870 
type of perturbation into equatorial meteorology. The 
easterly current near the equator was described by him 
as being on occasion the seat of a disturbance in the 
speed field in the lower atmosphere. The streamlines are 
not disturbed from their easterly direction, but maxima 
and minima of speed travel downstream in the current. 
Freeman places great emphasis on a “Jump”’ in speed 
corresponding to the passage of the trough in the normal 
type of wave. However, it is probable that this is merely 
an extreme form of a longitudinal wave common in the 
New Guinea area and known to forecasters there since 
the elaboration of the pilot-balloon network mm 1942 and 
1943. For convenience in this discussion, I shall call all 
easterly waves that are detectable in the speed field but 
not in the streamline field, Freeman waves, without 
necessarily agreeing with the dynamical explanations 
advanced by Freeman. As far as I know this type of wave 
is found only in the New Guinea area, and there is 
reason to suspect that the peculiar orography and geo- 
graphical position of that island play an important part 
in its genesis. However that may be, we may fairly add 
a new type of disturbance, mdependent of fronts or 
convergence zones, to those already known to affect 
the homogeneous air streams of low latitudes. 
It was one of the great advantages of the air-mass 
theory that it seemed to give a complete or almost com- 
plete explanation of a multitude of different weather 
phenomena in both high and low latitudes. For low 
latitudes, the climatological school also appeared to 
have a complete system of explanation. This is not the 
case, at least explicitly, with the newer concepts ad- 
vanced by the perturbation school. The ideas are new, 
the whole movement being less than ten years old. As 
a consequence there has been a tendency to emphasize 
methods of analysis, especially synoptic analysis, rather 
than to attempt complete theories. Further, the workers 
on perturbations of homogeneous streams have rightly 
adopted explanations from earlier meteorologists, where 
these have seemed to them to fit the facts, even though 
at the time it seemed difficult to reconcile those ex- 
planations with the newer outlook. We have already 
mentioned the fact that the equatorial front was ac- 
cepted by the perturbation school; it was, of course, 
regarded as a zone of intense horizontal velocity con- 
vergence in the lower atmosphere, and its role as an 
atmospheric discontinuity was minimized. Nevertheless 
the newer analyses in the vicinity of the equator were 
almost indistinguishable in practice from those of the 
more enlightened members of the air-mass school, such 
as Deppermann. Similarly, in their treatment of the 
monsoons of Asia, Africa, and Australia, the group 
adopted almost without question the explanations of 
the climatological school. Thus, although no unified 
theory has yet been presented, we may reconstruct 
such a theory from these adaptations, from hints in the 
literature, and from the evident dependence of the group 
upon the dynamical conceptions of Rossby and his co- 
workers in Chicago. 
In this reconstruction, the trade winds, considered 
as broad streams of air largely homogeneous in the 
lower horizontal planes, play a major role. Less empha- 
TROPICAL METEOROLOGY 
sis than formerly is placed on the meridional compo- 
nents of these winds. They are, indeed, regarded as part 
of the circumpolar vortex, which happens to be retarded 
with respect to the earth’s surface in low latitudes but 
which moves faster than the earth in higher latitudes 
and in the upper air over the tropical zone. The base 
currents of the general circulation, on this view, are 
primarily zonal. Meridional circulations arise as per- 
turbations on these currents; the only synoptic problem 
in tropical regions, with the sole exception of the study 
of the equatorial front, is to describe and explain the 
perturbation of the zonal currents in the tropics. After 
description, the perturbations are classified as we have 
seen; there are (1) the major “dynamic” perturbations 
of the zonal motion, the subtropical anticyclones and 
the semipermanent lows of high latitudes; (2) the great 
seasonal perturbations of the trades, the monsoons, due 
to thermal causes, as understood by the climatological 
school; (3) the perturbations of the trades themselves— 
in winter the polar troughs, im summer the easterly 
waves and Freeman waves; and (4) the minor diurnal 
and orographic perturbations, for which an explanation 
has already been advanced. Hurricanes and tropical 
storms arise in two ways: (1) as a result of dynamic 
instability in the trade current, leading to the growth 
in amplitude of the easterly wave and its transformation 
into a vortex, and (2) as a result of some unknown proc- 
ess affecting the equatorial front (or, if one wishes to 
be free of all frontal taint, the intertropical convergence 
zone). The causes of the instability are unknown, 
though Riehl has advanced two accounts of the suf- 
ficient conditions [54, 55]; of conditions both necessary 
and sufficient there has been so far no hint in the litera- 
ture. Whatever may be the difficulties of explaming the 
origin of hurricanes, typhoons, and other tropical 
storms, explanation of the major features of the per- 
turbations (again, with the exception of the equatorial. 
front) is achieved by the methods of dynamic meteor- 
ology as set forth by Rossby. The central notion of 
these methods is that the vertical component of vor- 
ticity is conserved during the isentropic motion of the 
air particles in the trade-wind zones. However, in the 
case of the easterly wave, the horizontal velocity diver- 
gence of the lower parts of the trade must be taken into 
account, and this makes the so-called vorticity equation 
so intractable mathematically that no quantitative 
evaluation of the wave characteristics can be carried 
out—the explanation must therefore follow the original 
qualitative discussion of Bjerknes [7], z.e., in terms of 
a latitude and a curvature effect. Greater precision was 
given to this latter explanation by Bjerknes and Holm- 
boe [8], but unfortunately the theory as handled by 
them leads to the result that all easterly waves of the 
type described by Riehl must deepen and give rise to 
vortices.® Freeman, moreover, has advanced an entirely 
5. Professor J. Bjerknes has directed my attention to the 
possibility of effecting a reconciliation between the theoretical 
results and the Riehl model by taking into account convergence 
due to friction in the lowest levels of the divergent part of the 
wave. He thinks that this might bring about a balance in the 
integrated mass divergence over the region and thus stabilize 
the wave. 
