1918] on The Winds in the Free Air 725 



Supposiui,' 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 trans- 

 mitted throug-li all the lower layers of the atmosphere, and unless 

 modified by 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 kilometre level owing to the greater density of the air 

 near the surface. If, however, the air to the north at every height 

 were at a lower temperature than the air at a corresponding height 

 over the place of observation, there would 1)e 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 kilometre 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 l)e 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 Ijegins to cause 

 a diminution of velocity. 



If, again, at the 9 kilometre 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 neutralize one another, 

 in which case we get a solid current of east wind between the strato- 

 sphere and the ground level. 



If there is no wind at the 9 kilometre 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 mechanism by which 

 the different types of vertical wind structure may 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 pijint 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 strato- 

 sphere 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 



