IV

CLIMATE

Climatically, as well as geographically, Bornu belongs completely to the tropic zone; although cut off from the warmth of the Equator, its average temperature throughout the year of 29° to 30° centigrade makes it not only one of the hottest countries in Africa, but even of the whole world.[118] Though the climate of Bornu may on the whole be called regular, considerable fluctuations in temperature nevertheless take place, as is proved by the records of the last sixty years.

Of all the travellers who have spent any length of time in Bornu, the only observations that extend over a detailed period are those of Barth and Nachtigal; moreover, those of later travellers, which embrace a few months at the most, in the main and on the whole, in so far as they are not of small value, confirm the former results. Barth’s observations have the advantage of extending over several years, of embracing all parts of the country which he personally visited, and of having been manifestly taken at places whose situation could not influence the result one way or other. But their reliability is somewhat prejudiced by the fact that the necessary thermometrical readings are sometimes discontinued for weeks at a time, and were resumed on odd days at quite different times. These irregularities happen to be of especial importance in the thermometrical readings, which were taken after midday or during the afternoon, when according to experience quite considerable increases of temperature may occur in a short time. Owing to the fact that the traveller, especially at the beginning of his journey, was not always in a position to be able to take observations during the hottest part of the day, many high maximum temperatures no doubt escaped him. Barth himself was well aware of the insufficiency of his data, for he refers to them throughout only as ‘Fragments of a Meteorological Register’.

Nachtigal’s observations are extraordinarily conscientious and arranged with greater regularity and include also barometrical readings. But they extend, as far as Bornu is concerned, only over seven months and omit precisely the period when the highest temperatures occur. Moreover, the place where the observations were taken, viz. the court-yard of Nachtigal’s house in Kuka, was such that it must have influenced at any rate the thermometrical readings. Inside the walls of the town, daily baking in the sun, the nightly radiation from the soil does not nearly reach the same intensity as in the open plain; this is strikingly apparent in Nachtigal’s thermometrical readings. There is an absence throughout the winter months of the low temperatures, shortly before or after sunrise, which have struck every one travelling in Bornu at this season.[119] Moreover it is not to be forgotten that the shade-tree, under which Nachtigal took his observations, influenced the result both of the maximum and minimum readings.[120]

The table given below makes it apparent that April, with an average temperature of 34° centigrade, is the hottest month of the year and December the coldest; further, that the chief rainy months, July, August, and September, show the least fluctuations in daily temperature, while the coolest months, November, December, and January, show the greatest. The reasons are not far to seek, they depend on the position of the sun together with the degree of intensity of its rays resulting from its position. At the end of March the sun approaches its zenith for the first time—i.e. as regards Central Bornu—the atmosphere begins to be saturated with moisture and this lessens the fall of temperature at night, which becomes still less marked in April, in which month prevails the most oppressive heat, not yet mitigated by the rains.[121] The first tornadoes now set in, and become more and more frequent, while their ever-increasing volume lessens the nightly fall of temperature, though it affects the daily temperature in the opposite direction, so that the daily fluctuation of temperature in July and August only reaches 9°. At the beginning of the month of October the tornadoes cease, and consequently there is an ever-increasing rise of temperature to correspond, which, however, very soon drops in the following months.

Month. Nachtigal’s Observations. Degrees in Centigrade. Barth’s Observations. Degrees in Centigrade. (Those given in Fahrenheit reduced to Centigrade.) Approx. avg.Approximate average reckoned from the observations of both (an addition of 3° Centigrade should be made to the temperature recorded by Barth at midday from May to August). Mon. avg.Monthly Average.

Avg. snr.Average of readings at sunrise. Avg. 2 p.m.Average of readings at 2 p.m. Hst. fl.Highest fluctuation in daily temperature. Avg. snr.Average of readings at sunrise. Avg. 12-1 p.m.Average of readings from 12-1 p.m. Avg. 1-2.30 p.m.Average of readings from 1-2.30 p.m. Hst. fl.Highest fluctuation in daily temperature. Avg. snr.Average of readings at sunrise. Avg. 1-2.30 p.m.Average of readings from 1-2.30 p.m.

January 18·9 30 16·2 15 35 33 (?) 24 17 32 24·5

February 19 30·2 16·1 15 31 35 19 17 33 25

March 21 37 36 16 21 36 28·5

April 25 37 43 18 25 43 34

May 23 40 17 23 43 33

June 24 37 20 (?) 24 40 32

July 24 31 9 24 34 29

August 23·3 29·3 9·2 22 30 23 31 27

September 24·1 31·4 10·8 22 31 36 11 23 34 28·5

October 24·5 35·5 16·1 24 36 15 24 36 30

November 20 33 16·5 16 32 27 18 32 25

December 18·5 30·4 16·3 12 31 26 15 31 23

Yearly average of temperature measured at sunrise and at 2 p.m. 21 35

Total yearly average 28°

Naturally the conditions for the northern and southern parts of Bornu vary somewhat, and local circumstances may also cause quite considerable fluctuations of temperature. Thus I noticed at Idjege in Southern Bornu, at the beginning of October, that the granite masses thickly studded with trees, on the slopes of which the place lies, had the result of causing an unusual increase in heat, which close to the surface of the soil measured 51° centigrade in the shade. While the nights here were even oppressively hot, a few days later on the plain at Dissa, not far from Idjege, the thermometer only showed 10° centigrade in the morning before sunrise, and by so doing registered the lowest degree of temperature, whose fluctuations far exceeded 30°. In general the heat reaches its highest point about 2 p.m. and then only declines very slowly, so that even in the evening abnormally high temperatures may be registered,[122] in fact the interesting phenomenon may occasionally be observed of a higher temperature at sunset than at midday.[123] If one takes the yearly average temperature according to Barth’s and Nachtigal’s tables, one cannot but notice that Nachtigal’s tables leave out of consideration just the hottest months. Having regard to this circumstance, one may reckon without much inaccuracy the yearly average temperature at from 29° to 30° centigrade.[124]

With regard to the second most important factor in the climate, viz. the winds, there exist no such detailed observations as with regard to the temperature; still the facts communicated on this subject by Nachtigal and recent French explorers, especially Foureau,[125] are sufficient to show that just as the atmospheric pressure is extremely regular so likewise are the winds. During the greatest part of the year, under the influence of the north-east trade-wind, north-easterly or easterly winds predominate, but south of 12° north latitude, where the wind comes mostly from the south with a decided inclination towards the south-east,[126] it is less constant, a fact which Foureau attributes to the counter-influence of the south-east trade-wind which reaches as far as here.[127] According to the universal verdict of all observers, easterly and north-easterly winds predominate especially in the dry weather, while in the rains, especially in July and August, westerly winds gain the upper hand, only to surrender their predominance to the easterly winds in October.

Views as to the daily fluctuation in the strength and direction of the wind appear to vary. Unfortunately Nachtigal’s tables[128] do not inform us whether observations with regard to the wind were taken also at night-time. The French naval officer Audoin, who personally took observations of this sort on Lake Chad, where the conduct of the watery plain is bound to give indications conformable to every movement of the wind, lays stress on the predominance of the north-east wind, especially from 11 p.m. to 9 a.m., while it is replaced by the south-west wind for the rest of the day.[129] The north-east trade-wind brings with it from the Sahara a great volume of dust, which often renders the atmosphere so opaque that the sun only appears through it like a dark-red disk. This phenomenon, which causes a peculiarly oppressive sensation,[130] is known throughout the whole of Western Tropical Africa under the name of Harmattan, but is especially characteristic of the Central Sudan.

Apart from the violent atmospheric disturbances which the tropical thunderstorms bring with them, the cyclones or tornadoes, coming sometimes from the south-east, sometimes from the north-east or south-west, especially during the times of the change in the wind, affect Bornu as well as the whole tropical district of the Western and Central Sudan.

In Bornu, where over wide stretches of country the weakening effect of dense forests is lacking, they attain an unparalleled violence. They give notice from afar of their arrival by thick clouds and whirling dust, and sweep everything off the earth that has not been made trim and taut. Sometimes cornfields which have been overtaken by them look as if a roller had passed over them. During the autumn of 1903 the roofs of the houses at Dikoa were blown off by a violent tornado. In general the cyclones are not preceded by discharges of electricity, but there are exceptions to this rule.

The tornadoes may be matched in violence by the thunderstorms which occur in Bornu. As is often the case in the tropics, they are heralded beforehand by the discharge of a large amount of electricity, a phenomenon which it is especially fine to observe at night, when the lightning flashes following each other uninterruptedly render it possible to read a book.[131]

Whilst on the subject of hydrometry, the question of cloud formation has next to be considered. The sky in Bornu, quite apart from the Harmattan, is very seldom quite clear; it is only in winter that there may be several successive days free from clouds; at other times ‘cirrus’ clouds at any rate are perceptible. The most wonderful cloud formations can be observed at the transition period shortly before the bursting of a thunderstorm. The results are analogous to those prevailing in Europe under similar circumstances; only in the Central Sudan the ‘cumulus’ formations differ in being grander, and they discharge themselves more quickly.

Fogs occur, but they are chiefly confined to the cooler parts of the year, and usually, as in northern latitudes, are connected with the presence of swampy districts, such as the damp low-lying shores of Chad. Besides it would be difficult, especially in the cooler morning hours, to distinguish sharply between fog and Harmattan.

Bornu lies exactly in the zone of the tropical summer rains, whose northerly boundary, naturally not to be sharply defined, coincides roughly with the fifteenth parallel of latitude, and thus includes also the most southerly part of the Sahara.[132] The moisture that falls there is only in the form of dew and rain. The mountains which border the country are not high enough to favour the formation of hail, which, however, is often enough to be seen in the neighbouring highlands of Central Adamawa and the grass-lands of the Kameruns that lie to the south of them. Dew appears—though only seldom—chiefly in September, October, and March; but then usually in profuse volume. Thus Denham in March and Barth in September experienced a quite extraordinarily heavy dew on the banks of Lake Chad, which they thought worthy of particular mention.[133]

The weather conditions accompanying the beginning or end of the rains usually occur with great regularity in Bornu. In this respect the country shows the typical Sudan climate, which is generally entirely without rain during the months of November, December, January, and February. The only noteworthy exception appears to be the actual borders of Chad. On the evening of the 22nd of January, an unusually cool day, and on the following morning, in the immediate vicinity of Chad, Foureau notes violent showers of rain, which were preceded in the afternoon by slight showers with some thunder and lightning.[134]

The rainfall is influenced in two different ways by the mountains on the southern border. Firstly, the volume of rain that falls here—though proper measurements of it have hitherto not been forthcoming—appears to be more intense than in the plains; and, secondly, the rains here set in earlier and do not end till later.[135] A greater or lesser distance from the Equator operates in a similar way. For the further south one goes, the longer is the duration of the rainy season and the greater the volume of the rainfall.

Apart from the above-mentioned exceptions, the first drops of rain do not fall in Bornu before the end of March. The downpour is heaviest in July and August. Though many of the tornadoes are extraordinarily severe and long-continued, yet the rain never lasts without intermission for a whole day or even a week, as it may do in the primaeval forest region of the Kameruns.[136]

In spite of the low rainfall as compared with that of Adamawa, which lies to the south of it, the climate of Bornu, except in the dry weather, thanks to its low-lying marshes, is distinctly unhealthy.[137] Besides the diseases, such as dysentery, which appear at all times of the year, malaria and its attendant evils are chiefly prevalent during the rainy and equinoctial seasons; Europeans especially are subject to it to an unusual extent, as are also the Arabs and Berbers accustomed to the healthy climate of the desert.[138] But even the natives are not entirely exempt from such diseases, which may assume the form of devastating epidemics.[139] The only part free from fever seems to be the uninhabited belt of marshy vegetation round Lake Chad, because infection from man to man is here impossible, although there are clouds of mosquitoes ready to convey the germs of the disease. On the same grounds the larger towns, which scorn all sanitary regulations, are the worst centres of disease. The irritant microbe which lives in the anopheles mosquito is paralysed at a temperature below 17° centigrade, and consequently dies during the cold nights of the dry weather, in the event of its host not spending the night in the warmth of a house.

The rainy season in Bornu is dreaded by the Berbers or Tripoli Arabs not only on account of the danger to their own health, but the condition also of the pack-camels bred in the Sahara is influenced very unfavourably by it, in fact it sometimes causes great mortality amongst them, so that the caravans, whenever they can, leave the country at the beginning of the damp weather.[140] It is not yet certain whether the camels fall victims to the same disease from which the animals bred in Bornu, horses and especially oxen, have to suffer. Inquiries into the question of whether it is spread in the form of a contagious disease like trypanosomiasis—the disease whose germ is conveyed by the tse-tse fly—have not yet been published, but at any rate myriads of flies belonging to the ‘Glossina’ species are to be found in the neighbourhood of the water-courses fringed with trees and bushes.

It is obvious that the moist heat of the rainy season is favourable to the growth of all kinds of sickness. Besides venereal disorders, malignant eye-diseases are very common, and, owing to the indolence of the natives, find every chance of being spread abroad; leprosy is by no means unknown.[141] One of the commonest evils is that caused by the guinea-worm, a skin-disease which, though not dangerous, is protracted; the worm enters the body through the use of unboiled water from puddles, water-courses, and even from wells, and almost every native has been attacked by it at some time during his life.[142] Cleanliness and the scrupulous observance of simple rules of health, including especially regular bodily exercise, afford, however, far-reaching protection against all these diseases, not excluding malaria itself, so that at any rate Europeans can secure themselves in some degree against sickness. All these districts are at all events far from being so unhealthy as the region of the West African primaeval forest, where the germ-destroying effect of the sun’s rays seldom penetrates.