EQUATORIAL METEOROLOGY 
By A. GRIMES! 
Farnborough, Hants, England 
INTRODUCTION 
Equatorial meteorology may be regarded as the mete- 
orology of the region about the equator where the 
Coriolis term in the equations of motion is no longer 
the only one which is of the same order of magnitude 
as the pressure term. Indeed, on the equator itself the 
Coriolis force is zero and the other terms must balance 
the pressure gradients which undoubtedly exist. One 
cannot quote any investigation which will indicate the 
limits of the region thus defined, but the general im- 
pression given by most writers is that the Coriolis force 
ceases to predominate between the latitudes of 15°N 
and 15°S, and it is to this region, roughly one-quarter 
of the surface of the earth, that the following discussion 
will be confined. The region, extending over the central 
portion of the Atlantic, Pacific, and Indian Oceans, is 
largely water broken by the land masses of central 
Africa, Central America and the northern part of South 
America, the East Indies, southern India, and Ceylon. 
Generally speaking, the land areas are sparsely popu- 
lated and meteorological history is comparatively short, 
extending back only so far as the beginning of inter- 
national civil aviation for most places in the region. 
This is not to say that no extended series of meteoro- 
logical records exists in the region, but the number of 
stations with reliable records over a period greater than 
ten to twenty years is pitifully small for the large area 
that has to be considered. The situation is steadily 
improving, because of the pressing needs of interna- 
tional aviation, and the meteorological services in the 
more backward—meteorologically speaking—of the 
countries in the region are being more and more ex- 
tended as time goes on. Nevertheless, the advancement 
lags considerably behind the development in the tem- 
perate zone. This is principally due to the high cost of 
meteorological equipment, such as the radiosonde, 
which puts modern methods of observation beyond the 
resources of the local governments. The number of sci- 
entific workers in the region is very small, and since 
even this small number exist almost entirely for the 
purpose of issuing forecasts to aircraft, it is not sur- 
prising that little meteorological information of any 
importance has emerged, and it is fairly certain that 
little will emerge until more equipment and more sci- 
entists become available to the region. The standard 
of observing, except at a few stations, is low in many 
cases because of the poorness of the equipment, and in 
many more cases because of the shortage of competent 
observers. Steps are being taken to improve the obser- 
vations, but it must be many years before adequate 
and accurate climatological statistics will be available 
for most of the region. 
1. Formerly Director, British West African Meteorological 
Services. 
The greatest impetus to the study of meteorology 
was given during World War II when the need for 
developing lines of communication through the region 
brought an influx of men and equipment far greater 
than in prewar days. During this period it was realised 
that our knowledge of the area was meagre, and a large 
number of reports, both statistical and otherwise, were 
published to remedy this defect. Not all the reportg 
were of great value but at least attempts were made to 
explain the phenomena of the equatorial zone on a 
scientific basis: if little in the way of coherent mete- 
orological theory emerged, there were at least some 
ideas suggested that might serve as a starting point | 
for further investigation. 
THE AIR MASSES 
Except for the occasional intrusion of polar air. into 
the South China Sea area, it seems certain that the 
only air masses affecting the region are tropical and 
equatorial. The tropical air masses are those of the 
trade winds giving on an average a belt of northeast 
winds in the Northern Hemisphere and of southeast 
winds in the Southern Hemisphere, both derived from 
the subtropical anticyclones. During the summer of the 
Northern Hemisphere the trade wind of the Southern 
Hemisphere crosses the equator everywhere, so far as 
can be determined, and becomes what is usually known 
as the monsoon. In the Northern Hemisphere winter 
the reverse process takes place everywhere except in 
the Atlantic and West African areas, and it is these 
annual movements of air which provide the variations 
in the weather of the equatorial zone. When the tropical 
air from either hemisphere has passed into the equa- 
torial zone and becomes relatively stagnant it is known 
as equatorial air and is therefore not an “‘air mass” as 
one usually thinks of the term. Such stagnant air, 
which frequently develops a light anticyclonic circu- 
lation over land masses, is invariably moist (or becomes 
moist) in the lower layers. This air is a common feature 
of the synoptic situation and is therefore of great im- 
portance. 
The properties of tropical air are quite well known 
up to 4 or 5 km, but observations are too few above 5 
km for reliable conclusions to be drawn. It seems that 
above this level the winds are mainly easterly. Some 
investigators consider that these winds play a large 
part in the variation of weather along the equator, but 
what part they play has not yet been demonstrated 
with any assurance. It is not in fact clear from the 
study of daily charts where the trade winds end and 
the overriding easterlies begin, because in most cases 
the maximum speed of the trade is reached between 
500 and 1500 m above the surface, and there is only a 
gradual variation of speed in the higher levels. When 
the tropical air has a continental source it is usual to 
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