Equatorial Westerlies over East Africa and their
Climatological Sigunificance
Kazuo NAKAMURA
ABSTRACT
This is an attempt to describe the nature of the Equatorial Westerlies over
East Africa and to discuss their origj皿s and their climatic significance． The
January−March Westerlies are distinguished from the May−October Westerlies by
their horiz6ntal and vertical distribution． The former is associated with a depres−
sion aloft over the African colltinent at abdut 150S， and the latter with passages of
deep troughs。 Although the Equatorial Westerlies generally tend to，produce rain，
there are regional variations in the relation between the Westerlies and weather．
Frequent incursions of the Westerlies result in heavy thunderstorms in the Kenya
Highlands， but the lee effect is evident in the region east of the Highlands．
INTRODUCTION
Itエs an undeniable fact that Westerly winds do exist as prevailing winds at
least in a certain season over a part of the Equatorial region・ But the theoretical
studies oロthe Equatorial Westerlies have so far been scanty． It is， the士efore，
necessary to clarify the nature of the Westerlies−一一the time of appearance， fre−
quency， their、spatial distribution etc．一一一〇ver specific regions based on observed
facts．
In East Africa， it has been noted empirically that the Westerlies may have not
alittle・influence on the weather of this region， where the climate is considered to be
controlled basically by the two easterly wind systems， the northeast and the south−
east monsooIls． Referring to the rai㎡all in Uganda， Henderson（1949）mentioned
that the frequent occurrences of heavy thunderstormS in August were related to the
incursions of Westerlies from the Congo region． According to Henderson， the
Westerlies may appear at any season of a year． h享alater year， Thompson stated
that the Westerlies over East Africa”provide the forecaster’with his happiest and
most confident predictions”（1957a）． But， contrary tb Henderson， Thompson ex−
pressed an opinion that the westerlies apPeared mainly in January and February
across southern Tanzania， and that this is the reason that the monthly precipitation
at Nairobi for January−February exceeds that of July。August・
The Westerlies in the Equatorial region in general are believed to be associ−
ated with rain． It will be mehtioned in a later section that this has also been ex−
Plained theoretically． However， Jo㎞son and M6rth（1960）made a statistical investi−
gation of the relation between the mean wind direction at the 750−550mb level and
the raininess in the vicinity of Nairobi． They concluded that there are many occa−
sions of rainless Westerlies in June−october・
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Flohn has written a number of papers concerning the spatial distribution of the
Equatorial westerlies mainly in January−February and in July’・August（Flo㎞・1949・
1951，etc．）． As for the African continent， he has drawn resultant streamlines at
different levels in the both extreme seasons． These figures reveal that East Africa
experiences the Westerlies， but・of different origips， in the both seasons（Flo㎞，
1960b》．
Thus， diverse remarks have been expressed even about the season of frequent
appearances of the Westerlies 6ver East Africa． Much less has been known about
their origins， their thermal or thermo−dynamic properties， and their relation to
weather． The purpose of this paper is to describe the nature of the Westerlies and
to discuss their origins and the relation between the Westerlies and weather．
DATA AND METHOD
Asparse network of meteorological stations， particularly of aerological
ろ0°
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stations， in the tropics is the principal reason that studies in tropical meteorology
is far behind that for the higher latitudes． As a matter of fact， in East Africa the
routine observations of upper air are conducted only at Entebbe， Nairobi and Dar es
Salaam．
However， many stations have carried out upper wind observations by pilot ba1−
loons． Such data have been published for the period 1927−43， for 25 stations in and
around East Africa． This enabled the author a basis for climatological analysis of
upPer winds over this region・
In contrast to the scanty upper air observations， of which the importance and
．necessity have been emphasized， East Africa has a dense network of rain gauges．
’℃1imate”and”weather”are often used as synonyms in the tropics． It does
not mean， however， that the daily weather can be neglected in the study of tropical
climatology。 The author not only treated monthly means but also intended to break
t尊em up into daily weather when necessary． Fo茨this purpose， so士ne unpublished
daily data for Nairobi were used． As for the daily weather maps for this region，
the recently published weather maps for the tropics as a whole during the IGY period
are readily available．
Because of the limitation of availatble data， this paper deals with problems of
the Westerlies over East Africa through a geographica1， rather than a meteorological
approach． Attention has always been paid to the spatial distribution of a certain phe−
nomena occurring simultaneously， to the relationship between phenomena which occur
simultaneously but in different forms in different areas， and also to the association
between those phenomena and land forms．
Many climatic phenomena in the tropics， including the Equatorial Westerlies，
have likely been explained by the ITC theory which seems to have firmly been estab−
lished． Bt｝t， since the・existence of the ITC itself is questioned ln East Africa ．
（Thompson，1957a）， the author does not intend to apply the simple ITC theory． It is
believed by the author that it is the very standpoint of climat．ology as a part of geo−
graphy to examine the peculiarity of the climatic phenomena in East Africa．
THE SEASONAL VARIATION OF THE WESTERLIES
Based on the above mentioned pilot balloon data， the percentage frequencies of
Westerlies（wind directions NW， W and SW combined）were calculated for each month．
The length of the period of data differ from station to station， and even at a station
the niumber of observation naturally decrease upward． This study deals with those
cases in which there are at least fifty observations．
Fig．2illustrates the annual march of the frequency occurrences of Westerlies
at three inland stations located at different latitudes；Kabete／Nairobi（OIol6 ’S），
Tabora（050 02 ’S）and Mbeya（08056’S）． Although there are some noticeable regional
differences， the figures clearly show a marked seasonality common to all the three
stations． The frequencies decrease appreciably in April and again in November−
December over all of East Africa． Consequently， the two seasons of the Westerlies
are distinguished． One season lasts from January to March and the other from May
to October．
The Westerlies are well developed at the southernmost location． At Mbeya，
the frequency exceeding 50 percent at the height of both seasons． But remarkable
differ6nces can be recognized between the characteristics of the Westerlies of both
seasons． One of the differences lies in the altitude and thickness of the Westerlies．
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Percent・g・frequ・n・i…fWesterli・・at（・）K・b・t・（01°16「S），
（b）Tabora（05002’S）and（c）Mbeya（08056’S）．
Westerlies during January−March are frequent in the south， but
Westerlies during May−October have a large merldlonal extent．
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In other words， the Westerlies in the northern winter are observed at a111evels from
2000to 5000 meters above sea leve1． On the other hand， the Westerlies in the
southern summer have a sharp lower boundary between 200αand 3000 meters。 They
also tend to show a steady decrease above 5000 meters，． It is， therefore， a unique
feature in East Africa that the Westerlies during this period are entirely embedded
between the lower and upper Easterly systems．
Another great difference is their spatial distribution． The Westerlies in the
northern winter appear most frequently in the southern part of Tanzania， but they
steadily diminish toward the north． At the latitude of Nairobi， they are only rarely
observed during this season． The Westerlies in the northern summer on the contra−
ry are quite evident at least in July and August・It extends in a large meridional
extension from 100S across the Equator into the Northern Hemisphere．
Some of these results obtained from the pilot balloon data are ascertained by
the radiosonde observations．
THE DISTRIBUTION OF THE WESTERLIES
The three inland stations shown in Fig．2indicate that the Westerlies in the
northern winter decrease from southern Tanzania toward the north． The same tend−
ency is evidenced also along the Congo、 border as well as along the India皿Ocean
coast． It is， therefore， conjectured that the Westerlies at this time has a zonal
distribution along th6 southern血ternational boundary of Tanzania． ．
On the other hand， it is true that the Westerlies in the northern summer are
on the whole more prevalent throughout the period May−October in southern Tanzania，
but they are frequently observed even in the Northern Hemisphere in July−August・
Even pilot balloon stations are lacking in the Northem Hemisphere， so that the exact
’northern boundary of the Westerlies cannot be determined． It is unlikely that the
Westerlies extend as far north as Sudan， where the ITC is commonly thought to be
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Avertical cross section along the Equator show評ng the percentage
frequencies of Westerlies in August．
Note the infrequent−appearance of Westerlies in the west．
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located at this time． Certainly at Khartoum， there are prevailing Westerlies below
the 850mb level in these months． But considering that the northern summerドWest。
erlies oVer East Africa are characteristically concentrated at levels of 700−600mb，
the Westerlies at Khartoum are most probably of a different origin．
The west−east extension of the Westerlies is worth noting． As many of the
pilot balloon stations are located near the equator， a vertical cross section along the
Equator is drawn in Fig．3． The figure shows the percentage frequencies of the
Westerlies for August． It is interesting to note that the Westerlies， which are com−
monly designated as’℃ongo air”in East Africa， are very rarely observed in the
・west． Kabete， the westemmost location， reportS some westerly winds in August，
but they are almost entirely confined to the lowest layer， that is a marked contrast
to the Westerlies血question that have a distinct lower boundary between 2000 and
3000meters． The frequency of the Westerlies tends to increase from Lake Victoria
eastward， to 70 percent over the eastern coast． As evident in Fig．3， the higher
parts of the Kenya Highlands， the Aberdare Mountains and Mt． Kenya， are probably
Kilimanjaro too， are under the direct influence of the Westerlies． It is also noted
that the height of the greatest frequency differs on the western side from that on the
eastern side of the Aberdare Mountains．
The same tendency of the distribution of the Westerlies are also recognized
over central and southern Tanzaniaゲ．Accordingly， it can be stated that the northern
summer Westerlies are observed over the whole area of East Africa east of the 33rd
meridian， and also that they have a much larger meridional extent than the northern
・winter Westerlies．
SOME CHARACTERISTICS OF THE WESTERLIES
Thermo−dynamic characteristics
It has been suggested that the Westerlies are cool and moist（Henderson，1949；
Thompson，1957a）． The greater moisture content of the Westerlies can be ascertained
even by the monthly means of relative humidity and mixing ratio at Nai士obi・i1 July ・
and August， the levels of 700，600 and 500mb are much more moist than in the other
months．
Based on the daily data for Nairobi in 1964， the author examined if there are
thermo−dynamic characteristics pertinent to the Equatorial Westerlies． There were
Temperature， relative humidity and miXing ratio
Table 1
by wind directions at 700−600mb in Nairobi
T
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July
Easterlies
Westerlies
Press．
X
T
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X
一42．60C
T
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一43．20C
300
400
500
600
700
800
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22％
0．2gr／kg
一32．8
20％
0．2gr／kg
一34．8
20％
一17．2
36
0．8
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24
0．6
一17．2
一7．4
59 。
2．6
一7．2
37
1．6
。7．2
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5．1
0．4
0．5
7．9
6．7
6．6
6．2
14．4
17．8
80
10．4
16．5
8．7
14．2
76
74
11．0
19．7
60
75
58
56
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7．1
83
87
22
38
64
88
9．6
17．8
71
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33days during the period from June to September on whidl Westerlies were reported
at both the 700 and 600mb levels， whereas there were only 13 days of Easterly winds
at these levels． The mean temperature， relative humidity and mixing ratio for the
both cases are given血Table 1・For comparison， the monthly means for July，1964，
are also included ln the table． Except for the lowest layer， there is little difference
in temperature． No significant difference can be found between the Westerlies and
the Easterlies． But the moisture content is apparelltly much greater in the Westerlies
than in the Easterlies． Thus， the Westerlies are decidedly more moist than the
Easterlies but the lower temperatures are by no means the characteristics which
dist血guish the Westerlies from the Easterlies．
250
3◎◎
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600
700
800
Fig．4
Skew T−log P Diagram for July 11，1964
thick solid line： ascent curve
thick broken line： characteristic curve
An example of an ascent curve is illustrated in．Fig．4． The absolutely unstable
SE monsoon is overlain by the Westerlies at 600mb． The Westerlies are relatively
warm， resulting in a nearly isothermal layer between the Westerlies and the SE
monsoon． The Westerlies are in turn overlain by the very dry Easterly currents
aloft． These characteristics are very often， if not in all cases， descernible． The
dry subsiding upper Easterlies have been discussed in relation to the dry climate of
East Africa（Taljaard，1955；Trewartha，1961）． But as far as the ascent curves
indicate， a stable layer between the Westerlies and the SE monsoon seems to be im−
portant． Clouds are formed in the SE monsoon， but their vertical development is
checked by the presence of this stable layer． This seems to be contradictory to the
commonly held concept that the Equatorial Westerlies are rain bringers． This will
be discロssed in a later section．
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Tlme section during the Westerlies season
Fig・5
A full barb indicates 10 knots Isollnes are 24 hr change of
lsobarlc surfaces
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Tlme sectlon showmg the elld of the Westerlles season．
There ls Ilo transitional season with weak and variable wlnds
The alternatlon from the SE to the NE monsoon precedes the
termlnation of the Westerlles
Duratlon
In 1964， the Westerlles began to appear over Nalrob1皿May． The flrst appear−
ance of the northern summer Westerlles ls nearly synchronous wlth the alternatlon
of the surface wlnds from the NE to the SE monsoon． But， dlfferent from the monsoons
whlch， once establlshed， are so steady that there ls little change ln the wlnd dlrectlon
for a long duratlon， the Westerlles are rarely observed for many days runnlng．
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Particularly， at Nairobi in May and June，the westerlies are only sporadically reported
and a duration is not longer than three days． The Westerlies appear very frequently
in July and August． Yet， it seems that the Wosterlies at 700−600mb do not last more
than a week before they give way to the Easterlies， though for a short while． This
fact may be important when the origin of the Westerlies is discussed．
The longest duration in 1964 is 18days from・July 22 until August 8・Not only
the duration is extraordinarily long， but many rainfall stations over a wide area of
East Africa recorded a remarkable increase in precipitation during this period． This
gives a good example to study the relation between the Westerlies and weather． Fig．5
shows the time section for this period．
The Westerlies at 700−600mb over Nairobi steadily decrease in September． ll
turn， different Westerlies begin to appear a声300−250mb． The relation between the
lower and the upper Westerlies is unknown． Th宇y seem to join to form a thick layer
of Westerlies in October． All of a sudden， the season of the Westerlies ceases血．
the middle of October． As shown in Fig．6， the alternation from Westerlies to East−
erlies is extremely discontinuous． About a week prior to this abrupt change in the
Φper circulation， the SE mgnspon is replaced by the， NE monsoon．
’ Wave・perturbations in the Westerlies
Aclose examination of Fig．5reveals that the wind direction of the Westerlies
・i・invariably・hanging． At・，th・7晦b leve1， they fluとtuat・between NW and SW fairly
regularly at peヰ・d・・f f・ur・r fiv・・ねy・・in’ acc・rdance with th・flu・tuati・n・at 700mb・
the Westerlies at 600 and 500mb also vary between NNW and W． This might be an
indication of probable existen6e of wave−1正ke perturbations within the Equatorial
Westerlies．
THE oRIGINs oF THE EQuAToR】［AL wEsTERLIEs
」
OVER EAST AFRICA
The origils of the Westerlies in the Equato士ial region in general are not yet
fully understood． There are diverse opillions・
1） It has usually been described that the Equatorial Westerlies are a part of
‘the trade winds， which， on entering the opposite hemisphere， are deflected because
of Coriolis force．
2） The Westerlies are the wind system between two Intertropical Convergence
Zones． hl respect to the formation of the two ITCZ’s， Fletcher（1945）is of the opin−
ion that a single ITCZ is split into two zones because・of a thermal ’cause， while Flo㎞
（1960a）stated that the thermal and dynamic effects of continents at both extreme
seasons result in the formation of the two ITGZ書s over the continents and their sur−
roundings． In any case， these statements are different from l）in that the Westerlies
may appear o皿both sides of the Equator at the same time．
3） There are eddies in the tropical circulation， and winds on one side of the
eddiξs are observed as the Equatorial Westerlies（Riehl，1948）．
As for the Westerlies over East Africa， they have been considered to be the
deflected NE or SE monsoons， or otherwise to enter from the Atlantic Ocean through
the Congo region・ 、 ・ ’
Considering the characteristics of the Westerlies over East Africa that have
been described in the previous sections， questions are raised concerning the concepts
−of 1）and 2）． For instance， if the northern summer Westerlies are the deflected SE
一51一
monsoon， the reason why the SE Trades in the lowest layer penetrate far int6 the ’
interior without’ b?奄獅〟@deflected and why deflection occu士S only at 700−600mb cannot
be explained， and Coriolis force fails to explain that the Westerlies can exist 6n the
both sides df the Equator at the same ti血e． Moreover， the Westerlies which are
concentrated at the 3000−4000 meter levels cannot be explained by the ITCZ that is a
phenomenon near the sufface．
The ’concept of the Congo origin must also be rectified as illustrated in Fig．3．
It is true that there are prevailing Westerlies at the surface over West Africa in the
northern summer， but there is no evidence that these are uplifted to become the
Westerlies at the 3000−4000 meter levels in East Africa．
oo
OO
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January l 958
赴
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January朋．19詣
Fig・7
Centers of highs and lows at 500mb in January，1964（toP）， and 500mb
．chart with Equatoria1．Westerlies over an extensive area（bottom，
reproduced from”World Weather Maps， Pt． II Tropical Zone）．
Aquasi−permanent low pressure belt Qver the continent along）50S
ls noteworthy． Westerlies dur血g this season are associated with this
low pressure belt．
一52一
Daily weather maps for this region are examined． The published weather maps
for the entire tropical zone contain both the surface and the 500mb maps from July，
1957until the end of 1958． They are by no means the most appropriate charts for the
purpose of studying the Westerlies at 700−600mb， but the 500mb charts are instruc−
tive． Since tropical African has a relatively dense network of observations at 500mb，
this part of the 500mb chart is thought to be credible．
Fig・7shows the centers of highs and lows at 500mb for the month of January，
1958．The most distinctive feature is， besides the Equatorial Low Pressure belt，
the concentration of lows over the continent at the latitude of about 150S． An example
of such a low pressure is given in the same figure． Westerly winds are widely ob−
served on the northern side of the low pressure． This type of pattern appears recur−
rently from January through March， diminishing rapidly in April． Consequently， the
prevailing Westerlies in January−March must be closely associated with the low
pressure・
r
A
《・”一
10E 20 o−−Mr
漂
30 40 50
July l5，1958
Fig・8
500mb chart showing Westerlies in the northern summer．
A trough gxtending from the higher latitudes is noted．
The 500mb charts display quite a different pattern when widespread Weste’rlies
appear in May−October． Fig．8is an example． In this case， a deep trough ex。
tending from an eastward movilg depression at the higher Iatitudes in the Southern
Hemisphere reaches the Equatorial region， accompanying the Westerlies far into
the low latitudes over the eastern portion of Africa． This suggests that some， if not
al1，0f the May−October Westerlies are related to the deep upper trough from the
winter hemisphere．
It has been known that an anchoring trough is formed along the eastern coast of
acontinent in the Westerlies of the higher latitudes． If this is also true for the
African continent， the higher frequency of the Westerlies over the eastern portion as
compared with the western po rt ion of the continent may be explained． The rhythmic
fluctuations of the Equatorial Westerlies， which the author was inclined to assume
as wave perturbations， might result from passages of troughs． This may also
account for the fact that the Westerlies are， unlike the underlying SE monsoon， not
persistent for long Periods． ・
一53一
o
THE EQuAToRIAL WEsTERLIES AND WEATHER
The Westerlies in the Equatorial region are generally considered to be impor−
tant rain b・ing…．Ad）mamic explanati・n ha・been giv・n f・r the ra血iness・f the
Equat。ria1 W・・terli・・（J・㎞・・n and M6茸h，1959；Fl・㎞・1960a）・J・㎞・・n and M6「th
（1960），h・wever， P・・ved that in Ea・t Af・i・a there are m・ny・cca・i・n・・f「ainless
Westerlies in the major dry，season． In fact， Mbeya， where the Westerlies appear
f。r a greater pa式・f a yea・， receive・practically n・rain during May曹Oct°be「・
」。hn・・n and M6・th・xplained thi・d・yness n・t by th・Westerli・・but by the SE m°n−
soon， which tends to be divergent over East Africa．
The author repbrted that there are great regional differences of weather in
E。。t Af。ica in July・nd Augu・t（1967）・The re1・ti・n b・tween th・w・・terli・・and
weath。。 i。， theref・re， n・t・uch that it differ・f・・m sea・・n t・・e琶・・n・but th・t it difr
fer、 f。。m regi・n t・regi・n． The relati・n m・・t be re−examined f・・m・uch a p°int°f
view．
Since there is a limitation of available data， a direct method to study the
relati。n i， n。t・1way・apPli・ab19． Theref・re，・・m・indi・ect m・th・d・are al・・used
as a supplement in the following discussion．
The Westerlies in January−March
N。t m。re than th。 f。11・wing・1imatic c・rre・p・nd・nce can b・kn・wn f・r the
P「ese 磨B di。t。ibu，i。n。f m。nthly predpitati。n rev・a1・aremarkable z・nal pattern
器畷儲Σ1・鶴盤ま瀦鼎盤潔1：繍ll臨識「n
tern・
ｯ盤1盤詳e舗潔犠灘i翌離。ゆ。thinth・frequenry・f
W。、terlies and in th。 m・nthly precipit・ti・n at many place・in・・uthern Tanzanla・
The Weste士1ies in May。October
A，m。nti。n。d in th・previ・us secti・n， the ea・tern half・f E・・t Af「ica is m°「e
。r less influenced by th・W・・terli・・． Th・influ・nce i・f・1・ev・n in th・N・「them
H。mi。phere・ in July and Augu・・． D・・pi・・thi・，・greater par・・f E・・t Af「ica「eceives
1’tt’e
ﾚ謙鷲：翻Re「血。 highland， inw。。・K・nya and m・・t・f ugand・4avr
m。ximum precipit・ti・n血July and Augu・t・Th・numb…fday・with p「ecipitat1°n ls
。1。udiness i、 alm・・t a・great・・，・r eveh greater than・th・t・f the「ainy seas°n『・
Thi， di，erepan。y b・tween・he a皿ual march…fprecipi・ati・n and・f・1°udiness lr
nal drizzles．
M。，tδf Tanzani。，、。uthern Kenya and n・rthea・t K・nya are literally d・y du「ing
㍑撒t㍊・鑑器躍t°誌羅a2｛盤綿器i鯉諾シ
dry・
−54一
Thus， the regional differences of weather at the time of frequent appearances
of the Westerlies suggest that the relation betWeen the Westerlies and weather is not
asimple one． If the raininess of the Equatorial Westerlies is true， what then is an
explanation for the rainless Westerlies？
First of a11， the relation between the rai皿fal1エn and around Nairobi and the
upper wind directions is statistically examined． Tropical rains are generally of a
local character， so that daily precipitation at a place is not representative・ There−
fore， the author introduced the concept of rainfa11血dex， which is the ratio of
stations reporting daily precipitation over l mm to the total number of rainfall sta−
tions in an area determined by a one degree grid． Similar concepts were introduced
by Bryson and Lowry（1955）and Johnson（1962）． The daily rainfall index was obtained
for the area i㎞cluding Nairobi（this area will be designated as S1−36 by its latitude
and longitude）， and the adjacent area to the north（SO−36》． Table 2 shows the rela−
tion between the rainfall index and wind directions at 700−600mb at Nairobi． It is
clear that the same Westerlies are definitely associated with widespread rains in the
northern area but not in the south． The boundary may be drawn along the parallel of
10S． ，
Table 2
The relation between wind directions
at 700−600mb over Nairobi and rainfall
index for SO−36 and S1−36 during the
period J皿e−September in 1964
Region
hlldex
Wind dir．
SO−36
25＞
25く
S1−36
25＞ 25く
Total no．
of cases
Westerlies
22
53
54 21
75
dasterlies
Q3
X7
Q8 4
R2
tn㎞own
@8
P1 4
P5
Furthermore， the rainfall i皿dex seems to become greater when the Westerlies
are observed and its daily fluctuations seem to be closely related with the above
mentioned perturbations（Nakamura，1967）． These facts also indicate the close as−
sociation of the Westerlies with rains．
Next， whether the thunderstorms in the Kenya Highlands correspond with the
Westerlies cannot be ascertained， because the daily upper wind data in this area are
unavailable． But， there are some indirect evidences． The precipitation in this area
begins to increase at the onset of the Westerlies season， and as the season ends，
there is a sharp decrease． The rains in this area appear to be caused by thermal
convection occurring at random， but the rainfall index implies that they， too， are
controlled by some factor on a large’scale． The index fluctuates periodically at the
interval’ of about 5−7days as d6es the upper wind directions． The number of sta−
tions reporting daily precipitation of over 30 mm is now taken as an index of after−
noon sh6wers． Fig．9illustrates the daily variation of the number for the Eldoret
di・t・i・．t（NO−35）・Th・increaS・・f th・number・・ρ・ur・， with・・me excepti・n・， at
about the time of the appearances ’of the Westerlies over Nairobi． Its fluctuation is
nearly parallel to the fluctuatioh of rainfall index in the Nairobi district and its
viCinity．「
↑he precipitation−altitude relationship is also worth’noting，・Weischet（1965）
一55一
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Number of stations in No−35 reporting daily precipitation over 30mm
and occurrences of Westerlieβat 700−600mb over Nairobi．
describes that， in the tropics， the annual precipitation generally reaches the greatest
value at the altitude of l300−1500 meters above sea level． This holds true for the
monthly precipitation in Kenya，iexcept for July and August， when the precipitation i箪
the Kenya Highlands is nearly uniform irrespective of the altitude． This implies that
the causeof the precipitation of these．．months may lie at a higher altitude than in the
other months．
From all of these facts， it may be safe to state that thunderstorms in the Kenya
Highlands are related to the Westerlies，．
The third question is how the dryness of the eastern part of Kenya is explained．
Again the daily upper wind data are unavailable in this area． But it is conjectured
from Fig．3that this part is certainly under the control of the Westerlies at least in
July and August． If the westerlies do exist over the area， the general rule of the
raininess of the Equatorial Westerlies is inapplicable for this area． This may be
explained， as has been done in connection with the dry climate of East Africa， by the
difluence of the SE monsoon and the subsidence of the upper Easterlies（Trewdrtha，
1961）．But， considering that the driest part lies just east of the East African High−
1ands， some orographic influence upon the Westerlies on the lee side of the High−
1ands is likely．
Fig．10 shows the distribution of precipitation and of the number of rainy days
in the vicinity of the Aberdare Mountains and Mt．Kenya during the last decade of
July，1964， when the well developed Westerlies prevail persistently・Both the pre−
cipitation and the number of rainy days are much greater on the western side of the
mountains， decreasing rapidly toward the east． Relatively greater precipitation is
found also on the southeast facing slopes of the mountains probably at a somewhat
lower altitude than the rainiest part on the western side． This幽may be an indication
of the j皿fluence of the SE monsoon． But the eastern side of the mountains are decid−
edly drier in the Westerlies season than in ：the rest of the year．
If this distribution pattern is due to the Westerlies， it must be reversed when
the prevailing winds turn into the Easterlies． Actually， the reversal of the pattern
took place in mid−October in 1964． Fig． ll shows a marked contrast of the rainfall
index between the westem and the eastern sides of the high mountains． In the
Eldoret district（NO−35）representing the western side， widespread rains are re−
placed by local rains after mid−October． OR the contrary， a remarkable increase in
the rainfall index starts in mid−October in the east（SO−37）．
As mentioned earli．er， the ascent curve at Nairobi shows a stable layer between
the Westerli．e． s and the SE monsOon． The author believes that this iS the result Qゴ
subsidence of the Westerlies on the lee side． If so， this stable layer is supposed to
be lacking on the windward side・In addition，，the subsidence in・the． upP曾r Easterlies
一56一
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Fig・10
July 20「31 in 1964・ ．’tt
The humber of rainy days（0−1，2−4，5−7，．8−10，11−12 days）is
indicated by circleS． The east宇rn side of the mountains are
apP苓eciably dry・
置
OO
NO．−35
鮒書￡
．5
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Fig．11
Daily rainfall iロdex in NO−35 and SO−37 during September 25−
November 5 in 1964．
ln the west（NO−35）， widesp士ead rains are frequent口ntil midつctober，
but in the east（SO−37）ar包iny season sets in．at this time． This
is associated with the．dras亡ic change． in the atmospheric circulation．
＝）
T7一
o
o
seems to weaken toward the west． As a result， vertically deVeloped clouds may
easily be formed in the west．
The drier lee side can be divided into tWo clima亡ically different areas． The
N。i。。bi di。t・i。t i・invariably・・vered by・tratifi・d・1・ud・with・cca，Si・nal n・ctumal
drizzles， whereas in the northern part even cloud．s are not formed． This difference
may be a result・f th・natu・e・f th・SE m・n…n，・u・h a・the regi・nal difference°f
diverg・nce， rath・・than th・n琴tu士・・f th・Westerlies・becau・e cl・ud・in th・Nai・・bi
district are formed within the layer of the SE monsoon．
上astly， the relation between the Westerlies and the dryness． in Tanzania must
be discussed． A satisfactory explanation cannot be given for this at the present．
H。wever， Fl・hn蟹s hyp・th・・is c・ncerning th・・elati・n b・tween the climat・1・gi・al va1−
ues of the vertical component and the meridionOI component of winds is suggestive
（F1・㎞，1960a）． H・・tat・・that an・quat・rward・・mp・n・nt ha・at・nd・n・y t・wa・d
subsidence， and vice versa． The northern summer Westerlies generally have an
・quat・ri・1・・mp・n・nt in th・S・uth・m H・mi・phere・H・n・r・th・W・・terlies them甲
selves cannot be rain bringers in Tanzania．
CONC LUSION
It is believed that the climate of East Africa is basically controlled by the two
monsoons which are prevalent over the region． But， in addition， frequent appear−
ances of Westerlies and their rain b血ging character have long been known empiri−
cally． Th・hithert・rep・rt・， h・w・ver，・・ntain・・me c・ntradi・t・・y remark・rega「d−
ing th・・ea・・n・f frequent apPearance， th・・patial di・tributi・n・th・i・・rigin and thei「
association with weather． The author has attempted to make full use of all the avai1−
・b1・data・f m・t・・r・1・gical・b・erv・ti・n・t・investigat・th・time and・patial di・t・ibu儒
tion of the Westerlies， and to discuss their origins and the relation with weather．
The pilot balloon and radiosonde observations clearly show that there is a
di。t血。t sea・・nality in th・frequen・y’・f apPearance・With th・tw・minim・in Ap・il
and血November−December， which are common to most of the stations in East
Africa， the westerlies in January−March are distinguished from those in May網
October．
The horizontal and vertical distributions of the Westerlies are entirely differ−
ent between the two seasons． The westerlies in Jamlary−March are most frequent
over southern Tanzania， steadily decreas血g toward the north， and they are equally
frequent at least to 5000 meters in altitude． By contrast， the Westerlies in May−
o。t。ber， parti・ularly・．in・July・and・Augu・t， are m・・t frequent・ver the ea・tern half・f
Ea。t Af・ica， with・1arg・m・・idi・n・1・xt・n・i・n f・・m・・uth・m Tanzania acr・ss th・
Equat・r at lea・t t・central K・nya． Th・W・・terli…fthis sea・・n・hara・teri・ti・ally
appear at the 3000−4000 meter正evel overlying the SE monsoon．
Emphasis has been placed on the May−October Westerlies， because of the
insufficient data concerning the January−March Westerlies・
th・M・y−0・t・ber W・s）be・1i…ver N・i・・bi are by n・means c・・1 n・r un・tabl・
。・mpared with th・E・・terlies． Rather， it mu・t b・n・t・d that・・tabl・1・yer is「e向
peat・dly rec・gnized b・tween th・W・・terlies and the underlying SE m・n…n・The
m。i・ture c・nt・nt i・ Significantly greater in th・Westerli・・than in th・Ea・terli…
In additi。n， th・tim・・ecti・n analy・is sug9・・t・the p・ssibility・f wave−lik・pertu・ba層
tions in the Westerlies． A drastic change demarcates the termination of the West−
erlies season．
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According to the daily 500mb charts， a low pressure， which is different from
the Equatorial Trough， is formed over the continent at about 150S in January−March，
and the Westerlies are observed on the northern side of the depression． On the
contrary， the May−October Westerlies seem to be associated with the passages of
troughs extending from the higher latitudes in the Southem Hemisphere， which are
likely to develop over the eastem coast．
It is considered that the relationship between the Westerlies and weather
differs from region to region， for the reason that there is a great discrepancy be−
tween the spatial distribution of the Westerlies and weather． Even under the probable
control of the Westerlies， the Kenya Highlands experience frequent afternoon showers，
while the Nairobi district is characterized by extensive stratified clouds but with only
insignificant rainfall． An extremely dry condition prevails just east of the East Af−
rican Highlands in northeastern Kenya and also in most parts of Tanzania．
In spite of the commonly held belief that the Equatorial Westerlies tend to pro−
duce rain， the author has indicated that all of the regional differences of weather are
related to the Westerlies， and that they can be explained without much difficulty．
The dryness in the east of the Highlands is possibly due to the subsidence of the West−
erlies on the lee side． The difference between the Nairobi district and northern
Kenya may be attributed to the region鼻1 differences of the nature of the SE monsoon．
The dryness of Tanzania might be explained by the fact that the prevailing wind direc−
tioll has an equatorward component， hence the tρndency toward subsidence．
However， mainly due to the limitation of available data， several questions
remain unallswered． For example， whether the August thunderstorms in the greater
part of Uganda are really associated with the Westerlies is still dubious， because the
Westerlies are rare in the Wes鱗 1・．・ ・1・
In the second place， the fact』that the Westerlies over Nairobi have on the aver−
age a southerly component’ at 700mb， but a northerly component at 600mb must be
taken血to consideration particularly when Flohn’s hypothesis is adopted．
Third， the author made an attempt to infer the general nature of the Equatorial
Westerlies over East Africa from the particular case of the well developed Westerlies
at the end of July in l964． But this particular case seems to have been accompanied
also by the relatively moist and unstable upper Easterlies． Some rectification of the
author「s findings might be needed in the future．
Furthermore， since East Africa has been believed to be devoid of any wave−
1ike perturbations including Easterly waves， a further investigation of the”Equatorial
Westerly waves”is required．
』ACKNOWLEDGEMENT
Gratitude is e琴pressed to・ Professor Taiji Yazawa of the Department of Geogra−
phy， Tokyo Metropolitan University for his advice and encouragement throughout
this study． The author is母lso indebted to Dr．B． W．Thompson and other members
of the East African Meteorological Department who kindly offered suggestions and
provided data．
REFERENCES CITED
Bryson， R，A．， and Lowry， W． P．，1955：Synotic Climatology of the Arizona
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Summer Precipitation Singularity， Bu11．Amer．Met． Soc．， vo1、．36，
p．329−339．・
Fletcher， R．D．，♂’1945： The General Circulation of the Tropical and Equatorial
lAt血osphere， Jour．Met．，vol．2， P．167−174
Flo㎞， H．， r1949：Eine aquatoriale Westwindzone als Glied der allgemein，en
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the W．M．0．and the Munitalp Foundation， Nairobi， p．244−252．
1，．1960b： Equatorial Westerlies over Africa， Their Extension and Signifi−
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Henderson， J．P．，1949：Some Aspects of climate in Uganda with special
Reference to Rainfal1， East African Met．Dept．，Memoirs， vo1．2， no．5．
Johnson， D． H．，1962：Rain in East Africa， Quart・Jour・Roy・Met・‘soc・，
vo1．88， P．1−16．
Jo㎞son， D． H．，and M6坤， H． T．，1960：Forecast血g Research in East Africa，
In”Tropical Meteorology in Africa”， p．56・137．
Nakamura， K．，1967： Climate of East Africa as Related to Equatorial Westerlies，
Geogr、 Repts．Tokyo Metropolitan Univ．，no．2， p．49−69．
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Africa， Notos， vo1．4， p．219−220．
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East African・Met．Dept．， Tech．Mem．，no．8．
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Trewartha， G． T．，1961： The Earth’s Problem Climates， Madison， p．121−137．
Weischet， W．，1965： Der tropisch−konvektive und aussertropisch−advektive
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SOURCES OF DATA
British East African Meteorological Service，1946：Upper Wind Frequencies，
Tables of Frequencies of the Speed and Direction of UpPer Winds Determined
by Pilot Balloon Observatio皿s at Stations「in British East Africa duri皿g the
Period 1927−43．
East African Meteoroiogical Depart血e堪，ユ959：・The Frequency oflTQtal Cloud
Amount at Stations in East Africa．
一 1960：Upper Air Data foτNairobi， Summaries of Radio−sonde Observations
of Temperature and Humidity a且d of Radar Wind Measurements at Standard
Pressure Levels， 1948−55．
，1961：Frequencies of Surface Wind Speeds and Directions．
，1963：Upper Air Data for Nairobi， Summaries of Radio−sonde Observations
of Temperature and Humidity and of ’Radar Wild Measurements at Standard
Pressure Levels，1957−62．
，．、1964： Collected Climatological Statistics for East African Stations．
一・ 60一
，1964： The Daily Rainfall of East Africa（monthly）．
， 1964： The Daily Aerological Data for Nairobi during 1964（unpublished）．
U．S．Weather Bureau，1960−1965：Monthly Climatic Data for the World．
Deutscher Wetterdienst，1967： World Weather Maps， Daily Sea Level and
500mb charts， Pt． II， Tropical zone， July，1957−Dec・，1958・
o
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