88 



TEMPERATURE. 



Table 50, 



Change of -Temperature from Month to Month. 



The minimum temperature occurs over the Barrier in July, at 78° N. in February, which 

 corre.sponds to August in the south. During the period of increasing insolation (September to 

 December in the south, and March to June in the north), the temperature rises at approxi- 

 mately the same rate in the north and south, 51°, and 49° respectively. The temperature, 

 however, continues to rise in the north after midsummer, and July is 5-9°F. warmer than 

 June, while over the Barrier the temperature commences to fall almost as soon as the insola- 

 tion decreases, and January is 5°F. colder than December. The continued rise of tem- 

 perature in the north after the solstice, coupled with the fact that the mean temperature 

 during the two warmest months in the year, July and August, is well above the freezing 

 point, is the key to the problem. Even during July and August, the greater part of the sea 

 in latitude 78° N. is completely frozen over, and where it is not frozen over, the temperature 

 of the sea is between 2 and 3°F. below the freezing point. Hence as the temperature of 

 the surface does not rise above the freezing point, the air during these months is warmer 

 than the surface, and therefore cannot obtain its heat locally. During July and August, the 

 north polar ocean is surrounded by snow-free land, the temperature of which is well above 

 the freezing point, and the winds carry warm air from the laud over the polar ocean, so 

 causinif a higher temperature than would be produced by the insolation received locally. The 

 course of the temperature in the north between the winter minimum and the summer maxi- 

 mum is now obvious. At the end of the winter, the north polar ocean is frozen over, and 

 surrounded by snow-covered land, which is locally colder than the (tcean further north. Thus 

 the first sunshine of the year falls on a snow surface at a temperature much below the freez- 

 ing point, hence the conditions are similar to those over the Barrier. The rise in temperature 

 is, therefore, similar in both north and south. But over the laud of North Asia and 

 America, the snow commences to melt exposing land and rock on which the insolation acts 

 with "reater intensity. The snow covering rapidly decreases, and by the end of May has 

 almost completely disappeared from the land, the temperature of which rises well above the 

 freezing point. 



The pfessure distribution is then favourable for a considerable interchange of air between 

 the warmer land and the colder ocean. The prevailing wind direction over the whole area 

 mav not be from the south, but there are sufficient southerly winds to carry a considerable 

 amount of warm air northwards with a consequent raising of the mean temperature over the 

 ocean. Thus after May insolation is not the only source of heat in 78° N., but to it must 

 be added the winds carrying warm air from the relatively warm continents in the south. 

 The summer temperature is, therefore, well above the freezing point, and the maximum occurs 

 a month after the insolation has commenced to decrease. 



