RAINFALL PATTERNS AT ABERCORN, NORTHERN RHODESIA By P. SYMMONS

NTIL recently accurate information about the weather of Central Africa U has been lacking. Even now little is known about that of the remoter areas such as the Northern Province of Northern Rhodesia. As a small contribution to filling this gap I have attempted to describe the rainfall patterns experienced at Abercorn. In particular, I have tried to determine the amount of rain falling and the intensity of fall in storms of different lengths, the mean dis- tribution of rainfall through the day, and the probability of spells of wet weather occurring.

This analysis is based on recording rain gauge records kept by the Inter- national Red Locust Control Service at their Abercorn headquarters for the rainy seasons 1954-55, 1955-56, and 1956-57. With data covering such a short period statistical methods cannot be applied and conclusions must be tentative.

The climate of tropical Africa is characterized by a division of the year into a wet and a dry season. At Abercorn the rains start in October with a few isolated showers and finish in the same manner in May. November to March inclusive are the heavy rainfall months ; June to September are com- pletely, or almost completely, dry. The rainy season is caused by the south- ward movement of the zone of convergence between the north-east and south- east trade winds with the unstable air so caused giving rise to rain storms of the thunderstorm type. Such storms were thought to be short, intense, and to occur regularly in the afternoon at about the time when the greatest heating of the earth’s surface would have produced warm unstable air. More recently it has been recognized that persistent cloud is common during the rainy season and that extensive periods of drizzle do occur. I t is thought that such weather may be frontal in origin, that is, it may be caused by weak low pressure troughs originating along the line of convergence of the trade winds.

AVERAGE RAINFALL DISTRIBUTION DURING THE DAY

In order to test these generalizations I have studied the way in which rainfall, rainfall intensity, and time during which rain occurs, vary through the day. As can be seen from Fig. I most rain does fall during the time when the greatest amount of heat is received from the sun though the rest of the day is by no means rainless. Between 11 a.m. and 7 p.m. 56 per cent of the average annual rainfall occurs. This is the wettest time, though another maximum occurs between I a.m. and 3 a.m. The late evening and mid-morning are the driest periods.

This rainfall pattern is caused partly by variation in the intensity of fall and partly by variation in the amount of time during which rain falls. Figs. z and 3 show that both intensity and duration of fall follow a similar pattern to total rainfall. In other words, between 11 a.m. and 7 p.m. when most rain

A V E R A G E D I S T R I B U T I O N OF R A I N F A L L THROUGH THE DAY

I I J C 5 6 7 6 9 10 II I1 I3 II IS I 6 17 I 8 14 10 2 , 11 1) I +

T I M E OF DAY AVERAGE D l S T R l 6 U T l O N THROUGH T H E DAY OF THE NUMBER O F HOURS DURING WHICH R A I N F A L L S IN A R A I N Y SEASON

A V E R A G E D I S T R I 6 U T I O N O F RAINFALL INTENSITY THROUGH

e THE DAY 0” 0.14 . I g 0.11 a. n’ - 2 0.18 .

- j 0 . 0 6 ’ < LL 5 0 . 0 3 < 4

0 I i j i 7 i q i’ i d i j it I S li 37 I.I 8.4 io i i it I > B T I M E O F D A Y

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falls, it rains harder and, taking the mean total for the season, it rains for longer ; the same is true though to a lesser extent of the I a.m. to 3 a.m. period.

Though the three factors, total rainfall, duration of rainfall, and rainfall intensity, follow a significantly similar pattern, there are interesting differences between them. Thus rainfall intensity starts to increase from about 5 a.m. onwards though rainfall and duration of rainfall do not get greater until 8 a.m. and 9 a.m. respectively. Also rainfall intensity gets less rapidly after I p.m- though the duration of rainfall does not start to decrease until 5 p.m.

CHARACTERISTICS OF STORMS OF VARIOUS LENGTHS

For ease of study, storms were divided into 4 lengths ; less than I hour, from I to 2 hours, from 2 to 3 hours and longer than 3 hours. As would be expected, there proved to be a large number of short storms ; more than half the falls lasted for less than I hour and many short sharp showers lasting no longer than 10 minutes occurred. Storms of this length accounted for less than a quarter of the total rainfall time but contributed more #an a quarter to the season’s rainfall total. Short storms proved to have a higher intensity of fall than longer ones (Table I).

SEQUENCE OF RAINY DAYS

An examination of the probability of a succession of rainy days, that is days when more than 0.01 in. of rain falls, shows that there is a tendency for rainy spells to occur. If rain falls there is a better than average chance that the next day will be a rainy one also. After two days rain a third is even more probable and the likelihood of a 4th and 5th wet day is greater still (Table 2 ) .

SUMMARY

Unfortunately the evidence is of too short a period to allow firm conclusions based on statistical analysis to be made. Also the pattern may be influenced by the relief of the area surrounding the station. However, the information does indicate that the idea that rainfall in the equatorial convergence zone is purely convectional and occurs regularly and almost exclusively in the early afternoon, is a misleading over-simplification. At Abercorn rain may fall a t any hour though storms are more likely and the fall is likely to be more intense between 11 a.m. and 7 p.m. than at any other time. Since the intensity of fall is greatest for short storms it seems likely that the afternoon is the time for sharp showers of convectional origin. Long periods of steady rain contribute the major part of the annual total and such storms may not be of convectional origin. The tendency for sequences of rainy days to occur suggests also that rainfall of other than convectional origin is important. Further investigation may show that the heavy rainfall in the early morning and the increase of rainfall intensity in mid-morning before the rise in rainfall amount, is significant.

There appears to be no satisfactory explanation of these patterns though with the rapid increase in the knowledge of tropical climatology and meteoro- logy, a reasonable theory may be put forward in the near future.

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TABLE I . Characteristics of Storms of Various Lengths

Storm size in hours 0- I 1-2 2-3 3+ Total

Average number of storms per season 141 42 I8 22 223 Average amount of rain per season in. Average time during which rain fell in hours

Average rainfall intensity per season in in.

I2 I 0 6 I7 ,45

per season . . .. .. .. . . 64 63 45 114 286 Mean

per hour . . .. .. .. .. 0.19 0.15 0.14 0.15 0.16

TABLE 2. Probability of Sequences of Rainy Days

Probability

Probability of a rainy day .. . . .. . . . . .. .. 0.61 Probability of a rainy day the previous day having been rainy . . .. 0.69 Probability of a rainy day the previous 2 days having been rainy .. 0.71

.. 0 ' 7 5 0.80

. . Probability of a rainy day the previous 3 days having been rainy Probability of a rainy day the previous 4 or more days having been rainy

. .

REFERENCES BULTOT, F. 1956 Etude statistique des pluies intenses au Congo Belge et au

Ruanda-Urundi. Institut National pour l'etude agro- nomique du Congo Belge. Bureau climatologique com-

KENDREW, W. G. 1953 The climates of the continents. Fourth edition, Oxford University Press

THOMPSON, B. W. Some reflections on equatorial and tropical forecasting. East African Met. Dept. Tech. Memorandum No. 7

RIEHL, H. . 1954 Troeical meteorology. McGraw-Hill

- munication No. 11

1957

RAINFALL AT GROUND-LEVEL . By F. H. W. GREEN

Nature Conservancy

S there a significant difference between rainfall measured in gauges exposed, I as is customary at meteorological observing stations, with the rims one foot above the ground, and that which actually falls on the ground surface ? The diagram given in G. Stanhill's letter to Weather in January 1958, may be open to question, but there is much evidence that his general conclusion that the average difference is about 5 per cent is close to the truth. While potential Evaporation (P.E.) has generally been considered to be more difficult to measure directly than rainfall, there is now some evidence from recent direct observations of the former which throws light on the accuracy of measurement of the latter.

Observations have been made, under the aegis of the Nature Conservancy, since 1954, with simple apparatus similar to that described by Gamier and Lewis (I954), and there has, considering the years as a whole, been quite good agreement between the observed values of P.E. and those computed by Penman's or Crowe's formulae. But it is a t once apparent from Table I

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