836 



NATURE 



[December 8, 19: 



have to bitiaden our survey to include these two 

 facton. 

 The primary facts aJviut the eirrulation of the waters 



of the N(»rth Atlantic < ' '■• most people, 



and for our jnirijosc > into two pro- 



cesses. I'irst, a jifiat mass ol Miinu aurface water is 

 driven westwards near the e(|uator by the Trade winds, 

 ultimately U-in;; deflertcd northward l>y the coast of 

 America and forming the Gulf Stream ; secondly, this 

 warm water is spread out south-cast of Newfoundland, 

 and is driven eastward by the prevailing winds towards 

 the European coast. ('onse<|uently, any increase in 

 the stren)?th of the Trade winds should l>e followed 

 after a considerable interval by a rise in the temperature 

 of the sea north of Scotland. This has very little 

 direct influence on the temperature of these islands, 

 but it intensifies the Icelandic minimum and draws it 

 south-eastward, causinj^ low pressure and much cyclonic 

 activity in the North Sea, with strong south-westerly 

 winds over Holland, Germany, and Denmark, and 

 northerly winds over Iceland. Thus, P. IT. Gall6* 

 found that when ships' observations showed an in- 

 creased strength of the Trades during spring and 

 summer, the following winter was warm over Holland 

 and Germany, but cold over Iceland and Greenland. 

 This refers to observations over the whole Trade belt. 

 Unfortunately, direct observations of the strength of 

 the North-east Trade are difTicult to obtain, but we 

 may employ instead the mean pressure at the Azores, 

 which may be taken as a measure of the develop- 

 ment of the North Atlantic anticyclone, with which the 

 Trade wind is associated. The interval between the 

 occurrence of high pressure at the Azores and of low 

 pressure near the Faroes is about a year, which is the 

 average time required for the completion of the oceanic 

 circulation between these points. 



The influence of floating ice on the pressure dis- 

 tribution is equally marked. The great ice-factory 

 of the northern hemisphere is the Arctic Ocean north 

 of the Eurasian coast, and deliver)' is effected by a 

 current which sets from near Spitsbergen down the 

 east coast of Greenland and round Cape Farewell. 

 Each spring this current carries great masses of sea- 

 ice, and in some years with strong north-westerly 

 winds much of this ice is carried to the coasts of 

 Iceland. From 1901 to 191 9 there were 43 months 

 during which ice lay off Iceland for more than five 

 consecutive days, and in the majority of these months 

 pressure in Iceland was more than 2 mb. above normal. 

 The mean deviation of pressure from normal during 

 the whole of the ice-days (to the number of 701) was 

 -h6-7 mb. Since there is no reason to suppose that 

 the high pressure brings the ice, this result indicates 

 conclusively that the ice and the cold surface water 

 associated with it are effective in raising the pressure 

 over Iceland and producing a northerly type of weather 

 in the British Islands. 



W. Weise * has recently made a further step. The 

 ice takes about \\ years to travel from the Arctic 

 Ocean north of Siberia, wlun it i^ formed, to the 



* On the relation betwwn lluctuations in the stmijjth of the Trade winds 

 of the North .\tlantic CVcan in summer .md departures from the normal of 

 the winter temperatures in Europe. Amsterdam, Proc. R. .-Vkad. Wetenschap, 

 vol. 18, No. 9. 



• Die Einwirkung dcs Polareises im GronUndischen Meeie auf die nord- 

 atlantische zyklonale Tatijjkeit. Berlin, Ann. Hydrogr., vol. 50, 1922, 

 p. i;i. 



NO. 2823, VOL. 112] 



East Greenland current, and he found that low tern 

 perature at Olxlorsk and Tururhansk in autumn i> 

 followed after this interv'al by a large amount of ice ea^t 

 of Greenland, and vice versa. Thus an in 

 in our spring weather is determined I* 

 years previously in the north of Sitieria. 



The influences which control British v 

 many and diverse, and it is not worr 

 small fluctuations of the solar constant < 

 response. Yet they can sometimes he traced ; lor 

 example, at times of increased solar radiation (and 

 many sunspots) the tracks of depressions appear i*- 

 be on the average rather further south than with 

 diminished radiation (and few sunspots). From 2 to 

 4 J years after sunspot maxima the Azores anticyclom 

 tends to spread northward in summer over Spain and 

 the Bay of Biscay, or even over the British I ' 

 conditions favourable to drought. Various 

 effects have lx;en suggested in British weather, bui 

 none are definite enough for u.se in forecasting. Tin 

 same conclusion must apply to " weather cycles." Tht 

 search for the golden cycle in weather is curiousb 

 similar in its history' to the search for the philosopher^ 

 stone — it has not been found, and we are more and more 

 compelled to the belief that it does not exist ; but in 

 the search for it much information of value in other 

 respects has been acquired. Periodicities in weather 

 there undoubtedly are, but they are usually either so 

 small in amplitude as to be of academic interest only, 

 or they show baffling changes of phase and amplitude. 

 Even the classical " Bruckner cycle " of T)^ years is 

 only recognisable when we add the rainfall of ten 

 consecutive years together, and its absolute uselessness 

 for forecasting is shown by the position of the dr\' year 

 1921 — one year before a maximum. The standard 

 deviation from normal of a month's rainfall in London 

 is about sixty per cent., while the variation attributable 

 to the Bruckner cycle is less than three per cent. .\ 

 similar criticism applies to Sir William Beveridge- 

 periodicities in the price of wheat. 



Ocean currents and floating ice are thus the most 

 important factors in British w-eather. Given a fore- 

 sight of these two elements, we could make a reasonable 

 guess at the general type of weather likely to prevail, 

 though not the changes from day to day. Both ocean 

 currents and ice are themselves also subject to modifica- 

 tion by pressure distribution, and consequently we 

 have a chain of cause and effect connecting a succession 

 of months or seasons. We know the normal oceanic 

 circulation and the normal pressure distribution. If 

 in a given month we knew also the deviation of pressure 

 from the normal distribution, we should be able to 

 infer the abnormalities which will be produced in the 

 oceanic circulation and hence to calculate the pressure 

 deviations for the following month. If the process 

 were sufficiently well understood we could carr\' our 

 calculations forward long enough to give useful fore- 

 casts ; at present the subject has scarcely reached even 

 the experimental stage. A large statistical basis i> 

 necessar}-, and it is only within the last few years that 

 this has begun to be supplied by the Riseau Mondial * 

 a compilation of monthly means of pressure, tempera- 

 ture, and rainfall over the globe. 



* London, Meteorologiral Office, British Meteorological and Magnetic 

 Vear-Book, Part V. 1910-1914 issued. 



