1918-19.] The Origin of Anticyclones and Depressions. 75 
tendency of these denser masses to press to the right must oppose to a 
slight degree the general movement of the stream and so promote 
gustiness. 
Glacial breezes, boras, and other katabatic winds are probably to be 
classed with the winds in the region of cooling of a cyclonic system, and, 
except in so far as disturbed by ground contours, should form notable 
exceptions to Buys Ballot’s law. Of course, in the extreme case of the 
disturbance of convectional equilibrium over a steep slope, gravitation 
plays an important part. But even on fairly level surfaces, or where the 
cooling effect does not create a “ hyper-adiabatic ” lapse rate, similar but 
lighter breezes will also openly defy the law of movement given by Buys 
Ballot. Lake-breezes and sea-breezes are probably of this type. 
It has been shown that the variations of entropy at any level in the 
air create pressure differences at other levels, which will affect the winds 
at those levels notwithstanding the comparatively slight amount of actual 
vertical movement. If we restrict our attention to one horizontal plane, 
and consider the effect on existing air currents at that level of the creation 
of a new area of depression or high pressure, it is at once obvious that 
the portion of an existing current which is approaching the new depression 
or receding from the new high pressure will be accelerated, while the 
portion which is receding from the depression or approaching the area of 
high pressure is retarded. If the new pressure distribution is maintained 
it has no resultant effect on the speed of the previously existing currents. 
For the work done by the pressure forces on a small mass M of the air 
as it moves from an isobar where the pressure is jp 1 to one where it is p 2 is 
P2 
fp2 
JPl 
(vdp + pdv), 
( vdp ±pdv) =gain of internal energy -l-gain of kinetic energy. 
If we neglect friction, conduction of heat, and radiation, the conditions 
are adiabatic, therefore V (and also the internal energy) is a function of 
p only. Hence in crossing the area enclosed by a given isobar the gain 
in kinetic energy is 
5 I f(p)dp - gain of internal energy = 0-0. 
Jp ‘ 
Hence the air always crosses the same isobar with the same speed, and 
no permanent acceleration is produced by the pressure forces. Friction 
and the conduction of heat tend always to reduce the kinetic energy, and 
so in the absence of radiation effects the air will cross any given isobar 
with diminished speed on each successive occasion of reaching it. 
