206 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



these ice-capped areas are regions of high rather than of low pressure. Previously it had 

 been supposed that the general low pressure which appears to become more and more pro- 

 nounced as one goes poleward prevails over all lands where continental ice-sheets now 

 exist. It was thought that only in this way could sufficient precipitation be obtained to 

 support the great ice-fields and send out the thousands of bergs and floes which wreck 



our ships. 



As soon as explorers began to study the winds of ice-capped regions with any care, 

 however, they discovered that the prevailing movement of the air is strongly outward. 

 At times it may be reversed, but not for long. This means, apparently, that high barometric 

 pressure prevails in the interior and is busily forcing the air outward except at times when 

 some general disturbance upsets the normal conditions. This is what would naturally be 

 expected, since vast areas of snow are bound to be extremely cold and hence are Ukely to 

 induce high barometric pressure. Hitherto, however, it has generally been supposed that 

 such almost continuous high pressure could not characterize these regions because there 

 would be no way of obtaining precipitation. Hobbs has attempted to explain this by an 

 ingenious hypothesis which holds that, under the peculiar conditions of glaciation and of 

 the newly discovered inversion of atmospheric temperature at high levels, fine crystals of 

 snow can be deposited even when the barometei- is high and the upper air clear. Whether 

 his theory is sound or not can not here be discussed, but it seems at least to be worthy of 

 careful consideration. Its chief value, however, lies in its emphasis of the fact that regions 

 of continental glaciation appear to be areas of high pressure. This being so, an increase 

 in the area of continental glaciation would mean an increase in the area of permanent high 

 pressure. At the height of a glacial epoch high pressure would prevail throughout the year 

 over most of the northeastern part of North America, and over northwestern Europe. 

 If other conditions remained in general the same as at present, the barometric gradients 

 from the great glaciated areas of high pressure to the low areas of the oceans and of equa- 

 torial regions would be steeper than now. Hence the winds would acquire great strength, 

 storms would be more numerous than now, and they would follow more southerly courses. 

 Forced far to the south by the great continental high areas both in northeastern America 

 and northwestern Europe, they would probably swing down into the Gulf of Mexico or 

 the Sahara Desert, as the case might be, and the climates of those southerly regions would 

 partake of the unstable and stimulating nature which to-day is so characteristic of the parts 

 of North America and Europe where the world's most progressive races dwell. 



If we are right in supposing that the climatic changes of which we seem to have found 

 evidence during historic times are of the same nature as those of the glacial period, we can 

 readily see how in the days of Yucatan's glory the storminess now characteristic of the 

 United States may have prevailed farther south. At all seasons, but especially during 

 the winter, cyclonic disturbances were probably more frequent and severe than now, the 

 winds presumably blew more forcibly, and the minimum temperature of Yucatan may have 

 fallen as low as freezing instead of only to 50°. The effect which such a southward shifting of 

 the stormy belt would have upon precipitation deserves careful consideration. At present 

 the "northers" bring very little rain to Yucatan, since they are cold winds moving rapidly 

 toward warmer regions. Therefore their capacity for moistm-e increases and they are not 

 likely to produce much rain. Neither their frequent prevalence nor the general shoving 

 southward of the dry subtropical zone would probably have much effect upon the winter 

 rainfall of the dry northern parts of Yucatan, but might much diminish that of the 

 forested regions. This is because the winter rainfall is here due to the trade winds._ If 

 cyclonic storms came farther south than at present, they would destroy the trade winds 

 in this latitude and would allow them to prevail only in regions farther south. Hence 

 there would not be any steady winds blowing in from the sea through the winter and causing 

 abundant precipitation. Tliis would give rise to a dry season longer and more intense than 



