NATURE 



[May 22, 1913 



ing over one another and setting up waves ; but the 

 observations of pilot balloons have shown that between 

 two currents from different directions there is either a 

 layer of calm, or else the wind changes round gradu- 

 ally; two very different currents are not found in 

 close juxtaposition : there is no abrupt transition 

 between them. 



To show the relation of the different types of wind 

 structure to the surface pressures a model has been 

 prepared ; on the map are shown a depression and an 

 area of high pressure, with arrows to show the wind 

 directions ; above the map is a sheet of glass to repre- 

 sent the first 5 km. of the atmosphere ; on this are 

 marked the winds one would expect to meet with 

 at this level under the conditions of pressure supposed ; 

 above this sheet of glass is another representing the 

 thickness of the atmosphere from the 5-km. level to 

 the stratosphere. The model is on the scale of one- 

 millionth, the vertical scale through the glass being 

 approximately the same as the horizontal scale. 



The churning up of the air resulting from the heat- 

 ing of the surface layers by contact with the earth 

 heated daily by the sun, does not presumably reach 

 into the stratosphere ; there being no vertical move- 

 ments, we should expect to find only such horizontal 

 movements as are consistent with a suitable distribution 

 of density. In the simplest cases the wind increases 

 in velocity until a maximum is reached just below the 

 stratosphere; above this the wind begins to diminish, 

 and sometimes falls off in a very marked manner. 

 There are occasions when all real wind seems to have 

 ceased, and the balloon as it ascends through this 

 curious region moves first in one direction and then 

 in another, so that the relation of wind direction to 

 height can only be represented on a diagram by a 

 disconnected series of points. 



What takes place still higher? Does this region 

 of calm extend to the very confines of the atmosphere? 

 We have practically no evidence to go on. In 

 February, 1909, a meteor left a magnificent streak 

 which was visible for two hours and a half ; this trail, 

 which was some forty miles above the surface of the 

 earth, moved in a manner suggesting very high wind 

 velocities, with sudden variations in the different layers 

 throue-h which it passed. But it is possible that the 

 streak of a meteor may partake of the nature of an 

 aurora, the luminous patches of which sometimes 

 move in a remarkable way, and probably under forces 

 other than those of the winds. 



Having for purposes of classification divided the 

 wind structure of the atmosphere into different classes, 

 I must now attempt to put them together, and to 

 show that some of the types that seem very different 

 are in reality closely connected. 



Following on inquiries made by Mr. W. H. Dines 

 on the correlation between the surface pressure and 

 various meteorological elements at a height of 9 km., 

 it was suggested by Dr. W. N. Shaw,"F.R.S., that 

 the changes of pressure to which our changes of 

 weather are due have their origin, not near the sur- 

 face of the earth, as hitherto supposed by many 

 meteorologists, but just below the level of the strato- 

 sphere, at a height of 9 km. or so above the surface. 

 This view is in accordance with the observed facts of 

 the wind distribution in the different layers of the 

 atmosphere. 



Supposing that on a certain day there is a pressure 

 distribution just below the stratosphere, which at that 

 level produces a westerly wind of a certain strength; 

 this pressure distribution will be transmitted through 

 all the lower layers of the atmosphere, and unless 

 modified bv other conditions will produce a west wind 

 at the surface; the velocity of this wind will, however, 

 be only about one-third of that at the 9-km. level 



NO. 2273, VOL. 91] 



owing to the greater density of the air near the sur- 

 face. If, however, the air to the north at every height 

 were at a lower temperature than the air at a corre- 

 sponding height over the place of observation, there 

 would be at all levels a tendency for easterly winds. 

 This will have the effect of reducing the westerly wind 

 as we descend through the atmosphere, and when the 

 surface is reached the west wind will have a much 

 lower value than it would have had were it only for 

 the increased density of the air. If the wind at the 

 9-km. level is not very strong, or if the tendency to 

 produce an easterly wind is strong, as would be the 

 case if the air to the north were very cold, we may get 

 a calm at the surface, or the calm may even be reached 

 at some distance above the surface, in which case 

 the tendency for easterly winds may actually produce 

 such a wind, which will increase in velocity as we 

 descend towards the surface under the layer of calm, 

 and be strongest a little above the surface of the 

 earth, at a point where surface friction begins to 

 cause a diminution of velocity. 



If, again, at the 9-km. level, there is a pressure 

 distribution producing an easterly wind, cold air to 

 the north will produce a tendency for an increase of 

 easterly wind as we descend through the atmosphere ; 

 but the greater density of the air at the lower levels 

 will produce a decrease of wind velocity from whatever 

 direction the wind may be coming ; the two tendencies 

 may neutralise one another, in which case we get a 

 solid current of east wind between the stratosphere 

 and the ground level. 



If there is no wind at the 9-km. level, cold air to 

 the north will produce easterly winds in the lower 

 levels, in which case we should find easterly winds 

 increasing in velocity as the surface is approached. 



These considerations give some idea of the mechan- 

 ism by which the different types of vertical wind 

 structure mav be produced. The wind increasing; in 

 height, the solid current, the wind decreasing with 

 height, are seen to fall into their places. The reversal, 

 with an east wind near the surface and a west wind 

 higher up, is only an extreme case of the slackening 

 of the westerly wind near the surface ; and the point 

 of reversal, far from marking a point of discontinuity 

 in the atmosphere, is seen to be merely the result of 

 forces extending right through the lower part of the 

 atmosphere, between the stratosphere and the earth. 



If the winds are resolved into components at right 

 angles to each other, that is north-south and west- 

 east components, it is found that in most cases 

 the west-east component decreases below the strato- 

 sphere and is a minimum near the surface, an east 

 wind in this case being considered as a negative west 

 wind. This is what should be the case if the ideas 

 I have been considering are correct, for the air to 

 the north is generally colder than the air over this 

 country. In the case of the north-south component 

 we find no such general rule, but this also is as it 

 should be, for the air to the east and west mav be 

 either of the same temperature, or warmer, or colder 

 than the air over the station ; in other words, there 

 is a normal north to south temperature gradient but 

 not a normal west to east gradient, in our islands. 



The sunnosed cases mentioned are, of course, simple 

 types, and it can readily be understood how varying 

 conditions of pressure and temperature n>iv in similar 

 ways produce varieties of vertical wind distribution. 

 In considering the pressure distribution just below the 

 stratosphere as the regulator of the winds and the 

 weather in the lower part of the atmosphere, I fear 

 I have nothing to add concerning- the laws governing 

 these pressure distributions; the idea is a new one, 

 and has yet to be worked out in its details, and to 

 stand the test of criticism and fuller investigation. 



