VI. 



ON ATMOSPHERIC MOVEMENTS.* 



(SECOND PAPER.) 



By Prof. H. von ITelmholtz. 



ON THE TIIEORY OF WINDS AND WAVES. 



In my previous communication made to the Academy on the 31st of 

 May, 1888, 1 endeavored to prove that conditions must regularly recur in 

 the atmosphere where strata of different density lie contiguous one 

 above another. The reason for the greater density of the lower stratum 

 is conditioned by the fact that the latter has either a smaller amount of 

 heat or a smaller velocity of rotation, if in fact both conditions do not 

 work together. As soon as a lighter fluid lies above a denser one with 

 well-defined boundary, then evidently the conditions exist at this 

 boundary for the origin and regular propagation of waves, such as we 

 are familiar with on the surface of water. This case of waves as 

 ordinarily observed on the boundary surfaces between water and air is 

 only to be distinguished from the system of waves that may exist 

 between different strata of air, in that in the former the difference of 

 density of the two fluids is much greater than in the latter case. It 

 appeared to me of interest to investigate what other differences result 

 from this in the phenomena of air waves and water waves. 



It appears to me not doubtful that such systems of waves occur with 

 remarkable frequency at the bounding surfaces of strata of air of 

 different densities, even although in most cases they remain invisible 

 to us. Evidently we see them only when the lower stratum is so nearly 

 saturated with aqueous vapor that the summit of the wave, within 

 which the pressure is less, begins to form a haze. Then there appear 

 streaky, parallel trains of clouds of very different breadths, occasionally 

 stretching over the broad surface of the sky in regular patterns. More- 

 over it seems to me probable that this which we thus observe under 

 special conditions that have rather the character of exceptional cases, 

 is present in innumerable other cases when we do not see it. 



* From the Sitzungs-iericlde of the Royal Prussian Academy of Sciences at Berlin, 

 July 25, 1889, pp. 761-780. 

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