of photographic exposure) for a very long time. 



By some experiments in shallow Danish waters 

 Knudsen found [1923] a minimum of the coefficient of 

 absorption in the green part of the spectrum and not in 

 the blue, which is probably to be explained by the occur- 

 rence of different kinds of solid particles in the water. 

 Colloids as well as coarser suspensions and microscopic 

 organisms may hinder the passage of the rays very appreci- 

 ably and cause another selective absorption than found in 

 optically pure water.') 



The absorption of the radiation from the sun directly 



') 111 this connection an effect of the r.idiation upon organisms 

 living in the upper strata of the sea may be worth mentioning. To 

 what extent such organisms wiM feel a heating above tlie temperature 

 of tlie surrounding water depends partly upon their capacity of reflecting 

 or absorbing lieat rays. If tlie body of a living creature has the 

 character of a "black body", in the physical sense of the word, the 



and from the sky causes a heating of the water to some 

 distance below the surface. We may easily obtain an 

 approximate estimation of this effect on the temperatures 

 in the ocean. 



From Langley's well-known curve illustrating the 

 distribution of energy (in the form of heat) throughout 

 the normal spectrum we may interpolate relative values 

 of the heat energy of the radiation reaching the surface 

 of the sea. We have done this for intervals of wave- 

 lengths corresponding to 0-025 //. Aschkinass and others 

 have determined the coefficients of absorption {t) for 

 various wave-lengths in pure distilled water. From these 

 data the values of t wanted have been found directly or 

 by linear interpolation, with sufficient approximation. By 

 means of the formula given above we may, then, com- 

 pute the absorption of heat per metre for different wave- 

 lengths and by numerical integration find the total absorp- 

 tion. The computations give: 



/, --0-71 A, 



— /-,, ^ 0-0012 /o 



0-0004 /„ 



'50 '31 



'l 00 ' 'lOl 



The indices correspond to L, i. e. the distance passed 

 by the rays through the water. It has been supposed to 

 be the same for all wave-lengths. It does not correspond 

 to the depth m below the surface of the sea unless the 

 rays hit the surface vertically (the sun in zenith). When 

 the sun is just above the horizon the direct sun-rays 

 penetrating the sea are refracted so much that their direc- 

 tion forms an angle /? of about 42' with the surface. In 

 this case m is equal to 0-67 L. When the sun is 60° 

 above the horizon, we have /J ^= 68" and m = 0-93 L. 



The value of /,, is very variable. It depends upon 

 the position of the sun above the horizon, cloudiness, 

 humidity etc. As a probable mean value we may put /,, 

 equal to 360 gr. cal. per day per square centimetre of 

 the sea surface of the North Atlantic in summer. Allowing 

 for the deviations of the rays' directions from the vertical 

 we obtain the following results: 



In the uppermost metre of the sea so much heat 

 (about 70 per cent of the total heat energy) is absorbed 

 by the radiation from sun and sky that the temperature 

 of the water on an average would rise about 2 — 3^ C 

 per day. The increase of temperature is very much less 

 because heat is lost by evaporation and radiation from 

 the surface. Wave motion will diminish the effect in 



sensation of heating in the day may possibly be quite distinctly felt. It 

 is perhaps not excluded that some of the vertical movements of animal 

 organisms — downwards in the day and upwards at night — may 

 partly be due to the stimulus of heating and not only to a reaction 

 against variations in the intensity of light or to the chemical activity 

 of the radiation. 



