240 TRANSHORIZON PARAMETERS 



[25]. It has also been applied by Flavell and Lane [6] in studies of the 

 effect of anticyelonic subsidence on tropospheric propagation. Values of 

 K can be derived very rapidly from upper-air data and, since plots of K 

 against height or pressure do not exhibit the large systematic decrease of 

 A^ with height in the conventional N{h) profile, the structure and motion 

 of meteorological features are clarified considerably. (Below the conden- 

 sation level, a lapse rate of — 20A''/km = dK/dh = 0.) In this respect, 

 K is superficially similar to the A unit [26] derived from the exponential 

 reference atmosphere. However, no quantitative results are yet avail- 

 able with this parameter and, furthermore, the method of deriving K 

 assumes a dry adiabatic lapse rate and a constant humidity mixing ratio. 

 The presence of a condensation level in the actual atmosphere is therefore 

 neglected. In addition, accurate values of A'' can only be derived directly 

 from the K profile in certain restricted conditions. Pending further 

 studies in this direction therefore, K and dK/dh remain more suitable for 

 qualitative synoptic studies than for quantitative predictions of field 

 strength variations. However, it is of interest to note that a close con- 

 nection exists between K and the composite parameter M discussed above. 

 Misme [27] has shown that the change of K in a given height interval, 

 AK/Ah, is of the form: 



AK/Ah = ki AN/ Ah - k2AW/Ah (6.3) 



where ki and ko are constants, and AW is a measure of thermal stability 

 as defined earlier. 



6.1.6. Vertical Motion of the Atmosphere 



The influence of stability has also been discussed by Moler and Arvola 

 [7], and Moler and Holden [8]. These authors assume that the average 

 lapse rate, dn/dh and the magnitude of local irregularities on the profile, 

 are primarily determined by changes in vertical velocity. Local centers 

 of convergence (low pressure cells) produce updrafts which result in con- 

 siderable mixing and the dissipation of any stable layer structure. Hori- 

 zontal divergence from local high-pressure centers create temperature 

 inversions and associated layer-type discontinuities in the n-profile. 

 These latter features are most pronounced in conditions of anticyelonic 

 subsidence. 



The direction and relative magnitude of the vertical component of 

 wind velocity can be estimated by techniques outlined by ]\Ioler and 

 Holden, and a correlation between hourly median field strength and calcu- 

 lated values of vertical velocity has been noted by these authors. These 

 fundamental studies represent an important attempt to explain signal 

 variations in terms of atmospheric motion, and a survey of available ex- 

 perimental evidence supports the basic assumptions in this approach. It 



