4:58 Periodic Convection Currents in the Atmosphere. 



with observation, as the wind at a considerable distance 

 above the ground is known to have approximately the 

 gradient magnitude and direction appropriate to the 

 pressure distribution at the same height. From the results 

 of this paper we should expect the divergences from this 

 rule to be important in temperate latitudes when the period 

 is not long compared with 15 hours. 



The type of disturbance here considered cannot be 

 expected to bear any resemblance to the cyclones and 

 anticyclones of Western Europe ; for it deals only with 

 a standing oscillation over a fixed area, whereas a cyclone 

 is not usually generated in the region where it is observed. 

 It may, however, find a counterpart in the annual variation 

 of pressure in the continents of North America and As;a ; 

 the well-known tendency for wide-spread depressions and 

 elevations to form over these in summer and in winter respec- 

 tively, agrees w ith the theory, as does the average direction of 

 the observed winds round them. The further result, pre- 

 dicted bv the theory, that the pressure difference between two 

 points at the same great height should have the opposite sign 

 to that between their projections on the earth's surface, 

 awaits confirmation, as the upper-air observations usually 

 record only the relation between temperature and pressure, 

 the height being found only by calculation from the 

 pressure and not directly. Wind velocity at a great 

 height should also be opposite to that near the surface *. 

 The Southern Hemisphere can yield little direct infor- 

 mation, as everything is there dominated by the planetary 

 circulation. 



The physical interpretation that has been given of these 

 results suggests that making allowance for compressibility 

 and the difference between the troposphere and stratosphere 

 will not affect their character, save by numerical factors 

 that will not change the order of magnitude. The wave 

 velocity is of order (#H)% where H is the height of the 

 homogeneous atmosphere, for all modes of variation of 

 temperature with height ; so that possible temperature 

 variations will not lead to any change in the order of 

 X 2 ^/t/(4&) 2 — 7 2 ), which determines the mode of approximation 

 to the result. 



* The existence of the counter-trade winds above 3000 metres, 

 observed by A. L. Rotch and L. Teisserenc de Bort, Hann-Band der 

 Meteorologischen Zeitschrift, 1906, p. 274, on the whole confirms this. 



