The Sea-water and its Physical and Chemical Properties 59 



the direct incident radiation coming from a whole quadrant is concentrated into a 

 fairly narrow beam range from 0° to 48-5°, while at angles of incidence more than 65^ 

 the intensity of the entrant radiation is rather small. Schmidt (1908) showed by 

 actinometric measurements at the surface of pure water that the same conditions apply 

 for the total solar radiation as for the D line of sodium (n = 1-333). More recent 

 measurements by Poole and Atkins (1926) and by Whitney (1938), as well as by 

 Angstrom (1925) using the pyranometer, show that the theorectical values for re- 

 flection are also obtained essentially in practice. However, the reflection is more or 

 less strongly increased by waves on the surface of the water; it may be increased in this 

 way by more than 50% (Lauscher, 1944). 

 (c) The Behaviour of the Water Surface for Diffuse Incoming and Outgoing Radiation 



As well as the direct sunlight, which may be regarded as unilateral parallel radiation, 

 there is also a general diffuse radiation for which conditions relative to the sea surface 

 are rather different. The diffuse radiation on the surface of sea includes: (1) diffuse sky 

 light (daylight) which is essentially short-wave radiation (between 0-38 ju and 0-75 /^i) 

 and is only present in the day time; and (2) the long-wave radiation from the atmosphere 

 which is long-wave (maxima at 7-5 /z and 12-5 /x), and is present both day and night. 

 Each single beam of the diffuse radiation that is incident on the surface of the water at 

 an angle / is partly reflected following Fresnel's law and is thus subject to a corre- 

 sponding reflection loss as shown by the values given in Table 23. Since the diffuse 

 radiation comes from all directions and the radiation with a greater angle of incidence 

 is more strongly reflected, it is necessary to find the sum of the losses for each angle of 

 incidence in order to determine the total loss by reflection. The calculation of this 

 total from the values r(i) given in Table 23 gives the reflection losses (forn = 1-333) as 

 0-660, that is 6-6% of the diffuse radiation is reflected from the surface of the water. 

 Considering the refractive index to be slightly different for different parts of the 

 spectrum this value varies between 5% and 10%. 



Mention should also be made here of the properties of water as a source of radiation 

 (Schmidt, 1915). Since the extinction coeflftcient of water for long-wave radiation is 

 particularly large and the thermal radiation from the surface of the sea contains only 

 longer wavelengths (around lO^u) it can be expected from Kirchhoff's law that as a 

 source of radiation water would behave as a black body. Nevertheless, water radiates 

 less than a surface of the same temperature since each beam coming from the interior 

 of the water mass will suffer a reflection loss at the surface which will reduce the 

 intensity of the total from the surface outgoing radiation (Fig. 33). 



In addition to this reflection loss the intensity of the radiation suffers a further de- 

 crease since in passing through the surface to the air it must spread out into a larger 

 space. The radiation from water within a space angle of 2 x 48° 35' = 97° 2' is spread 

 out over a full 180°. If this is taken into account (Schmidt, 1916) it is found that for 

 a temperature range of 0-20°C the outgoing radiation from a water surface is about 

 9-10% less than that from a black surface. Since the radiation from a black body 

 according to the Stefan-Boltzmann law is given by £" = aT'^ where a = 1-374 x 10~^- 

 cal cm"2 sec"^ grad"^ the radiation from a flat water surface will be given by ^4 = 

 0-904CTr''. Angstrom has found experimentally that for long-wave radiation the effici- 

 ency of emission of sea-water is 96% of that of a black body. The constant in the above 

 equation should therefore be not very different from 0-95 for the temperature range 



