LXXn PRESIDENTIAL ADDRESS. 



It will be noticed that the temperatvire at the Equator scarcely 

 varies at all between summer and winter, remaining about constant 

 at about 80 degrees Fahrenheit. 



Now, at the North Pole in summer, the mean temperature is 

 about 30 degrees F., whereas in winter it is minus 41.8 degrees F. 

 Obviously, then, the difference in temperature between the North 

 Pole and the Equator in winter is greater than that in summer by 

 no less than 71.8 degrees F. 



This steeper grade of the isotherms in winter is accompanied by 

 an equally steeper grade of the isobars in winter than in summer. 

 This obviously is the reason for the acceleration of atmospheric cir- 

 culation in the Northern Hemisphere during the northern winter. 

 Similar phenomena have been observed in the Southern Hemisphere. 

 There, at the South Pole, the temperature at the end of January 

 is thought to be about 21 degrees F., and in July it is considered 

 to be about minus 28 degrees F., when reduced to sea-level. Of 

 course, as the South Pole itself is not at sea-level, but at an alti- 

 tude of about 10,260 feet, the actual mean temperature there in 

 July may be as low as minus 70 degrees F. At the South Pole, 

 therefore, as at the North Pole, there is a considerably steeper 

 isothermal, and consequently isobaric grade in winter than in 

 summer. The isotherms and isobars are particularly steep along 

 the main Antarctic shore-line, for the obvious reason that, whereas 

 in winter the temperature a few miles inland may be minus 30 or 

 40 degrees F., the temperature of the open ocean, a few miles to 

 the north of the shore, does not fall below 28 degrees F. — a differ- 

 ence of temperature of no less than 58 to 68 degrees F. in a distance 

 of 20 miles or less. 



One can easily understand why, with these abnormally steep 

 barometric grades, and the general dome shape of the Antarctic 

 Continent, with its almost universal seaward slope, the blizzard 

 wind develops such intense speed near where land and water meet. 



The questions now suggest themselves — (a) Why is the atmo- 

 spheric pressure lower at the poles in winter than in summer, and 

 (b) wliy, generally, is the atmospheric pressure so much less 

 towards the poles than elsewhere, particularly in the case of the 

 South Pole 1 Hann suggests that the former question is to be 

 answered on the lines that the phenomenon is due to some dynamic 

 effect to be connected with the increased rapidity of circulation of 

 the atmosphere in winter time.^ 



' His remarks are of such importance that they are quoted here in full; — 



JD)\ Julius Hann. Ilandhuch dcr KUmatologie. Bd. III., s. 601. 



" Im amerikanischen Polarj^ebiet beiresrnen Wir der intercssanten Erscheinung', dass das 

 Minimum der Temperatur mit einem Minimum des Luffdrucke'i zusammentallt. Obiu-Ieich fiber 

 Nordgronland und Melville Island ein Kiiltepol liej^t (mit - 35 bis - 40° C.) und die Erstreckung 

 und Machtigheit der kalten Luffniassen daselbst vielleicht gro.sser ist als uber dem allerding-s 

 intensivcren asiatiscshen Kaitepol, sinkt dofh der Luffdruck und erreicht im Januar ein Minimum. 

 Wir miissen dies wohl aut dynami-che Ursachen zuriickfUhren ; die Druckabnahme getfen das 

 Zentrum des Polarwirbels komiTit dann trotz der Storunaren der n' rnialen iTUckverteilung ,auch 

 an der Erdoberflftche zur Geltung-, was in Asien wegen der frrossen LuftdruckanhSufunj,' in den 

 imtersten Niveaus, die namentlich auf oroj^raphische B( dinfruneen zurilckzufuhren sind, nicht 

 mdglich ist. Uber den gleich-formigeren Flachen des amerikanischen arktischen Gebietas greift 

 der Folarwirbel bis gegen'die Erdoberflache hinabdureh. 



