546 BIOLOGY Of THE SEAS OF THE U.S.S.R. 



Saline Central Caspian waters slowly fill the Ural trench, flowing in between 

 Kulaly Island and the Central-Zhemchuzhnaya Bank close to the Buzachi 

 peninsula. A compensating current of fresh water runs mostly along the 

 western shore of the Northern Caspian. 



Vertical transferences of water masses 



The vertical mixing of the Caspian Sea water masses is well assured, with 

 comparatively small differences in the density of the surface and deeper layers 

 of water owing to winter cooling, to the effect of on- and off-shore winds, 

 to the heating of deep waters owing to adiabatic processes, and as the result 

 of turbulence. 



Temperature conditions 



Temperature conditions in the Caspian Sea are very peculiar and are deter- 

 mined by a sharp difference in temperature between its southern and northern 

 parts in winter and a levelling-up of the temperature in summer. On the other 

 hand, strong annual fluctuations of temperature are characteristic in the 

 upper layer of the Sea, with uniform temperature in its deeper part. The fact 

 that the Caspian Sea extends for 1,200 km from north to south determines 

 also the climatic differences on land adjacent to the Sea. The average annual 

 air temperature at the mouth of the river Ural is 7-8° C, and at Pehlevi 15-6°. 

 However, in some years it may reach 19-5° (Inlet of Astrabad). In January the 

 average temperature at the mouth of the river Ural is — 10-5°, and at Pehlevi 

 +5-9°. In July the difference between the air temperatures of the shores of 

 the Northern and Southern Caspian is only 3° to 3-5°. 



Since the Sea is heated mostly from its surface, the difference in the air 

 temperatures of areas adjacent to the northern and southern parts of the Sea 

 controls the difference in the surface temperature of the Sea. The nature of the 

 distribution of surface temperature and its seasonal changes are well illu- 

 strated by Fig. 260. Almost all the northern part of the Sea is commonly 

 (with variations in different years) covered with ice for four months a year 

 (December to March). The ice-cover attains a thickness of 40 to 50 cm, and 

 in the northeast even of 70 cm; the temperature of the water drops to —1°. 

 For the surface layer of the Sea January and February are the coldest months, 

 and July and August the warmest. The heating of the Sea in spring and its 

 cooling in autumn start at the coastal shallows, gradually spreading to the 

 centre and into the depths. In the hottest time of the year the surface tempera- 

 ture may rise to 30° and even 30-8°. The seasonal range of temperature fluctua- 

 tions is sharply pronounced in the upper layer and grows gradually less and 

 less with depth, and finally at 400 to 450 m it completely fades away ; below 

 this lies a layer of a practically constant temperature, with a somewhat 

 higher temperature in the Southern Caspian depression (a little below 6°) as 

 compared with that of the Central Caspian (slightly below 5°). As in any 

 other sea, time is required for the heating to be transferred into the depths 

 and with increasing depth this delay becomes greater. Knipovitch has shown 

 that in 1914-15 the maximum heating of surface water occurred at the end of 



