40 



BJ0RN HELLAND-HANSEN 



|REP. OF THE "MICHAEL SARS" NORTH 



A detailed investigation of the oscillations in question 

 cannot properly be carried out unless observations are 

 made at several stations simultaneously. The close co- 

 operation of several vessels is therefore necessary, and 

 continuous observations of the vertical distribution of 



temperature and density ought to be made in conned ion 

 with direct current-measurements, while at the same time 

 sufficient material for a detailed representation of the 

 solenoid field and its variations should be secured. This 

 seems a task worthy of international co-operation. 



VI. THE TEMPERATURES IN THE SEA. 



29. Gain and Loss of Heat. 



The heating of the sea as a whole is almost exclu- 

 sively due to absorption of radiating heat from sun and 

 sky. The quantity of heat radiation penetrating the surface 

 of the sea has an order of magnitude of 10^ gram calories 

 per year per square centimetre of the surface. A heating 

 of the surface merely by contact with warmer air is of 

 comparatively small importance (cf. section 11). Some heat 

 from the interior of the earth is conveyed to the sea, but 

 ti only amounts to between 50 and 80 g. cal. per year 

 per cm.- of the bottom. Heating by transformation of 

 kinetic energy or by chemical processes is also of quite 

 a subordinate importance compared with the first-named 

 source of heat. 



The cooling of the sea takes place exclusively from 

 the surface. It is to a great extent caused by emission 

 of dark heat rays. In some regions the evaporation is 

 very considerable and deprives the sea of great quantities 

 of heat which is transferred to the atmosphere as latent 

 heat of the water vapour and is liberated when the vapour 

 is condensed. Contact with colder air (convection) also 

 causes a sinking of the temperature of the sea. 



When we consider the sea as a whole and regard 

 the most important factors only, the total loss of heat due 

 to emission, evaporation and convection must, on an 

 average, be equal to the gain of heat by absorption of 

 radiating energy. There are, however, great local and 

 temporal changes in the relation between the different 

 factors. Within a limited region regular periodic (daily 

 and seasonal) variations in this relation may be consider- 

 able, but irregular meteorologic variations in cloudiness, 

 wind, humidity etc. may, also, have a great effect upon 

 the temperature of the sea. It would be difficult to unravel 

 the temperature conditions in the sea even if this be 

 motionless, but it is still much more so on account of 

 currents and turbulent motions. 



We shall, here, only attempt to make an approximate 

 estimation of the relation between the various quantities 

 of heat which are of greatest importance as to the exchange 

 of heat to and from the ocean in general. To begin with, 



we shall consider an area-element of the surface along a 

 meridian from 70^ N to 70^ S and estimate the average 

 quantities of heat gained and lost per square centimetre 

 of this element. By Q, we denote the average quantity 

 of heat gained by absorption of sun rays coming directly 

 and as diffuse radiation reflected from the atmosphere 

 and the clouds. The heat consumed by evaporation may 

 be termed Q.,, that lost by excess outwards of long-wave 

 radiation Qg, and heat lost by convection to the atmosphere 

 Q4. Leaving other processes out of account, we have: 



The radiation from the sun reaching the outer part of 

 the atmosphere of our globe corresponds to 2 g. cal. /cm. ^ 

 min. (the "solar constant"). Some of this radiation is 

 reflected to space again and is lost for the heating of the 

 earth and its atmosphere. The greater portion is partly 

 absorbed by the atmosphere, and the rest reaches the sea 

 or the solid crust of the earth. The amount of heat which 

 is absorbed by the sea from the direct sun-rays depends 

 upon the altitude of the sun and the transmissibility of 

 the atmosphere. It is subjected to great variations. To 

 this direct radiation from the sun we have to add the 

 diffuse radiation from the atmosphere (about 8 per cent 

 of the direct radiation from the sun, according to L. V. 

 King) and the clouds during the hours of daylight. 

 On the other hand, about 17 per cent of the total radia- 

 tion falling upon the surface of the sea is reflected again 

 [W. Schmidt, 1915). Starting from data compiled by Kia\- 

 B.ALL, we find as an average for the above-mentioned 

 area-element from 70"" N to 70° S in the Atlantic: 



Qj = 275 g. cal. cm."- 24 hours. 



G. WusT [1920] has made a careful study of the eva- 

 poration from the sea. He has calculated the mean evapo- 

 ration in mm. per 24 hours within zones comprising 10° 

 of latitude. From Wusi's data we may find the correspon- 

 ding quantities of heat, and we obtain the following value 

 as an average for our area-element: 



Q2 — 120 g. cal./cm.- 24 hours. 



