420 



NATURE 



[August 30, 1 888 



arrangement or the number of the maxima situated over 

 the Continent. 



The essential condition for successfully constructing 

 the isobars is a knowledge of the various types that pre- 

 sent themselves, so that we may discover by a simple 

 inspection of the charts of Europe and America what 

 general type is in question. We will give two examples 

 in which M. de Bost shows that the reconstruction of 

 the general chart according to this method would have 

 enabled him to foretell two important storms. 



On December 2, 1886, the general conditions were as 

 shown in Chart No. 1. 



" By confining our attention to the indications given 

 solely by the chart of Europe," he writes, '' we might 

 expect in France cold weather with cloudy sky and sleet 

 showers. On the 4th, a depression which was foretold on 

 the 3rd had spread over the British Isles, where it brought 

 bad weather ; but the barometer rose rapidly over the 

 west of Europe. Supposing that the high pressures would 

 extend over Central Europe, we might expect a spell of 

 fine and cold weather. Instead of that a rapid fall of 

 the barometer occurred over the north of Europe, and 

 the wind shifted to south-west. On the 7th and 8th this 

 condition was intensified, and one of the most violent 

 storms that we have experienced for a long time struck 

 us, the barometer at Mullaghmore falling to 27*45 inches." 



go 100 



80 60 40 20 



20 40 SO SO 100 



120 WO SO 60 40 20 20 40 60 



Chart No i. — 762 mm. = 30 'o inches. 



80 100 



of the month the existence of an area of high pressure 

 over the west of Europe, and it appears to extend to some 

 distance over the ocean ; low pressures are observed over 

 the north of the Continent. This condition persists, with 

 rather cold weather, and on the 7th, in France, a continua- 

 tion of dry weather is foretold. In England, the forecast 

 bears principally on the consequences of the move- 

 ment of a small barometric minimum which exists over 

 Scotland. 



On the 8th a complete change of system occurred ; an 

 important depression reached Europe over Portugal, and 

 the low pressure extended over the north of the British Isles. 

 This depression did not fail to bring a storm from the 

 north-east. While the situation in Europe was considered 

 as fairly stable, and the low pressures of the north of 

 Europe were considered to be chiefly in operation, an 

 important minimum which was advancing from the cen- 

 tral part of the ocean suddenly appeared, and produced 

 a complete change of conditions. And yet the predictions 

 of the European meteorologists were certainly the only 

 ones that could have been made from the study of the 

 various daily weather reports. But if we construct the 

 chart of the 5th of October, we shall recognize that a 

 centre of low pressure was very close to the south-west of 

 Spain, and was directly threatening Europe. A mere 

 glance at the Chart No. 2 would have been sufficient to 



120 WO 80 60 40 SO 20 40 60 SO 100 



If we refer to the general charts, and particularly to the 

 chart of December 2, we shall see that the barometric 

 maximum of Asia is in its place, and that over the United 

 States there is a large area of high pressure. This is 

 rather extended, and leaves no room for the low pressure 

 of the region of the Lakes except in the vicinity of New- 

 foundland. As there is also a rather important baro- 

 metrical maximum to the west of Europe, we conclude 

 that very important low pressures must exist over the 

 ocean off the American coast. Under these conditions, 

 in order to have fine weather, it is necessary that the area 

 of low pressure of Central Europe should shift towards 

 Siberia, so as to allow the maximum to advance over our 

 regions. On the 5th, the increase of the high pressure of 

 Asia towards St. Petersburg and Finland clearly indicates 

 that the area of low pressure cannot shift in its entirety 

 towards the East. The rise of the barometer over the west 

 of Europe must not therefore be taken as a sign of lasting 

 fine weather, but as the result of the approach of the low 

 pressure from the ocean. From these conditions M. de 

 Bort shows that the forecast to be drawn for Western 

 Europe was entirely opposite to that which resulted from 

 the study of the conditions over Europe alone. 



Another example of the utility of the construction of the 

 charts over the ocean is afforded by the very sudden 

 change of weather that occurred on the 8th of October, 

 1887 (Chart No. 2). 



The European weather charts indicate at the beginning 



change entirely the weather forecasts in Western Europe, 

 and would have given ample warning of the approach of 

 the storm of the 8th. 



As to the reasons which would allow us to trace the 

 isobars over the Atlantic in the way they are represented 

 on the chart ; the importance of the barometric maximum 

 situated over the British Isles and the west of Europe is 

 such that we must infer from it that the normal maximum 

 of Madeira was displaced. This conclusion was confirmed 

 by the observations from Madeira, where the barometer 

 was below 29/9 inches, with a south-east wind. Secondly, 

 there were no important low pressures in America, there- 

 fore these must exist over the ocean and near Europe, as 

 the observations from Nova Scotia show higher pressures 

 than those from Canada. Everything concurs, therefore, 

 in indicating with certainty the presence of a large baro- 

 metric minimum over the centre of the ocean, and the 

 importance of this indication for the prediction of the 

 weather in Europe cannot be contested. 



From these examples M. de Bort concludes— (1) that, 

 with the aid of telegraphic reports from America, and 

 the knowledge of what is taking place over Europe and 

 Siberia, we can trace the isobars over the ocean with 

 much chance of success ; (2) this trace being made, we 

 may take useful advantage of it to reveal the true 

 character of the general condition of the atmosphere, 

 which our charts, limited to Europe and the British Isles 

 alone, are powerless to indicate. 



