May 22, 1913] 



NATURE 



309 



The third class comprises those cases in which the 

 wind, after reaching the gradient velocity in the first 

 I km. or so, falls oft more or less rapidly in the upper 

 air. This class is almost entirely associated with 

 easterly winds on the surface, when there is high 

 pn-ssure to the north and low pressure to the south. 

 An east wind is usually, though not always, a shallow 

 one; a south-west gale increases in the upper air, 

 but when an easterly gale is blowing, causing such 

 high seas and such dangers to shipping, it is curious 

 to reflect that such a short distance up we should 

 meet with light breezes, or even a complete calm. 



We now come to the class of reversals when the 

 wind in the upper air is very different in direction 

 from that near the surface, and when it often bears 

 no relation to the surface pressure distribution. In a 

 typical case, after an initial increase for a short dis- 

 tance above the surface, we find the wind gradually 

 decreasing as we ascend, until a layer is met with in 

 which there is a complete calm; above this we find 

 an entirely different wind, which usually increases as 

 we go higher, as in the case of winds in the second 

 class. It looks at first sight as though there were 

 a discontinuity in the atmosphere, but I hope to show 

 later that this is more apparent than real. A typical 

 example of a reversal occurred on November 6, 1908, 

 when the surface wind was easterly with a velocity 

 of 17 metres a second, just below 1 km.; above this 

 it fell off to a complete calm at 3 km. ; at 4 km. 

 there was a light north-west wind, which increased 

 to a wind of 15 metres per second at 10 km. The 

 weather map for this dav is remarkable : over this 

 countrv there is no sign oh the surface of the westerly 

 wind above, but it appears 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 rela- 

 tively 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 km. ; above the calm another southerly 

 wind is met with; in this case the surface wind is 

 probablv related to the high-pressure system over 

 Germany ; the upper wind to the depression approach- 

 ing frotn'the Atlantic. There was another somewhat 

 similar case on November 16, 1908, though with winds 

 from a different direction ; the northerly surface wind 

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

 unexpectedly, came a thin stream of southerly wind, 

 above which again was a north wind, increas- 

 ing 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 Atlan- 

 tic ; the intermediate southerly wind was perhaps the 

 last remaining effect of the anticyclone over the Con- 

 tinent, while'the upper wind was the outflow from 

 above' a depression near Iceland, a wind which be- 

 longs 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 res-ion; 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 frequent. 

 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 differ- 

 ence 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 tend- 

 ency 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 considerable distance 

 from their point of origin. Such conditions of wind 

 are those productive of thunderstorms. This may also 

 possibly account for the superstition that gun-firing 

 produces rain; the sound of guns is only carried to 

 great distances under the conditions I have men- 

 tioned, which are precisely the conditions favourable 

 for heavv rains. 



\ north-east wind with rain lasting many hours is 

 a common and a very unpleasant type of weather ; it 

 is not obvious where the moisture comes from with 

 such a wind, for the air from the dry regions of the 

 Continent could scarcely become saturated in its short 

 passage over the North Sea. I believe the moisture 

 comes from the Atlantic in a south-westerly wind in 

 the upper air. Balloons cannot be followed for any 

 length of time in such weather, but I have sometimes 

 observed that the north-easterly wind slackens con- 

 siderably below the cloud level, and sometimes when 

 breaks in the clouds have enabled balloons to be fol- 

 lowed a little farther, there have been unmistakable 

 sie;ns of reversal. A careful watch for upper clouds, 

 scon through rifts in the lower cloud sheet, will often 

 indicate an upper southerly wind. So sure do I fee! 

 of these facts that, though living only twelve miles 

 from the Channel, I never hesitate to send up an 

 instrument-carrying balloon in rainy weather with a 

 north-easterly wind, feeling certain that, though the 

 balloons may go towards the sea at first, they will 

 ultimately return and fall on dry land. My confidence 

 is usually justified by the balloons coming to earth in 

 the Midlands or eastern counties. 



The last type of wind structure to be considered is 

 the outflow that seems to take place from the upper 

 layers over a low-pressure system, causing west to 

 north winds in the upper air on the east and south 

 sides of the depression. Depressions out in the 

 Atlantic, which cause south-west winds on the sur- 

 face £rive rise to west or north-west winds in the 

 upper air over England; even cyclones so far off as 

 Iceland produce such winds, and as they pass along 

 the Arctic circle, between Iceland and Norway, they 

 show their presence by an upper northerly wind over 

 this country. As the upper wind is often quite 

 different from that on the surface, reversals are fre- 

 quent in this class, and are associated as usual with 

 rains and with thunderstorms in the summer. It 

 mav be that much of the rain that falls in the cyclonic 

 depression is due to the rising of this outward flowing 

 current over the very different surface current on the 

 oast and north-east sides of the depression. 



In connection with the subject of reversals, I may 

 mention the wave and ripple clouds that form such 

 beautiful skyscapes. It used to be supposed that these 

 were formed by winds from different directions flow- 



NO. 2273, VOL. qi] 



