O N THE WET SEASON IN NORTHERN RHODESIA By J. H. CHAPLIN, Northern Rhodesia Meteorologicul Service

I; the three countries included in British Central Africa, Northern 0 Rhodesia is the one which has rainfall most closely confined to a definite wet season. Summer rainfall is the general rule, but Southern Rhodesia is also affected by winter spells of low cloud and rain or drizzle (known locally as g ~ t i ) , while Nyasaland's topography is such that isolated winter rains can sometimes be expected under certain conditions.

Northern Rhodesia lies between approximately 8 and 18 degrees South and 22 and 33 degrees East. The Congo ' pedicle ' cuts into it, making two north- west,ern districts, both of which contain high ground. There is also high ground, rising in parts to 7,000 ft, on the eastern borders. The rest of the land lies be- tween 3,000 and 5,000 ft, apart from the valleys. The main river is the Zambezi, which, like its major tributaries, the Kafue and Luangwa, first flows southwards, later turning to the east and passing to the sea. The northern area of the country is mainly drained by three lakes-Bangweulu, Mweru and Tanganyika. The rainfall increases from less than 30 in. in the south to above 55 in. in the north.

The advent of the rains is so closely linked with the life of the African farmer that late rains may well cause famine, even if the delay is one of only a few weeks. A considerable corpus of native lore and magic consists of methods of changing the adverse weather and bringing rain. Fortunately for the climatologist, the commencement and duration of the wet season has also been of interest to the men most closely concerned with the welfare of the country- the administrators and missionaries. The result has been that, although the country is young, there are a number of rainfall records for periods of 25 years or more. The early establishment of the railway and the growth of mines, both having staffs interested in rainfall from the engineering point of view, has also added largely to the stock of information.

The purpose of this note is to illustrate the spread of the onset of the rains and their recession. Data have been derived for 38 stations, for a standard z5-year period, from 1925-6 to 1949-50 (the climatic year in the country runs from July to June). Gaps in records and doubtful readings account for fewer years being used for some stations.

I t is obvious that arbitrary criteria must be used to define the wet season and after several trials, the information given here is based on the following definition : ' The opening date of the wet season is the first day in a period of four days on at least three of which more than 0.01 in. of rain was recorded ; the total rain for the period being 0.40 in. or more.' The closing date is similarly described, as the last day of a period conforming to these limits.

There are unlikely to be criticisms of the need for arbitrary criteria ; the British definition of drought has only to be recalled to show that there must be a limit somewhere. However, it seems important to illustrate that the major

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part of the year's rain does in fact fall between the dates thus defined. Table I has been prepared for eight stations scattered throughout the country. It shows the mean numbers of rainfall-days, and the mean rainfall, for both the whole year and the wet season.

TABLE I . Comparison of annual and wet-season means of rainfall-days and amounts

Lati- Longi- Mean no. of days Mean rainfall (in.) tude tude Height No. of

"S 'E f t Year season yo Year season Yo years Wet Wet Station

Abercorn

MDika (N. Province) .. 8.53 31.22 5,474 21 113.8 102.6 90.2 46.21 43'29 93.7

~ -r (N. Province) .. 11.50 31-27 4,598 25 93.4 84.4 90.4 44.65 42-90 96.1

(N. Province) .. 11-11 28.53 3,759 24 89.0 80.9 90.9 43.41 40.34 92-9 Ft. Rosebery

Ft. Tameson

(S."Province) .. 17.52 25.51 2,977 24 59.1 51.6 87-3 28.80 26-18 90.9

(Centr. Province) 15-24 28.19 4,198 24 69.3 63-4 91.6 31-59 30.17 95.5

IandProtectorate) 15-17 23-05 3,459 21 91.1 83.6 91.7 37-43 36-22 96.8

(W.Province) .. 11-43 24.26 4.447 25 135.4 129.2 95.4 53.03 51.53 97.2

Lusaka

Mongu (Barotse-

Mwinilunga

Naturally, should there be a large number of days with only small amounts of rainfall outside the selected season, the ratio of days in the rainy season to the total number of rain-days will be lower than the ratio of the wet-season rainfall to the annual rainfall. It is also reasonable to expect that rain falling outside the chosen dates will affect the ratios for stations with small annual rain- fa l l amounts to a greater extent than for stations with large rainfalls. Not- withstanding these limitations it will be seen that the season selected according to the proposed criteria accounts for at least 90 per cent of all rainfall-days, and about 95 per cent of the total annual rainfall.

Table 2 and Figures I, 2 and 3, contain nearly all the information it is the object of this article to present. Figure I displays the mean opening dates, Figure 2 the mean closing dates, and Figure 3 the mean duration of the wet season expressed as a percentage of the year. However, it may be worthwhile to those unfamiliar with the country to mention two points of interest :

TABLE 2 . Wet-season data for selected stations in Northern Rhodesia ~~ ~

No. of Opening date Closing date Station years Mean Earliest Latest Mean Earliest Latest

I 2 3 4 5 6 7 8

Abercorn Mpika . . Ft. Rosebery Ft. Jameson Livingstone Lusaka Mongu Mwinilunga

.. .. .. 22 I r thNov. 9.10.42 9.12.32 27thApr. .. .. .. 25 19th Nov. 31.10.39 23.12.37 5thApr. .. .. .. 24 10th Nov. 7.10.31 3.12.15 5th Apr. .. .. .. 23 14th Nov. 21.10.25 9.12.32 3rd Apr. .. .. .. 24 15th Nov. 16.10.41 24.12.40 16th Mar. .. .. . . z+ 13th Nov. 18.10.48 25.11.37 20th Mar. .. .. . . 23 28th Oct. 7.10.25 20.11.45 2nd Apr. .. .. . . 25 9th Oct. 6.9.25 20.11.35 17th Apr.

30.3.48 9.3.27

13.3.45 3.3.37 9.2.46

23.2.33 28-3-44

1.3.27

18.527 3.5.32 5.5.43

30.4.39 21.4.40 24.4.32 23.4.32 5.5.32

~

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I

Fig. I . Mean Opening Dates o f the Wet Season

Fig. 2 . Mean Closing Dates of the IVet Season

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(a) Columns 4 and 5.7 and 8 of Table z serve to emphasize the reality of the chosen dates for individual years, by showing, for example, the generally late closing date of the 1931-2 season, and to a lesser extent the late opening, of the 1932-3 season.

(b) The isochrones of closing dates over the west of the country are of interest in revealing the apparent reluctance of the rains to leave the Zambezi Valley. It is probable that this delay is associated with the flooding of the region. During the flood-time, beginning in early March, it becomes a lake some 40 miles wide or more in places, a situation affecting the pattern of life of the Barotse tribes of the area to a considerable extent. This flooding provides abundant sources of moist air for the formation of convective clouds and further rain.

Fig. 3. Mean Duration of the Wet Season

Those familiar with the problems of central African meteorology may well be asking the question : ' How does this fit in with the Inter-tropical Conver- gence Zone ? ' I would suggest that the isopleths may bear some relation to the Zone's activities, and that the application of this method for a limited dis- tance into adjoining countries may be of interest in studying its movements.

ACKNOWLEDGMENT

I am grateful to Mr. J. E. Stevens, Chief of the Northern Rhodesia Meteoro- logical Service, for his permission to use the records of the Service, and to publish this article. The opinions expressed here are my own.

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