864 
only the wind structure of those entities but also the 
associated temperature and humidity distributions and 
the typical cloud and precipitation regimes. For kine- 
matic and thermodynamic complexes of this kind were 
precisely the new elements the Bergen group had intro- 
duced into the synoptic meteorology of high latitudes 
under the name, ‘‘air mass.”’ Both the trade and mon- 
soon of the climatological school of tropical meteorolo- 
gists fitted the prescription of an air mass perfectly. The 
trade, for example, had a specific source region: the 
subtropical anticyclone. Its path from the source was 
well determined and the character of the surface over 
which it had to pass, the great tropical ocean, had 
already been investigated and its thermodynamic effect 
evaluated. These factors together gave to the trade its 
uniform, persistent features; the relatively steep lapse 
rate in the lower layers could clearly be attributed to 
the fact that it moved across the sea-surface isotherms 
toward the heat equator; it was clearly a cold mass of 
maritime subtropical origin (‘‘tropical”’ in the new vo- 
cabulary) and the presence and variation of the con- 
vective cloud in it confirmed, through indirect aerology, 
the thermodynamic classification. The trade at its 
source was a stagnant slowly subsiding mass, almost in 
equilibrium at all times with the sea surface; here there — 
were no clouds or perhaps small amounts of low strati- 
form cloud. As it moved toward the equator, however, 
the mass became heated by the warmer waters in lower 
latitudes so that a relatively steep lapse rate was charac- 
teristic of the lower layers, while the air above continued 
to be warmed by subsidence in the antitrades, thus 
stabilizing the trade-wind inversion. The closer the 
mass approached the equator, the more intense became 
the heating from below and the less the subsidence aloft; 
the result was that a deeper lower layer was overturned 
by convection, the trade-wind inversion was found at a 
higher level, and became weaker as it approached the 
doldrum region corresponding to the heat equator. The 
clouds used in indirect aerology confirmed this history, 
for as the mass moved toward the equator the cumulus 
reached to higher and higher levels and showers asso- 
ciated with cumulonimbus became more frequent. This 
picture not only fitted the older climatological view of 
the trade remarkably well, it also removed some of the 
difficulties of the older theory by emphasizing the role 
of the subtropical highs as source regions. The most 
typical trade would be found at the eastern ends of the 
anticyclones, because there the trade had the largest 
component toward the equator, and moreover the re- 
construction of the vertical motions around a sub- 
tropical cell, suggested by J. Bjerknes, required that the 
predominantly downward motion be confined to the 
eastern ends. Toward the west the circulation in the cell 
required an increasingly great upward component in the 
middle atmosphere—it was in those regions, empirical 
investigations showed, that the trade wind was more 
variable, the trade-wind inversion frequently weak or 
absent, and the weather worse than the older theorists 
could allow. 
The monsoon, wherever it occurred, in Africa, South- 
eastern Asia, or Australia, was also an air mass. It 
TROPICAL METEOROLOGY 
was, in fact, a trade that had passed from one hemisphere 
to the other, and thus, under the influence of a Coriolis 
force of opposite sign from that in its place of origin, 
acquired a westerly, instead of an easterly, component. 
This mass was accelerated under the influence of a 
solenoidal field that could be attributed ultimately to 
the different absorptive properties of land and sea, as 
required by the older theory. It was considered to be 
unstable to great heights and to have, in the upper 
layer, a higher specific humidity than the trade because 
it had been subjected to the destabilizing influences of 
the equatorial passage for a long time; further, it was 
moving on to a content during the warmest season, 
and hence might be expected to show cold-mass charac- 
teristics to a high degree. 
What, then, could be said about fronts in the tropics? 
On the new view, the air masses were tremendous 
volumes of air, moving away from their source regions 
and preserving the marks of their origin in their homo- 
geneous properties and of their history in the modifica- 
tions of their lower layers; ultimately they came into 
juxtaposition with other air masses along more or less 
sharp boundaries, the fronts, where the meteorological 
elements would necessarily show rapid space variations, 
particularly in the horizontal planes. Were there such 
boundaries to the trades? This question had already 
been answered for the Pacific. In 1921 Brooks and 
Braby [11] had described in the western South Pacific 
what they called a zone of convergence between the 
trades of the two hemispheres. The title of their paper, 
“The Clash of the Trades in the Pacific,” is suggestive. 
In the central Pacific the mean trades from the two 
hemispheres meet at a small angle along a line that 
corresponds very well with the mean position of the 
equatorial low-pressure trough. Farther west, however, 
the mean trades meet at a larger angle and, m the 
extreme west, near the Solomons and New Hebrides 
during the wet season, the boundary is well marked, 
separating westerly “monsoons” to the north from 
easterly trades to the south. Rainfall follows the in- 
tensity of the convergence? as judged from the angle of 
impact of the currents. In the east, the ram zone is 
narrow and corresponds in position with the boundary; 
in the west, the rain zone, while roughly corresponding 
with the boundary, is broader and, in the mean, the 
rainfall is heavier. Finally, the tropical cyclones of the 
South Pacific seem to originate near or at the boundary, 
to move along it for a time, but m most cases to curve 
away from it into higher latitudes. Brooks and Braby 
had done their work and prepared the paper, it seems, 
before they had heard of the new polar-front theory; 
before publication, however, they added a footnote, 
drawing attention to the similarities of the boundary 
between the trades and the polar front of higher 
latitudes of which they had just heard a description by 
V. Bjerknes at a seminar in England. They then pro- 
2. In the paper by Brooks and Braby [11], it is clear that 
“eonvergence’”? means convergence of the streamlines, not 
horizontal velocity convergence. 
