868 
theory in the tropics could be extended almost indefi- 
nitely. To anyone, like the writer, who has had to review 
the literature on tropical fronts published in the last ten 
years, the total effect is one of intolerable confusion. 
And this confusion exists at the present time, un- 
resolved. Where possible, I have purposely chosen the 
latest quotations for inclusion in the above list, which 
is, of course, very incomplete. Thus Garbell’s textbook 
was published in 1947; Forsdyke’s summary of tropical 
meteorology in 1949. Both these works will give the 
reader some idea of the complexities introduced by 
members of the air-mass school in an endeavor to re- 
move the difficulties associated with the attempt to 
forecast tropical weather by high-latitude techniques. 
Forsdyke, indeed, frankly admits in places the impos- 
sibility of using standard methods. On page 37 of his 
paper he says, 
The pressure distribution is of little or no assistance in 
forecasting the movement of the intertropical convergence 
zone. The streamline chart is the best aid, the frontal zone 
usually being located at the line of separation between winds 
having northerly and southerly components respectively. 
Over wide areas, however, the surface winds are often the 
only winds available; hence it is usually the surface front 
which is so placed, whereas observation shows that the worst 
weather often occurs up to 200 miles away from the surface 
front. It may be assumed that the front will move toward 
the side on which the winds are weaker, but it is difficult 
to estimate the speed of motion from the observed winds. 
Often the front appears to jump. 
It is easy to be wise after the event, but one may 
legitimately speculate on what the history of tropical 
meteorology would have been, during the last fifteen 
years, if the distinction between the observed wind- 
shift line accompanied by cumulonimbus, the old “line 
squall,” and the theoretical front of the Bergen group 
had been kept in mind. Would not the early workers, 
realizing that the empirical front of the tropics was not 
necessarily a discontinuity in the density field, have 
also realized that frontal dynamics would not be ap- 
plicable to it, that new synoptic entities were bemg 
investigated and that therefore new explanations were 
required? That is the position in which we now find 
ourselves. At a time when the tropical observing net- 
works are beginning to deteriorate, when we have fewer 
and less accurate observations than at any time since 
1939, we are realizing that tropical meteorology, in 
investigating these entities, has a contribution to make 
to general meteorology. Nowhere in the atmosphere 
are we better able to study the direct effect of horizontal 
velocity divergence in the lower atmosphere, free from 
other effects, than in the tropics. The tropical “fronts,” 
which have been dismissed as particular, well- 
understood examples of a common atmospheric dis- 
continuity, are now seen to be completely without 
dynamical explanation. When the explanation is forth- 
coming, it cannot but react on the theory of high-lati- 
tude fronts. 
TROPICAL METEOROLOGY 
THE PERTURBATION METHOD 
One of the most serious objections to either the 
doldrum or the frontal explanation of the origin of 
tropical storms is that a proportion of the hurricanes 
that affect the West Indies and the southern United 
States originate within a comparatively strong east 
current of great depth prevailing during the wet season 
over the Caribbean. The storms deepen rapidly and are 
often of great violence. They are usually of smaller 
diameter than the hurricanes that move in from the 
east and which clearly originate in the doldrum region 
of the eastern Atlantic. In 1940, Gordon E. Dunn 
published a paper dealing with the origin of these 
storms [25] and the ideas expressed in it have led to the 
development of a new outlook on tropical problems. 
Dunn, using 24-hr isallobars to eliminate the great 
semidiurnal oscillations of pressure, showed that: 
1. The deep easterly belt of the Caribbean in the 
wet season is subject to pressure waves of small ampli- 
tude, which move from east to west. 
2. Ahead of the wave the trade-wind inversion is 
relatively low, and the weather is fine, with small clouds 
and little precipitation. 
3. Behind the trough the trade inversion is high or 
absent and the region is the seat of many shower-clouds. 
He speaks of this as the ‘rolling away”’ of the inversion. 
4. Some of the waves, and a proportion that increases 
as the season advances, grow in amplitude and give rise 
to small but violent hurricanes. 
5. The isallobaric highs and lows associated with the 
waves, whether hurricanes are formed or not, follow 
tracks that correspond with the mean hurricane tracks 
of the appropriate month in the season. 
Subsequent writers elaborated this picture of the 
easterly wave. The most active worker was Riehl [53] 
who investigated the vertical structure of the wave 
more closely and showed that it was easier to detect in 
the wind field than in the pressure field and aloft than 
near the surface. Later the easterly wave analysis was 
applied to the North Pacific and the North Atlantic. 
More recently easterly waves have been identified in 
the Indian Ocean [29] and in West Africa [63]. 
In opposition to earlier writers on the Caribbean, 
such as Kidson [37] and Scofield [64], Dunn insisted 
that no temperature discontinuity could be found either 
in the easterly wave or in the developing hurricane. 
Later research has confirmed his contention and there 
seems now to be no dispute on this point as far as the 
West Indian hurricanes are concerned. Radar investiga- 
tion of the storms as they pass over the southern sea- 
board of the United States, however, seems to show that 
there are well-marked cloud and precipitation discon- 
tinuities in the mature storm [73]. As this topic will be 
discussed in the next section, I wish only to point out 
here that Dunn was explicit only on the absence of 
temperature discontinuities. He admits, however, that 
the southwest wind at Panama in the wet season may 
be colder in the lower layers than the easterly on the 
other side of the equatorial convergence zone. One gets 
the impression that he is willing to admit a frontal origin 
