TEMPERATURES AT THE SEA SURFACE 51 



two different assemblies of organisms e.g., fish of a given 

 species are known to frequent exclusively water of a given 

 temperature, or, more accurately, water with a given range of 

 temperature. 



TEMPERATURES AT THE SEA SURFACE. 



Lines joining places in the ocean with identical temperature 

 are called isotherms. The first chart showing isotherms was 

 published by Maury in 1852. The surface temperature varies 

 from an annual average of 27-4 C. to - 17 C., the warmest 

 water being in 5 N. Lat., the coldest from 80 N. to the Pole 

 and 75 to 80 S. Lat. Generally speaking, the temperature is 

 influenced mainly by the ocean currents, and this is especially 

 noticeable in the tropics. 



In high southerly latitudes there is a parallelism between 

 the isothermal lines and the degrees of latitude. 



More than half the surface of the sea has an annual average 

 temperature exceeding 20 C. (68 F.). The percentages of 

 the great oceans which have this or higher temperature are 

 Atlantic 50*1, Indian 517, and Pacific 58-4. 



Kriimmel estimates the average surface temperature in the 

 Northern Hemisphere to be 19-20, in the Southern i5'97; the 

 air temperature averages for the Northern Hemisphere 15- 1, 

 for the Southern 13-6 C. 



Of the three great oceans, the Pacific has the warmest 

 surface water, with an average of 19-10 C., the Indian being 

 17-03, and the Atlantic, the coldest, 16-91 C. 



Practically three-fifths of the Pacific Ocean lies between 

 30 N. and 30 S. Lat., whereas less than half of the Atlantic 

 is so situated. 



The coldest surface temperature hitherto recorded is 

 ~3'3 C. off New Scotland, the highest, in the open ocean in 

 the West Pacific, 32-2 C. The highest marine temperature is 

 not, as is usually given, the Red Sea (34*4 C.), but the north 

 end of the Persian Gulf, with 35-6 C. (96 F.). The annual 

 variation of temperature in British waters varies from 10 to 



