722 • Mr. C. J. P. Cave [April 11, 



that in Germany, where the pressure was highest, the westerly wind 

 must have been descending and must have divided into two currents, 

 one flowing on as a westerly wind over Eastern Europe, the other 

 flowing back as the easterly wind recorded in this country. 



There are other cases of reversal which are not so simple as the 

 one described above. In many cases this type is associated with 

 small depressions, or with small areas of high pressure which seem 

 to be relatively shallow. The surface winds are related to these 

 shallow systems, while the upper winds are controlled by larger areas 

 of high and low pressure, shown on the weather maps at places lying 

 farther from the point of observation. 



On September 30, 1908, for instance, a southerly surface wind, 

 after remarkable backing, gave place to a calm at 3 kilometres ; above 

 the calm another southerly wind is met with ; in this case the surface 

 wind is probably related to the high pressure system over Germany ; 

 the upper wind to the depression approaching from the Atlantic. 

 There was another somewhat similar case on November 16, I'JOS, 

 though with winds from a different direction ; the northerly surface 

 wind backed, and a calm was met with ; above this, very unex- 

 pectedly, came a thin stream of southerly wind, above which again 

 was a north wind, increasing in velocity with height. In this case 

 the lowest wind was part of the circulation of an anticyclone which 

 was approaching these islands from the Atlantic ; the intermediate 

 southerly wind was perhaps the last remaining effect of the anti- 

 cyclone over the Continent, while the upper wind was the outflow 

 from above a depression near Iceland, a wind which belongs to 

 another class to be noticed later. 



In cases of reversal we find that the warm wind flows over the 

 top of the one that comes from a colder region ; there must somewhere 

 be a line where the warm current is rising, where it must be cooled 

 dynamically, and where its moisture may condense into cloud or rain. 

 It is interesting to note that in most cases rain occurs somewhere in 

 the region of the reversal, and in summer, thunderstorms are fre- 

 quent. Thunderclouds may often be seen to be in a wind coming 

 from a contrary direction to the wind on the surface, and it seems 

 possible that for anything like a sustained thunderstorm something 

 in the nature of a reversal must exist ; it is difficult to see how a 

 difference of potential, sufficient to produce lightning, can be kept up 

 unless winds from different directions are bringing masses of air at 

 different potentials near to one another. 



It has been noticed in Hampshire that when the sound of 

 gun-firing in the Channel is distinct, it is, in summer, a sign of 

 thunder ; an explanation may be hazarded : if there is a reversal so 

 that the upper wind is coming from the south, the sound waves 

 travelling from this point with a slight upward tendency will be 

 refracted on entering the upper current, and thus, instead, of being 

 dissipated in the upper air, may again reach the surface at a con- 



