52 A TEXTBOOK OF OCEANOGRAPHY 



12 C. in the Channel to 13 to 14 C. in the southern North 

 Sea. The greatest known variation is in Japanese waters off 

 Yezo, where the annual range is from -2*8 to 28*3 C. i.e., 

 a variation of 31 'i C. Annual variations in temperature are 

 perceptible to a depth of 150 fathoms. 



It is interesting to compare the decrease in temperature 

 with the depth with the decrease at the surface as we pass 

 from Equator to Pole, and this has been done for the North 

 Atlantic : 



Zone o-io" 10-20 20-30 30-40 40-50 50-00 60-70 



Temp. C. ... 26*83 25*60 23-90 20*30 i2'94 8-94 '4*26 



At 7J N. the German Antarctic Expedition found in July, 

 1911, the following temperatures in the Atlantic : 



Depth in metres ... o 100 200 400 800 1,000 

 Temp. C. 26*86 18*57 1071 7*70 5-13 4*81 



At 100 metres the temperature is similar to that in 40 N. ; 

 at 200 metres to that in 50 N.; and from 700 to 800 metres 

 to 60 N. 



When the water has the same temperature from the surface 

 to the bottom it is said to be homothermous ; when there are 

 differences in temperature it is heterothermous. 



When the upper layers are warmer and the lower colder it 

 is anothermous ; when the upper layers are colder and the 

 lower warmer, the term katathermous is employed. 



When the upper layer is warm and the temperature sinks 

 and then rises again in the lowest layers, the term dicho- 

 thermous is used ; when the middle layer is warmer than either 

 the lower or upper, the water is said to be mesothermous. 



Although the surface temperatures of the ocean are higher 

 on the average than one would expect, the temperature of the 

 mass of water is low, since the great bulk of the oceanic water 

 i.e., practically all below 150 fathoms is not much influenced 

 by the sun's rays. Attempts have been made to calculate the 

 average temperatures of the main oceans at various depths 

 i.e., at 100, 200, up to 3,000 and 4,000 metres, and from the 



