AMELIORATIONS OF PRESENT ARCTIC CLIMATES 389 



i) An upper layer of very cold water whose salinity is relatively 

 low (ranging from 29 to 32 parts per thousand in the more surficial 

 portion, and somewhat higher in the bottom portion) and whose 

 density, because of this low saHnity, is less than that of the warmer 

 but more saline layer next below. This upper layer is the source of 

 the ice-laden outlet streams of the Polar Sea previously mentioned. 

 The freshening water which reduces the salinity of this layer comes 

 from precipitation on the sea surface, from the drainage of adjacent 

 lands, and from the melting of ice. 



2) A middle layer of warmer but more saline and denser water. 

 Its saHnity ranges from about 35.1 to 35.3 parts per thousand 

 (Nansen). 



3) A bottom mass, very much thicker than both the foregoing 

 layers taken together, and denser than either, because it is both 

 cold and saHne. It is the abysmal water of the Polar Basin. So far 

 as known, it occupies all the basin but the upper 800 to 1,000 meters. 

 Nansen gives its mean salinity as 35.29 parts per thousand; in other 

 words, it is slightly more saline than the middle layer. Its mean 

 temperature is about 1.56° C, which is not so low as the basal por- 

 tion of the upper layer. It holds notable quantities of the atmos- 

 pheric gases, which implies that even these basal waters take part 

 in a cyclic motion that has its contacts with the air at some stage. 

 They are not stagnant waters, although their motion is probably 

 very slow. 



THE SOURCES AND RELATIONS OF THE LAYERS 



The two variables by which the waters of the ocean are distin- 

 guished are temperature and saHnity. The variations of tempera- 

 ture are functions of insolation and radiation and, in a general way, 

 vary with latitude. The variations of salinity depend on the excess 

 or deficiency of precipitation over evaporation, which, is in turn 

 dependent on the ascent or descent of the regional atmosphere. 

 The mean salinity of the ocean is usually taken as about 35 parts 

 per thousand. The salinity varies also with latitude, but in a differ- 

 ent way and with greater departures from the general rule. In the 

 very high and the very low latitudes, precipitation generally exceeds 

 evaporation, and the sea-waters suffer dilution. In a belt between 

 these, conveniently known as the 30° belt, evaporation is generally 



