Yol. 57.] CLIMATE OP THE PLEISTOCENE EPOCH. 413 



Olacial phenomena may be accepted, must have profoundly affected 

 the distribution of barometric pressure in the Northern Hemisphere, 

 and have set up changes of a far-reaching character. Before attempt- 

 ing, however, to restore hypothetically the meteorological conditions 

 ■of the Pleistocene Epoch, it seems desirable, as briefly as possible, 

 to call attention to the salient features of those of our own day. 



Through the courtesy of Mr. W. N. Shaw, F.E.S., of the British 

 Meteorological Office, I have been permitted to consult the ' Tagliche 

 eynoptische Wetterkarten ' of the North Atlantic and the adjacent 

 continents, issued under the joint supervision of the Danish and 

 Hamburg Meteorological Offices, and the Bulletins of International 

 Meteorology of the War Department of the United States, in the 

 library of the first-named institution ; and I have also taken 

 advantage of the maps in the reports of the Challenger Expedition,^ 

 and in the recently published ' Atlas of Meteorology.' 



These maps show that in certain portions of the earth's surface 

 the meteorological conditions are more or less permanent. This is 

 especially the case in the Southern Hemisphere, as will be seen in 

 figs. 4 & 5 (pp. 414 & 415), and figs. 6 & 7 (pp. 418 & 419), repro- 

 duced from the ' Atlas of Meteorology,' by the kind permission of 

 Dr. A. Buchan, F.R.S., and Mr. J. G. Bartholomew, E.It.S.E.^' 



South of lat. 40° S. there exists, at all times of the year, a 

 trough of low barometric pressure, with a complementary belt of high 

 pressure to the north of it. The latter is not continuous during 

 the summer months, but tends then to break up and leave the land; 

 it not only covers a much larger area during the southern winter, 

 but extends at that season farther northward, encroaching upon 

 and pressing northward the low-pressure belt of the Equatorial 

 region. The relative position of these belts of high and low 

 pressure in the Southern Hemisphere, and the east-and-west align- 

 ment of the isobaric lines to the south of lat. 40° S. throughout 

 the whole year, is accompanied by a continuous prevalence there of 

 westerly winds. 



The state of things in the Northern Hemisphere, although con- 

 forming to the same general principle, differs in details, and the 

 meteorological conditions are less permanent. Belts of high and 

 low pressure, corresponding to those of the Southern Hemisphere, 

 may be traced, but owing to the great amount of land north of the 

 Equator, they are not continuous, and their relative position and 

 alignment varies at different seasons.^ The continents are hotter 



^ Report on Atmospheric Circulation, vol. ii, maps 1-52. 



^ Charts, whether isobaric, isothermal, or isohyetal, that are based on the 

 annual averages, are, in enquiries of this kind, more or less misleading, as in 

 many regions the atmospheric conditions differ at different seasons. An 

 isobaric chart for the whole year, for example, shows the greater part of Asia 

 as anticyclonic, but for three or four months during the summer the barometric 

 pressure is there abnormally low (see fig. 5, p. 415). In considering the 

 meteorology of the Pleistocene Epoch, it will be necessary, therefore, to deal 

 separately with summer and winter. 



^ The normal planetary circulation of the atmosphere' is modified in tlie 

 Northern Hemisphere by the existing distribution of land and sea ; and it must 

 have been so too, I think, during the Pleistocene Epoch, even if it was, at that 

 time, of a more intense character. 



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