476 THE ICE AGE IN NORTH AMERICA. 



The relatively small extent of sea in middle latitudes of the 

 northern hemisphere, in comparison with the southern, must 

 tend to warm the seas of the former, even if the quantity of 

 warm water from the tropical seas reaching them be equal. 

 Thus, generally in the middle latitudes the evaporation goes 

 on at a higher temperature from the seas of the northern than 

 the southern hemisphere. Now, this has a very great influ- 

 ence on the resulting precipitations ; when the evaporation 

 goes on at or near 32°, there is much more probability that the 

 resulting precipitation will be snow and not rain, even on low 

 lands ; the higher the temperature at which the evaporation 

 takes place, the greater must be the height at which snow can 

 fall, on account of cooling by expansion. 



Not all cold seas are favorable to glaciation. If they are 

 surrounded by land on which the winters have a temperature 

 considerably below 32°, they will be covered with ice, aud thus 

 evaporation will be checked just at the time when it is most 

 favorable to snow-fall. The ice of the seas will be covered 

 with snow, the temperature of the air over it may be very low, 

 but the snow-fall will not be great, and thus the conditions not 

 favorable to glaciation. Such is the condition of many seas of 

 the northern hemisphere, as the Arctic Ocean north of Siberia, 

 the Kara Sea, the bays and inlets north of the North American 

 Continent, the Sea of Okhotsk, etc., which are covered with ice 

 during many months. These conditions are favorable to cold 

 of many months' duration, but not to a large snow-fall aud 

 the resulting glaciation. The observations made at many 

 points off the coasts of Siberia and the North American Archi- 

 pelago have shown that the snow-fall is exceedingly light. The 

 seas between 45° and 70° of southern latitnde are deep and not 

 surrounded by land, and thus by far not so ice-bound, both on 

 account of the absence of very low temperatures favorable to 

 the formation of ice, and of the rupture of ice, when formed, 

 by winds and currents.* 



Thus it is shown that the depth and relative extent of 

 the southern ocean furnish a sufficient cause for its present 

 glacial conditions. 



* "American Journal of Science," vol. cxxxi, 1886, pp. 175, 176. 



