Not my understanding of the subject at all. …
Originally posted by Encyclopedia Britannica:
Relationships to surface features
Upper-air circulations play a key role in the development and propagation of surface pressure systems. The surface pressure is simply the weight per unit area of the air above the surface.
Thus, in order to change the surface pressure, it is necessary to change the mass content of the overriding air. Because large amounts of mass are transferred by upper-air flows, such flows are capable of forcing significant changes in surface pressure.
The transfer of mass by upper-air circulations occurs in a very nonuniform fashion. As a result, mass tends to build up (mass convergence) in some regions and be depleted (mass divergence) in others.
If these convergences or divergences occurred uniformly at all levels above a given surface location, it would be easy to anticipate the resulting surface-pressure changes. Such uniformity, however, is not observed. Rather, divergences in one portion of a column of air are largely compensated by convergences in another. Thus, changes in surface pressure may result from the subtle differences between these conflicting mass divergence/convergence patterns.
(Picture to be added later)
Figure 7: Vertical cross sections through a wave system depicting typical divergence/convergence…. Also included are the directions of the vertical air motions required to satisfy mass conservation principles.
Such vertical motions are typically very small compared to horizontal motions yet are important because they are largely responsible for the formation or dissipation of clouds. Clouds form in regions of upward motion if sufficient moisture is present and dissipate in those of downward motion.
In tilting wave systems, this accounts for the occurrence of inclement weather with low pressure and fair weather with high pressure.
In a non-tilting system, which is characteristic of an occluded cyclone (see above), the bulk of the upper-air divergence and convergence occurs between surface low (L) and high (H) centres, with little happening immediately above the centres.
This signifies a propagating wave–a very slow one in some cases–with little intensification of the surface centre. In fact, decay (filling) of the surface centre might occur because of frictionally induced mass convergence in the low and divergence from the high, which is not compensated for by corresponding upper-air mass divergence and convergence.
In the tilting system, much of the upper-air divergence and convergence lies above the surface centres. In this case, opportunities are much better for decreasing or increasing the mass above the cyclone or anticyclone, respectively, and thus for intensifying the surface centre. Such a wave also will propagate in the general direction of the mean tropospheric flow.