I 



414 ZONES AND REGIONS [Pt. Ill, Sect. II 



For example, the isotherm for January of 0° C. has its most northerly point in 

 America, to the north of Sitka, at about 58°; eastwards it falls rapidly down to 

 St. Louis, its most southerly American point, at 38° 5', runs with a slight ascent 

 to Washington at 39", to Philadelphia at 40 , ascends considerably in the Atlantic 

 Ocean, reaches the south coast of Iceland at about 63° 30', exceeds 70" on the 

 west coast of Norway and there reaches its most northerly point, then runs 

 directly southwards along the west coast of Denmark, across Central Europe 

 (Wilhelmshaven, Bamberg, Munich), then bends towards the east, passes south 

 of Sofia at about 42°, its most southerly point in Europe, then continues, with 

 weaker undulations, in the easterly direction, reaching its most southerly point 

 about 32°, in Central China, where it is 38° south of its most northerly point, 

 then it gradually ascends, traverses South Korea, and reaches its most northerly 

 Asiatic point in the northern part of Nippon at about 38°, which lies therefore 

 only about 6° above its most southerly point and considerably more to the south 

 than on the west coasts of America and Europe. 



In the southern hemisphere, north of the Antarctic circle, only a part of Tierra 

 del Fuego and a few insignificant groups of islands, for instance South Georgia, 

 have a July temperature of o' C. or less. 



The July isotherm of 20° C. is the most uneven in tlie northern hemisphere, 

 though without executing such sharp curvatures as the isotherm of o'C. It 

 reaches its most southern point on land in California at 31° (descending much 

 more to the south on the ocean), ascends almost due northwards up to about 55° 

 in the north-west, tiien proceeds westwards with repeated undulations through 

 Canada (Winnipeg 50", Quebec), through Boston at about 42° 20', crosses the 

 Atlantic Ocean somewhat north of 40°, aligns the north coast of Spain (about 44°), 

 the west coast of France (Bordeaux), bends eastwards through Paris to Moscow, 

 then straight through Siberia, ascending in East Siberia to Yakutsk (about 62°), 

 its absolutely northernmost point, where it is 31° north of its southernmost point, 

 then falls on the coast of China to south of 40°, its most southerly Asiatic point, 

 and reaches the south point of the island of Jesso. 



In the southern hemisphere, the Januarj' isotherm of 20" reaches the west 

 coast of America at about 20°, that is to say within the tropics, its northernmost 

 point ; it falls along the Andes to South Argentina, runs southwards along the 

 west coast of Africa from the tropic of Capricorn to the Cape of Good Hope, 

 then eastwards, without reaching Natal, to tlie south coast of Australia, to which 

 it nearly entirely clings, and passes through the north of New Zealand. 



The diurnal oscillations of atmospheric temperature are usually greater 

 in temperate zones than in the tropics. The_v attain their maximum 

 in deserts. Thus G. Rohlfs read - 0-5° C. on his thermometer in the 

 morning of the 25th of December, 1878, at Bir Milrha, south of Tripoli, 

 and in the afternoon of the same day +37-2° C The diurnal fluctuations 

 mainly depend on the radiation of heat — that 'of the sun by day, that 

 of the earth by night^and are accordingly much greater with a clear 

 than a cloudy sky. In the former case, particularly in dry districts, they 



' Peschel, Physikalische Erdkunde, Leipzig, 1881, II, p. 174. 



