524 



NATURE 



[June 24, 1920 



motion from east and north. On the southern 

 side of the line the tropical orig^in of the air is 

 recognised by the corresponding signs — its gener- 

 ally higher temperature, its greater humidity, its 

 haziness, and its prevailing motion from west and 

 south. There can then be no doubt concerning 

 the origin of the line. Heavy, cold air flows out 

 along the ground from the polar regions. It is 

 separated from the overlying warmer air by a 

 surface of discontinuity, the height of which above 

 the ground decreases very slowly until it cuts the 

 ground along our line of discontinuity. Thus this 

 line shows how far the cold air has succeeded in 

 penetrating; it is a kind of ■polar front line. 



Along the whole of this front line we have the 

 conditions, especially the contrasts, from which 

 atmospheric events originate — the strongest 

 winds, the most violent shifts of wind, and the 

 greatest contrasts in temperature and humidity. 

 Along the whole of the line formation of fog, 

 clouds and precipitation is going on, fog prevail- 

 ing where the line is stationary, clouds and pre- 

 cipitation where it is moving. 



The line has a wavy form, and is in a continu- 

 ous undulating motion, thereby sweeping over 

 the whole of what is called the temperate zone. 

 The wavy form comes from alternately cold and 

 warm tongues of air, which extend themselves 

 towards the equator or the pole. The whole 

 system is moving from west to east, while 

 the line, at the same time, changes its form, 

 especially when great masses of accumulated cold 

 air are expelled from the central polar regions. 

 The more wavy the form of the line, the more 

 tempestuo,us and variable is the weather. At the 

 northern ends of the warm tongues the air motion 

 which characterises cyclones is recognised, and 

 the corresponding areas of rain are seen so far as 

 it has been possible to mark them from the few 

 observations ; these are the places of great storms 

 and low barometric pressure. The broad tongues 

 of polar air, on the other hand, bring the clearing 

 up between the successive storms and the corre- 

 sponding higher barometric pressure. 



Two expanding tongues of cold air may occa- 

 sionally cut off from its base an interjacent tongue 

 of warm air. Then the storm at the polar end 

 is no longer supplied by warm air, and soon 

 loses its power ; this is the death of a cyclone. 

 A tongue of polar air which has extended itself 

 too much towards the tropics may be cut off in 

 a similar way ; or, as the consequence of a new 

 outbreak of polar air, a more retired front may 



be formed behind one too far advanced. In 

 this way great isolated isles of polar air are 

 formed in lower latitudes ; this gives the formation 

 of great anticyclones, which generally bring 

 settled, good weather. Thus anticyclones are born 

 as cyclones die. Cyclone and anticyclone and all 

 meteorological events of the temperate zone are 

 in the most intimate way related to the polar front 

 and its motion. 



This expulsion of great masses of polar air, 

 which leads to the formation of anticyclones, also 

 enters as an essential element into the great atmo- 

 spheric circulation. There is a practically continu- 

 ous flow of warm air along the ground from the 

 "highs " of the sub-tropic calms towards the polar 

 regions. This flow concentrates itself in the warm 

 tongues, and continues into the polar regions in 

 upper levels. Here the air is cooled, and eventu- 

 ally reaches lower levels. Thus increasing masses 

 I of cooled air are accumulated behind the polar 

 I front. This must continuously advance, with the 

 ! effect that the tracks of the corresponding cyclones 

 I are always moved further towards the south. 

 Finally, at the place of least resistance, great 

 i masses of cold air break through and are expelled 

 I in the direction of the tropics. The polar front 

 i performs a corresponding retreat, the cyclonic 

 tracks are again displaced to the north, and the 

 i type of weather is changed. Then the same action 

 ; repeats itself. This intermittent form of the 

 I great atmospheric circulation is especially pro- 

 I nounced in the winter. During the summer the 

 I polar front is far back, and the high temperature 

 of the continents exerts a considerable influence ; 

 then occasionally a continuous return of polar air 

 may be established along the west coast of the 

 continents, leading direct into the trade winds. 



These results cannot fail to exert a considerable 

 influence upon the methods of weather forecast- 

 ing. All meteorological events of the temperate 

 zone, great and small, are derived from the general 

 atmospheric circulation described above, as we 

 know it from the motions of the polar front. If 

 we succeed in watching it effectively it should be 

 possible not only to give short-range forecasts a 

 hitherto unattained accuracy, but also to com- 

 plete them by long-range forecasts giving 

 the general character of the weather perhaps for 

 weeks ahead. These two kinds of forecast could 

 be extended to all regions of the temperate zone 

 — oceanic as well as continental. The required 

 survey of the polar front is merely a question of 

 organisation. 



The Cardiff Meeting of the British Association. 



ill be issued at the end of 



PREPARATIONS are going steadily forward 

 -*- in Cardiff for the forthcoming meeting of the 

 British Association from Tuesday, August 24, to 

 Saturday, August 28 inclusive. Owing to the 

 crowded state of the town, the question of accom- 

 modation is causing diflficulties, but these will, it 

 is expected, be satisfactorily overcome. A list of 

 NO. 2S43, VOL. 105] 



hotels and lodgings 

 the present week. 



The Marquis of Bute has generously offered to 

 give a garden party at the Castle on the Wed- 

 nesday afternoon, and the invitation has been 

 gratefully accepted by the Lord Mayor on behalf 

 of the local executive committee. Owing to the 



