December 8, 1923] 



NATURE 



earthy especially the tropics, respond readily to these 

 solar variations, while in other parts the solar variation 

 is almost completely masked by secondary^ modifica- 

 tions. Hence we may classify weather influences into 

 two classes : solar, in which the influence of solar 

 variation is directly recognisable, and terrestrial, 

 depending on causes which at first sight are entirely 

 due to the influence of the land, sea, or atmosphere. 

 These two classes shade into each other, with no 

 definite line between them. 



The weather of the British Isles, apart from seasonal 

 temperature changes, is almost entirely terrestrial in 

 its control, being dependent on the distribution of 

 pressure over the North Atlantic and Arctic Oceans 

 and the continent of Europe. This distribution is 

 constantly changing, and we experience a succession 

 of " depressions " and " highs " which pass across or 

 near these islands, bringing our notoriously variable 

 weather. Careful examination of a series of daily 

 weather charts shows, however, that the most rapid 

 changes are generally only in details, the main features 

 of the pressure distribution changing comparatively 

 slowly, and four main types of weather have long been 

 recognised. These are named after the cardinal points 

 whence blow the winds characteristic of the type — 

 southerly, westerly, northerly, and easterly. 



In the southerly type, pressure is high over Europe 

 and low over the North Atlantic. Depressions are 

 continually appearing over the ocean, but their centres 

 fail to reach the British Isles, so that we are situated 

 between a depression to the west and an anticyclone 

 to the east, a condition which brings southerly winds 

 and warm weather. The amount of rainfall depends 

 on whether the high or the low pressure predominates 

 over the British Isles ; it decreases from west to east. 

 In the westerly type, pressure is high in the south and 

 low in the north, and depressions appear from the 

 Atlantic and pass rapidly eastward, generally along a 

 track somewhat to the north of Scotland. This type 

 brings fresh westerly winds ; spells of a day or two of 

 very fine weather alternate with spells with more or 

 less rain, the actual amount being least when the 

 storm tracks lie farthest north. If in addition to high 

 pressure to the south an anticyclone develops over 

 Iceland, with a trough of low pressure between, the 

 depressions pass directly across the British Isles and 

 very rainy weather is experienced, sometimes with de- 

 structive gales. On the other hand, when the southern 

 anticyclone extends so far north as to include Great 

 Britain, very fine weather is experienced, which, if 

 prolonged, as in 192 1 , may give rise to a serious drought. 

 A moderate development of the westerly type forms 

 about seventy per cent, of our weather. 



The northerly and easterly types are more or less 

 reversals of the southerly and westerly types ; the 

 easterly type proverbially brings us our most un- 

 pleasant weather. These weather tj-pes may persist 

 for periods varying from a few days to weeks, and 

 unfortunately there is generally little means of knowing, 

 when a type first becomes established, how long it is 

 likely to continue. Some cases are more favourable, 

 notably the formation of a large anticyclone centred 

 directly over the British Isles. In such a case a long 

 spell of fair weather is very probable, and when this 

 distribution was established on September 26, 1921, 



NO. 2823, VOL. I 12] 



a long-range forecast of fair weather for the next 

 fortnight over eastern and central England was issued 

 by the Meteorological Office, and was justified by 

 results. The original division of weather into four 

 types has been extended by the critical analysis of long 

 series of daily weather charts, the most recent classifica- 

 tion being that made by E. Gold,^ and much material 

 is now available for statistical study. 



An understanding of British weather depends on an 

 understanding of the causes which lead to the establish- 

 ment and modification of weather types. We may 

 carr\' the study a step further by referring to the 

 conflict between " polar " and " equatorial " air, 

 envisaged by the Norwegian meteorologists in their 

 researches on cyclones ; but a better insight into 

 ultimate causes is given by the older conception of 

 " centres of action," according to which the dominant 

 factors in the pressure distribution are three : the area 

 of low pressure which is generally found in the neigh- 

 bourhood of Iceland or southern Greenland, and 

 is termed the Icelandic minimum ; the area of high 

 pressure which occupies the eastern Atlantic near and 

 south of the Azores, termed the Azores anticyclone ; 

 and in winter, the area of high pressure which is centred 

 in Siberia and extends in a west-south-westerly direc- 

 tion towards the Azores anticyclone. 



If we could forecast the position and intensity of 

 these three centres of action during any month, we 

 should be well on the road to true long-range weather 

 forecasting. That is not yet the case, but a certain 

 amount of information has been gleaned which en- 

 courages further research. The Siberian winter anti- 

 cyclone appears definitely to be due to the extreme 

 " continentality " with its low temperatures and 

 hindrance to the outflow of air ; it is, in fact, a " pool " 

 of cold air. Such a continental anticyclone itself 

 intensifies the cold of winter, and once formed is 

 diflicult to dislodge. There is nothing more favourable 

 to the establishment of anticyclonic conditions than 

 a covering of snow, and an extensive snowfall over 

 northern or eastern Europe early in the winter may 

 cause a persistent westerly extension of the Siberian 

 anticyclone, influencing the weather of the British Isles 

 for several months. 



The other two centres of action are still more im- 

 portant, and they are intimately connected with each 

 other. When pressure is high at the Azores it is 

 generally low near Iceland, and vice versa ; when the 

 Azores anticyclone advances northward the Icelandic 

 minimum generally does likewise, but there are excep- 

 tions to both these rules. There have been cases, 

 notably February 1895, when the distinction between 

 the anticyclone and the depression was wiped out, 

 and for a whole month pressure was higher at Iceland 

 than at the Azores. The existence of these two centres 

 of action is connected in some way, not yet fully 

 understood, with the general circulation of the atmo- 

 sphere, but there can be no question that the details 

 of their position and intensity are modified by local 

 effects, and especially by variations of sea tempera- 

 ture, and in the case of the Icelandic minimum 

 by the presence or absence of floating ice, and we 



> London, Air Ministry, Meteorological Office. Geophysical Memoirs, 

 No. 16. Aids to forecasting : types of pressure distribution, with notes 

 and tables for the fourteen years, 1905-1918. By £. Gold. Loudon, 1930. 



