TEMPERATURE OF THE HYDROSPHERE 183 



cases there is an abrupt increase between 45° and 35° both north 

 and south latitude, where the temperature is sometimes nearly 

 doubled in going toward the equator. Of course, the sur- 

 face temperature is not divisible into a series of regular zonal belts, 

 but is in reality quite irregularly distributed owing to the influence 

 of currents, etc. Considering average surface temperatures as a 

 whole, it appears that the Pacific is the warmest of the four oceans, 

 its mean being 19.1° C. whereas the mean of the Atlantic is only 

 16.9° C. The mean of the Indian Ocean is only 17° C. in spite of its 

 high maximum of 27.88°. The Pacific is the great tropical ocean, 

 so far as surface temperatures are concerned, owing to its great 

 expanse in the equatorial region, where the Atlantic experiences its 

 greatest contraction. Of the total surface of the Pacific 59.5 per 

 cent., or about 3/5, lies between 30° N. and 30° S. latitude. This is 

 well brought out by the wide distribution of coral reefs in the Pacific 

 as compared with their occurrence in the Atlantic Ocean ; these reef- 

 building coral polyps being confined to relatively shallow waters. 

 A similar difiference is shown in the temperature of the air over the 

 two oceans, as noted by von Tillo, who found the temperature of the 

 air over the Atlantic 2.6° lower than that over the Pacific. As will 

 be noted later, the waters of the Atlantic, taken as a whole, are 

 warmer than those of the other oceans. 



Vertical J\Triation of Temperature. As has been noted al)ove, 

 the heat conductivity of water is very low% and were it not for the 

 absorption of the sun's rays by the deeper strata of water, and the 

 existence of vertical convection currents, there would be little change 

 in the temperature of the deeper waters. Kriimmel has introduced 

 terms to designate the downward changes in temperature, for all 

 heterothermal water bodies, i. e., those in which the temperature is 

 not uniform throughout, or Jwmothermal. When the temperature of 

 the water decreases downward we have the anotliermal arrange- 

 ment which normally prevails in the open ocean and in some intra- 

 continental seas of low latitudes ; when, on the other hand, it in- 

 creases downward, we have the katothermal arrangement, which is 

 generally associated with a katohaline state. This condition exists 

 in the intracontinental seas of higher latitudes during winter. A 

 dichothermal arrangement, with a colder stratum between warmer 

 upper and lower strata, is characteristic of such seas during the 

 summer months, while the reverse, a mesothermal arrangement, with 

 warmer strata enclosed between upper and lower colder layers, is 

 found in polar waters, and may even extend into the other oceans. 



In the anothermal arrangement of the ocean waters the decrease 

 is, in general, comparatively rapid and uniform for the first 500 



