PART II. OUTLINES OF TROPICAL CLIMATOLOGY

ERRATA.

p. 9, line 9 from foot, for “Camerun” read “Cameroon”; and line 12 from foot, for “Shilling” read “Shillong.”

p. 58, line 11 from foot, for “Sangor” read “Saugor.”

p. 63, in table, line 11 from top, for “Ayra” read “Agra.”

p. 84, line 1 in table, and p. 85, line 3 in table, for “Mazattan” read “Mazatlan.”

OUTLINES OF TROPICAL CLIMATOLOGY.

SECTION I. General Considerations.

A very broad belt of the earth’s surface is occupied by countries that may be said to possess hot climates; as they include not only those within the Tropics, but also the sub-tropical zones. Practically speaking, the whole of Africa, much of South America, the Southern States of North America with the West Indian Islands, Asia Minor, Arabia, Persia, India and the Malay Peninsula, the greater part of China, Australia and the islands lying between it and the continents of the northern hemisphere, may be said to be included within this general term. In so wide an area, it is needless to remark that the widest range of climatic conditions may be found, the only condition common to all being that of subjection to a fiercer heat than that to which we Europeans are accustomed, for some or all the months of the year. It must be remembered, too, that climate is determined not only by the relation of a place to the parallels of latitude, or to speak more exactly, to isothermal lines, but is also affected by the elevation of a site above the sea. Even under a vertical sun, an ascent of 18,000 feet will land the climber in an absolutely arctic climate, as far as the temperature of the air is concerned; but it is only in this one item that we have any similarity to arctic conditions, as the sun’s rays blaze down even more fiercely than they can at the sea-level, where they have been tempered by passage through miles of denser air and watery vapour.

On the crest of the Wakujrui Pass, at 16,500 feet, while the air temperature at noon stood at 20° below freezing point, the sun thermometer registered 165°, in May, 1886, when the writer was crossing the great divide between India and Central Asia; and yet it was sufficient to remove one’s hat for a minute to realise that to do so might result in sunstroke. Moreover, apart from the effects of the rarification of the air on respiration, radiation was so rapid as to be painfully apparent; one side of the hand turned to the sun would be scorched, while the other chilled so rapidly that the sensation conveyed was that of being in contact with a cold liquid; and one was constrained to wrap up even the face as closely as possible, though the air was fortunately well-nigh still, whereas cold of similar severity at the sea-level is quite tolerable as long as there is no wind. Thus, in the consideration of a given climate, not only geographical position, but also elevation above the sea must always be taken into account.

To enter into a detailed account of the climatic conditions of the enormous area under consideration is, of course, out of the question, as it is a subject on which a special encyclopædia might well be compiled, so that only the outlines of the subject can be touched upon in the present short pamphlet. Roughly speaking, we may say that the climates under consideration have a mean annual temperature at the sea-level of not less than 64° F. (18° C), while in the equatorial zone it reaches 80° F. (27° C.); but the difference between the maximum and minimum temperatures in tropical countries is rarely as marked as in sub-tropical localities, as the range of temperature in the latter is usually far greater than in the former, so that in spite of the lower mean, far higher temperatures are recorded for certain regions well outside the Tropics than can be anywhere found within them. At Jacobabad, in Upper Sindh, for example, a place some 500 miles outside the tropical zone, the enormous shade temperature of 127° F. (52·7° C.) has been registered, and readings of 115° F. (46·1° C.) are quite common during the hot season over large areas of subtropical India. With the exception of certain parts of the Soudan, such temperatures are hardly to be met with in the truly tropical zone, and even these are but barely within it.

The tropical zone may be defined as that within which the sun is at some time of the year vertical at noon; or in other words, comprises a belt extending about 23¹⁄₂° of latitude on either side of the Equator. To the north and south of it the sun approaches and recedes from the vertical once during the year, and there are accordingly but two distinct seasons of summer and winter; but at the Equator the sun necessarily passes overhead twice during the twelve months, and there are accordingly four seasons, none of which, however, owing to their shortness, can be very sharply differentiated from the other. The northern and southern limits of the Tropics coincide pretty closely with the isotherm, for the coldest month, of 68° F. (20° C). On the great oceans the coincidence may be taken as practically absolute, especially along the northern isotherm, but both isotherms show a tendency to turn towards the Equator as they approach the western shores of the great continents; so that the breadth of the tropical belt is considerably contracted in these positions, and the same remark applies, though to a lesser extent, to the mean annual isotherm of 68° which bounds the sub-tropical zone. The tropical zone on the West Coast of America is contracted to little more than 30° in place of the normal 47°, while as far as mean temperature is concerned, the temperate zone extends as far north as 20° S. latitude, well within the geographical Tropics. On the West Coast of Africa the contraction is equally marked, but mainly at the expense of the northern isothermal boundary, while the sub-tropical boundaries, on the contrary, spread out, so as to leave only the extreme northern and southern points of the continent outside their limits. A third narrowing is to be found at the western side of the irregular land mass formed by Australia, the Malay Peninsula, and the intervening islands, but is much less marked, amounting to a few degrees only. The comparative coolness of the western sides of the great continental masses is due to the existence of northerly currents of cold water coming from the frozen seas of the south pole, which wash the western coasts, while along the east coast there sets a current of warm water coming from the Equator.

The principal factor in the determination of climatic characteristics is the fact that while the land heats and cools with great rapidity, the sea does so more slowly, but holds the heat better. This is due to two circumstances. In the first place, it requires more heat to warm a given weight of water than an equivalent mass of the various substances which constitute the land. In the second place, while both are alike bad conductors of heat; water, being a fluid, is mobile, and in the colder parts of the globe where the surface is colder than the intermediate depths, convection comes into play, apart from which the least movement at the surface is sufficient to distribute the heat gained from the sun to a greater depth than is possible in the case of the solid constituents of the earth’s crust.

The second great determinant of climate is the fact that the temperature of the air is determined for the most part by that of the surfaces with which it is brought into contact, rather than by the passage of the sun’s rays through it, as is well shown in the arctic air temperatures of great elevations in tropical latitudes. From this it follows that the atmosphere is mainly heated by the sun’s rays indirectly, from below where it is in contact with the directly heated, solid and liquid surface of the globe.

In becoming heated it necessarily expands, and becoming lighter that the stratum immediately above it, ascends, drawing in, to take its place, the cooler air that has been in contact with surfaces of air or water less strongly heated. Now because, as we have seen, water heats and cools more slowly than land surfaces, along a coast there is always a tendency to the production of a sea wind from the cooler sea to the hotter land during the latter part of the day, and inversely of a land breeze from the more rapidly cooled land to the slowly cooling sea in the early morning; and in the Tropics, where the sun’s rays are sufficiently near the vertical to produce a rapid and marked effect, these diurnal land and sea breezes form a characteristic feature of littoral climates, and go far to render life tolerable in them. Hence also it follows that everywhere littoral and marine climates tend to uniformity, not only as to diurnal but also as to annual variations, while continental climates tend to wide variations of temperature, and the seasons differ to a degree never experienced in places on or near the sea. As an example may be contrasted the climates of Madeira and Peshawar, in the former of which the difference between the mean temperatures of the coldest months is under 13° F. (7·2 C), while in the latter the difference amounts to 40° F. (22·2 C), or three times as great in the continental climate.

It is further noteworthy that while, in the continental climate, the hottest and coldest months coincide with the summer and winter solstices, in the marine climate they lag a month or two after; the coldest month being February and the hottest August in Madeira, owing to the slowness with which the water surrounding the island gains and parts with its heat.

Just as the alternating heat of day and coolness of night produce, in littoral regions, the daily land and sea breezes, so the greater heating of the world’s surface over the Tropics produces, throughout the year, a steady flow of air from the north and south, to take the place of the air that has become rarified, and so floated to a higher level. These winds—“the trades”—are not, however, directly from the north and south respectively, but have also a great deal of easting in their direction, a circumstance which is explained by the fact that the air, coming as it does from latitudes where the circumference of the earth is much smaller than at the Equator, is moving from west to east, only at the comparatively slower pace of the rest of the earth’s surface at that latitude; and as they do not at once acquire the quicker motion of the latitudes to which they have travelled, they lag behind the points of the earth moving beneath, and so give the effect of an easterly breeze, just as would be the case with a vehicle driving rapidly eastward through still air.

Between the two belts influenced by the trade-winds there is naturally a broad zone, known as “the doldrums,” where the opposing air currents tend to neutralise each other, and which is naturally characterised by periods of prolonged calm, alternating with light and variable airs. The middle line of this zone will obviously be that in which the sun is vertical at noon, and will consequently lie north or south of the Equator according to the season of the year. For the typical development of the “trades” the absolutely uniform surface conditions of the ocean are indispensable, the variable conditions of soil and vegetation on land areas impeding their full establishment, by introducing local variations of capability of heat absorption and radiation; but in spite of this, though less definite in force and direction, winds of the same general direction are dominant over the comparatively uniform surface of the South American continent. When, however, we find a sufficiently large land area more or less surrounded by sea, the different heat-absorbing capacity of solid and liquid surfaces may suffice not merely to neutralise, but to reverse the normal direction of the tropical and sub-tropical air-currents. Under such conditions, where the land surface is sufficiently large, the much more rapid heating of the land surface during summer brings the air in contact with it up to a much higher temperature than that of the neighbouring seas, and accordingly over India and Southern China we find that about May or June, by which month the land has had time to become sufficiently heated, a strong south-west current—the monsoon[5]—is established, which, carrying with it air saturated with moisture by its contact with the sea, determines the rainy season, and by the gradual cooling of the surface thereby produced, brings about its own termination. In the winter, on the contrary, over these regions, the prevailing wind resumes the general direction of the trades.

Another effect of rapid local heating of the air, the cause of which, however, is ill understood, is the occurrence of the revolving storms which are met with during the summer months in low latitudes, and are generally spoken of as “cyclones” in the Indian Ocean, as typhoons in the China seas, and as hurricanes in the West Indies. These storms are determined by the formation of areas of low barometric pressure, to meet which the air, converging from all sides, takes on a circular motion, or vortex, round its centre. Nearly all such storms have a double motion, the vortex itself being not stationary, but travelling over the surface of the globe in a definite direction. However obscure their origin, the laws of these storms are now well understood and are as follows: In the Northern Hemisphere, the vortex revolves in the opposite direction to that of the hands of a watch, and in the southern in the same direction as they do; while in both the motion is not truly circular but spiral, in such manner that a particle carried by the wind, after circling round the centre several times, ultimately finds itself carried to the centre of low pressure. In the same way the centre of low pressure, with its accompanying vortex, always travels at first from east to west, and then curves away from the Equator, to ultimately take an easterly direction as it dies out. The dimensions of the vortex, and the area influenced by such storms, may vary from a few yards to several hundred miles, but in all cases their force, within the vortex, is very considerable.

Small atmospheric disturbances of this sort, often of no more than 50 or 100 yards in diameter, are very common on the hot, dusty plains of Rajputana and the Punjab, and are most instructive to watch, as they are exact reproductions, on a small scale, of the awful visitations that from time to time devastate huge areas of the earth’s surface.

After a period of exceptional heat and stifling stillness, the still leaves of the dried-up trees are agitated by light puffs of air of irregular direction, then away in the east is seen a column of dust, and this steadily advances till one finds one’s self for a few minutes buffeted by a violent, fiery wind and choked with dust. When it has passed and the air has again cleared, this is succeeded by a refreshing relief of the previously intense heat. When of very small dimensions these miniature cyclones are locally known as “devils,” and their form, narrow below and spreading out like a funnel above, can be studied at leisure. The boundaries of the expanded upper part are indistinct and fade gradually into the steel-grey of the surrounding glare, but below the contour of the column is well-nigh as sharp as if it were composed of solid materials, and may sweep along close by the observer without involving him.

When of larger dimensions, so that the boundaries of the revolving column of dust and air are beyond the range of vision, they are known as “dust storms,” and in spite of the temporary discomforts they cause, are gladly welcomed, on account of the relief they bring from the suffocating heat that originated them. When on the larger scale that is met with in the equatorial zone their violence is well-nigh incredible—trees are torn up, houses levelled, crops destroyed, and massive bodies, such, for example, as large anchors lying on the quays of a dockyard, trundled along as if they were straw hats in an ordinary gust of wind.

From a sanitary point of view these storms are usually beneficial by their effect in clearing and cooling the air; but this is unfortunately only small and temporary, so that they are of little interest to the hygienist. In any case the prophylaxis against their effects is purely mechanical, and consists in crawling, if possible, into the nearest cave or cellar.

Over the great Asiatic continent, especially north of the Himalayas, a strong northerly current is produced about mid-summer by the area of low pressure caused by the intense heating of the Siberian steppes, which, owing to the length of the days at these high latitudes, are exposed to the sun’s rays for practically the whole twenty-four hours, and the current thus initiated makes its influence felt for hundreds of miles to the south of its point of origin, and no doubt reacts upon and modifies other periodical forces of the same character. A useful law to remember is that discovered by Professor Buys Ballot, which is to the effect that, if you stand with your back to the wind, the barometer will be lower in your left hand than in your right, in the Northern Hemisphere, and vice versâ south of the Equator.

The effect of ocean currents bringing with them masses of hot or cold water from other latitudes has already been alluded to, a familiar example being the mildness of our own climate under the influence of the Gulf Stream.

In the Southern Hemisphere currents of cold water sweep up from the Antarctic regions along the western shores of the great masses of land, counter-currents of warm water flowing down from the Equator along their eastern sides. In the Northern Hemisphere the reverse is the case, the cold Arctic currents clinging to the eastern, and the warm equatorial to the western shores of the continents. The more detailed distribution of these currents, however, can be better gathered by a little study of the current chart found in any good atlas, than from any description, however elaborate.

Climate may also be profoundly modified by the distribution of mountain chains, the cold summits of which determine the precipitation of the moisture brought up from the sea, so that while their seaward slopes may be inordinately rainy, the country beyond may be completely parched; and apart from such marked contrasts as are produced by the interposition of great ranges of hills, large differences of rainfall may often be found in stations a few miles apart. Cherapunji, in Assam, which is said to hold the world’s record for heavy rainfall, is but 40 miles from Shillong, the rainfall of which is by no means excessive; and again, Debunja, in the Cameroons, which is said to hold the second place with a rainfall of 897 cm., is close to Cameroon, where the rainfall is less than half that figure.

Another climatic factor of great importance is the amount and character of the vegetation, for it is a well-ascertained fact that not only does a heavy rainfall determine luxuriant vegetation, but the converse is also true, and it is probable that the barrenness of certain regions is due rather to improvident deforestation than to original natural dryness. Certain experiments, indeed, go far to show that it is possible to materially alter the climate of even comparatively small areas by judicious tree-planting; and it is also certain that the presence of even small patches of verdure may make a marked difference in the temperature curves of places within but a few hundred yards of each other. In the Upper Nile valley, for instance, astounding differences in the temperature and humidity of the air have been found to exist in places, respectively barren and cultivated, quite close to each other, and there is little doubt that local differences of this sort are worthy of more detailed study than they have as yet received, and would often be of value in determining the most suitable sites for habitations and stations.

From a sanitary point of view the variations of the barometer are of little interest, as at any given level they are never sufficiently great to have any physiological effect on the human organisation, and hence the elements of climate that interest us most are temperature and moisture, for the determination of which all that is required are a maximum and minimum thermometer, a pair of ordinary wet and dry bulb instruments and a rain gauge.

In forming an opinion of the characters of any given climate the temperature of the air is alone of any great importance, the data afforded by the sun thermometer and that used for determining radiation being of comparatively little interest, so that the following data are the most important:—

(1) The mean temperature of each month. This can only be given accurately by self-registering instruments, but in the absence of these is usually taken as

max. t. + min. t. + t. at 9 hours + t. at 21 hours4

(2) The mean monthly daily range of temperature, or what is practically as valuable, the mean maxima and minima.

(3) The “relative humidity” of the air, or the proportion of moisture actually present to the amount that would suffice to produce saturation, for each month of the year.

(4) The monthly rainfall.

(5) The number of rainy days in each month.

(6) The average condition of the sky, whether clear or overcast, in each month.

(7) The amount and daily distribution of wind, its direction being for us of little moment.

In speaking of air temperatures it must always be understood that temperatures in the shade are referred to, and in systematic scientific observations the greatest care must be taken that not only shall the instruments be thoroughly protected from the direct, but also from the reflected rays of the sun; and that means are also taken to secure a free current of air over them.

If an absolutely exact determination of the temperature of the air in any given situation be required, the observation should be taken by swinging the instrument, attached to a short cord, rapidly round the head, and by this method it is possible to secure a close approximation to the actual air temperature, even in the open and under the direct rays of the sun.

In considering the effects of climate on human beings it will be well to commence with a short consideration of the way in which they are affected by each of these elements of climate.

Temperature.

—In health the temperature of the human blood varies but little, whatever may be the climatic conditions to which we are submitted, the normal point being generally taken as 98·4° F. (37° C), though it may range half a degree or so above or below this level without prejudice to health or comfort. The mechanism by which this uniformity of internal temperature is maintained, in spite of the widest differences in the temperature of our environment, depends upon an automatic regulation of the nutritive processes going on within the system.

The various muscular and nervous actions, going on constantly throughout life, derive the force necessary for their production from the oxidation of the various articles contained in our food, and as the oxidation of all its digestible constituents is really nearly as complete as if they had been subjected to combustion, the body gains nearly as much heat from the consumption of its food as if the latter were actually burned. In climates where the temperature of the air is less than that of the blood, a good deal of heat is conducted away from the body by its contact with the air; but where, as in hot climates, the difference is but small, or the temperature of the air may even exceed that of the blood, it is obvious that some further mechanism is required if the temperature of the body is to be kept at the normal level of health. This requirement is met by evaporation from the surface of the body, the amount of which is regulated automatically by a special set of nerves which are known as the vaso-motor system, whose function it is to regulate the calibre of the blood-vessels throughout the body, according to the varying nutritive necessities of the several organs to which they are distributed.

We are all familiar with the fact that either extreme heat or violent exertion will alike bring about free perspiration, and in both cases the object is the same, viz., to cool down the body which tends to become overheated; in the one case by the warmth of the surroundings, and in the other by the activity of the chemical changes going on within the body to provide the force required for the various muscular and nervous actions involved in the work performed. On the other hand, under the influence of cold the skin becomes bloodless and dry, very little blood being allowed to circulate at the actual surface, the bulk of it being kept to the deeper parts of the body, well beneath the protective coating of fat which lies immediately below the skin. Buried in this fat, and opening by delicate tubes on the surface, are enormous numbers of small glandular bodies—the sweat-glands—each of which, when sufficiently freely supplied with blood, pours out a fluid consisting mainly of water, but containing also a little common salt and minute quantities of other mineral constituents, as well as a trifling amount of organic or animal matter, which has served its purpose in the organism and is thrown off in this way as being of no further value.

In the conversion of water into vapour a large amount of heat is absorbed, and it is thus equally possible for the body to be kept at a temperature lower than that of the surrounding air as it is, under more ordinary conditions, to maintain it at a higher; but the endurance of intense heat throws an even greater strain on the organism than that of severe cold, as it cannot be combated in the same way by covering the body with non-conducting clothing, and under such circumstances exertion, involving as it does a further production of heat within the body, becomes well-nigh insupportable. The evil effects of intense heat become all the more marked in proportion as they are prolonged, and the effects of air temperatures approaching or exceeding that of the normal blood continuously, for many days or weeks, without any relief at night, are most debilitating, and render any considerable amount of muscular exertion not only painful but dangerous, even to natives of the country, who, indeed, thoroughly recognise the fact and abstain during such periods from any laborious tasks not absolutely necessary. The exhaustion and incapacity produced by extreme heat are naturally specially marked in persons in whom the sudorific system is ill developed, and there is no doubt that those who suffer from this defect in any marked degree should avoid subjecting themselves to such conditions and be content to remain in more temperate climes. It is obvious that under such conditions a failure in the action of the sweat glands must necessarily result in a rise of the body temperature, and there can be little doubt that this is what takes place in certain cases of simple “heat apoplexy.” This failure of the sudorific system appears to be specially favoured by the overcrowding of too many persons within a limited space. It is only, however, when the temperature stands for long periods above 90° or 95° F. (33° C.) that these distressing effects of heat are at all commonly experienced; most Europeans bearing heat up to this limit even for prolonged periods, if not with comfort, at least without serious detriment to health, and much higher temperatures are well borne during the day, provided that the daily range of temperature is sufficient to secure a definite relief during the night. It will thus be seen that a wide diurnal range of temperature will go far to neutralise the bad effects of a high mean temperature, and the importance of securing information on this point in estimating the possible effects of a given climate on health is therefore obvious.

The proportion of moisture present in the air has at least as important a bearing on health as its temperature. It is obvious that when the air is actually saturated with watery vapour evaporation from the surface of the body must necessarily be stopped, and with it the natural provision for preventing an undue rise of the temperature of the body. Actual saturation combined with high temperature is, however, fortunately rare for anything but short periods, as the absolute amount of water requisite to saturation increases rapidly as temperature rises; and hence warm air, but partially saturated, and therefore still active as an absorbent of evaporation from moist surfaces, may contain a far larger absolute amount of water than saturated air at a lower temperature. In practice, saturated air is only to be found in situations where it is brought into contact with colder surfaces, such as that of the earth, cooled by radiation during the night, as is seen in the production of dew; the dew point being, in fact, the temperature at which the amount of water present in the air suffices to saturate it. Apart from its diminution by the formation of dew, the absolute amount of moisture present in the air, depending as it does on but slowly changing conditions, can naturally also change but slowly, but the relative moisture, or the percentage of the amount required to produce saturation, which is actually present always varies greatly during the twenty-four hours in all places where the diurnal range of temperature is at all considerable; so that relative moisture, when given for any day or other period, always refers to an average. As a rule it is only the stratum of air of a few yards in thickness that is cooled by contact with the soil during the night and is hence concerned in the formation of dew, a fact which is prettily illustrated by the low-lying bands of vapour which hang over the landscape after a clear night on any fine morning in the Tropics, and which clear off as if by magic as the returning sun once more warms up the soil and air. Wherever radiation is impeded by the shelter of trees, by artificial shelter, or by sufficiently dense masses of cloud, the temperature of the soil and air falls but little, and hence, under such circumstances, dew does not fall.

For practical purposes there are no better hygrometers than those that depend on the hygroscopic properties of certain organic substances, such as hair and catgut, and it has been shown by Sresnewsky that the alteration in length of a hair caused by its absorption of moisture is directly proportional to the natural logarithm of the degree of relative humidity, so that such instruments can be graduated for use as scientific instruments. Rapidity of evaporation is, however, proportional not to the relative humidity, but to the difference of the tension of watery vapour present in the air with that of its tension when saturated—in other words, the difference of tension of watery vapour at the temperatures of the dry and wet bulb thermometers, a form of expression which admits of degrees of humidity at different temperatures to be directly compared. In practice, however, this datum is rarely to be found in climatic tables, which is of the less importance, as in its effects on our organisation a difference of 2 or 3 mm. of mercury, from the pressure of saturation at a low temperature, will give a pleasant sensation of dryness, while at a high temperature the same deficit of pressure would be felt intolerably close and sultry. Extreme conditions of either humidity or dryness are, of course, alike unhealthy, though much of the respiratory irritation ascribed to too dry air is, I believe, more truly referable to the dust which usually accompanies such atmospheric conditions; but in any case, there can be no doubt that alike in hot and cold climates it is far healthier for the air to be too dry than too moist. With the effects of damp cold we are all of us only too well acquainted in England, and those who have experienced the effects of damp heat will never need being reminded of its debilitating effects. Fortunately, however, relative humidities exceeding 80 per cent. are but rarely to be found accompanied with really high air temperatures, and are seldom met with except in localities blessed with a copious rainfall, which by cooling the air goes far to render matters tolerable.

The most trying of all climates, however, are those where high temperatures and relative humidity are combined with an absence of rain, and under such circumstances a relative humidity of far less than 80 per cent. gives rise to intolerable closeness and oppression, especially when combined with stillness of air. Typical examples are the autumnal climates of the Red Sea and Persian Gulf, the unbearable character of which is notorious. At Abusher, in the Gulf, for example, in the month of August rain never falls, there is little or no breeze, and the mean maximum temperature is 96·5° F. (35·7° C.), while the relative humidity averages 65 per cent., and though neither figure separately is remarkably high as compared with what may be met with elsewhere, the entire combination of conditions is generally admitted to constitute one of the most unendurable climates in the world. On the other hand, in the Algerian Sahara in the summer months the relative humidity may fall as low as 16 per cent., but provided that an unstinted supply of water, to supply the loss by evaporation, be obtainable, most people find crisp, dry heat of this sort rather stimulating than otherwise, and even where the temperature is so high as to become most trying to endurance, the mortality returns of such situations show that dry heat is really favourable to health. The reason of this is obviously found in the fact that a few hours’ exposure to the sun’s rays in such climates suffices to kill the germs of nearly all specific contagious diseases, and that the breeding of mosquitoes, which are now known to be the carriers of several of the most important and deadly of tropical diseases, is further summarily stopped. A further contributory reason is also found in the fact, that the population is driven to sleep in the open air instead of within more or less ill-ventilated houses, and hence obtains the inestimable benefit of the freest possible ventilation during a large portion of the twenty-four hours, besides reducing the chances of the direct infection of the healthy by the sick to a minimum. As a degree of relative humidity of the air so low as to be in itself irritating to the respiratory mucous membranes is almost unknown, we may practically consider that dryness of climate is everywhere synonymous with healthiness.

Effects of Amount and Distribution of Rainfall.

—As already remarked, it is quite possible for a climate to be damp and yet have little or no rainfall, but such instances are rare, and on the other hand, a heavy rainfall necessarily brings about a coincident increase of relative humidity. Rain is, moreover, necessarily combined with a cloudy sky, whereby the heating of the soil during the day and its cooling by radiation during the night are alike impeded. The immediate effect of a shower of rain is to cool the air, and this for a double reason: first, coming as they do from the higher strata of the atmosphere, the temperature of the raindrops is necessarily much lower than that of the earth’s surface; and secondly, from the multiplied surfaces of the descending drops and from the wetted earth there necessarily occurs a rapid evaporation, whereby a further large amount of heat is absorbed, but unless showers recur at sufficient intervals to continuously diminish air temperature, the temporary remission is apt to be dearly paid for by a period of heat combined with high relative humidity, with its attendant discomforts of reduced evaporation from the surface of the body, prickly heat, and the other discomforts inseparable from tropical damp.

The influence of rainfall on health necessarily depends to a great extent on the configuration of the land, but assuming that the latter admits of adequate drainage, more depends upon its distribution than upon its amount; for the sanitary influences of rain are, in the main, mechanical, and depend on the “laying” of the dust and the washing away of infective and otherwise deleterious material. A heavy shower of sufficient duration will carry away, viâ the river to the sea, the deleterious products of human occupation during a preceding drought; but to do this the rain must be heavy while it lasts, for a prolonged drizzle in a warm climate simply turns the soil into a particularly efficient cultivation ground for the germs of infective diseases, and the attendant gloom of the sky stops entirely the beneficent germ-killing power of the sun’s direct rays. A prolonged drizzle, never exceeding the absorbtive and drainage capacities of a given site, marks the maximum of unhealthiness in all climates, and is possibly even more obnoxious when associated with heat than with cold: so that the most pleasant tropical climates are those that combine frequent short but heavy showers with intervals of bright sunlight, a continuously overcast sky being everywhere unfavourable to health.

To judge, then, the influence of rainfall on health, we require three data—the total rainfall, the number of rainy days, and the aspect of the sky in any given season of the year; for the beneficent influence of light on the animal organisation is at least as marked as it is on plants, though while the latter fact is a matter of common observation, the former does not meet with the recognition which it deserves. We bleach our celery by protecting it from the light, but are apt to forget that, while the consequent reduction in the amount of its characteristic essential oil makes its eatable, the plant could hardly survive but for the application of lime and other artificial antiseptics, which we are obliged to apply to make up for the lack of the natural protection. Whether the process of “earthing up” be soothing to a celery plant or otherwise is a question of which we have no means of judging, but there can be no doubt that in such matters man is far more practical in the treatment of plants than of himself, and that in tropical climates he often suffers by shrinking too much from the immediate effects of the sun’s rays. In this as in all other affairs, moderation is, of course, desirable, but the commoner mistake is undoubtedly to shirk too much all exposure to the sun, whereas those whose avocations take them most into the open are generally the healthiest in the Tropics as elsewhere. Contrast the ardent sportsman who spends the broiling days of May and June in the pursuit of large game, with the lady who spends her days in a darkened bungalow, and there can be no question as to which suffers the most from “the effects of climate”; nor is the difference, as is often suggested, purely one of sex, for it will be noticed that female medical practitioners and missionaries and other ladies whose occupations involve their being much in the open are commonly at least as healthy as men similarly situated.

In all hot countries the period of the rains is the sickly season, but this is due not so much to any direct evil effects of damp on the human system as to the fact that the agents and carriers of disease, i.e., low plant organisms and mosquitoes and other suctorial insects, find in heat moisture and puddles the conditions that best favour their growth and multiplication; in other words, the unhealthiness of this season can be largely obviated by suitable measures of sanitation, so designed as to impede this growth and multiplication of noxious agencies in the immediate vicinity of human habitations.

Effect of Winds on Health.

—Save only in so far as it necessarily raises and transports dust, and that the latter may consist not only of mineral particles but may contain also deleterious organic matter and the germs of certain diseases, the action of wind, being equivalent to so much the freer ventilation, may always be considered desirable in hot climates. Given a free current of air, the highest air temperatures are borne with comparative comfort, whereas in stagnant air the sense of oppression is unbearable. The existence of a steady breeze from a known direction also makes it possible to artificially cool houses by placing in the doorway facing the direction of the wind wetted mats, which cool the air passing through them by the agency of evaporation. It further makes it possible to live in comfort without the use of punkahs and other artificial means of keeping up a free current of air; indeed, as a matter of fact, the habitability of places situated in the Tropics depends largely on the amount and continuity of the breeze.

As has already been remarked, the amount of dust present in the atmosphere depends mainly, in the first place, on dryness of the air, and in the second on the force of wind; but it is also a fact that under certain conditions, dependent probably on electrical manifestations, a very still atmosphere may yet carry in suspension a large amount of dust, and its presence may become inimical to health by causing irritation to the respiratory organs as well as to the eyes and lining membrane of the nostrils. This is specially liable to be the case when the suspended particles are sharp and angular, as in the case of the micaceous dust, with which, during the dry, hot weather, the air is often loaded in certain sub-Himalayan stations, producing in many persons soreness of the eyes and troublesome, dry cough.

Systematic observations on the amount of solid matter suspended in the air are as yet entirely wanting, but there can be little doubt that they would, if available, be valuable to the student of public health; for the high mortality among tradesfolk whose occupations involve the respiration of a constantly dusty atmosphere is thoroughly well known, and it is most improbable that what is true of dusty trades is not also true of dusty places.

Although generally admitted, especially as an article of popular belief, the influence of the varying electrical states of the atmosphere is as yet so ill-understood that nothing definite can be stated on the subject.

SECTION II. On the Special Characteristics of the Climates of Certain Hot Countries.

There is perhaps no one topic of human conversation that comes in for more discussion than that of the weather, and yet it is no exaggeration to say that there is no other as to which exact information is so scanty or so little accessible, and it is believed that even the following scanty notes are more complete than can be found in any one work in the English language.

If the intending emigrant to foreign parts desires to find out something of the conditions under which he will find himself, he may perhaps, after much trouble, unearth some undigested data on the subject from the Agents-General of Colonies, &c.; but he will find it far easier to ascertain the amount of piece goods bought by the “Borrioula Gah” tribe than the mean temperature of their capital; and, practically speaking, the only library containing anything approaching an adequate collection of the literature of the subject is that of the Royal Meteorological Society, to whom and to their courteous librarian, Mr. W. Marriott, my best thanks are due for their ready assistance in the compilation of the present chapter. Owing to the very varied sources from which the information has been drawn, any attempt at close uniformity of treatment is out of the question, but wherever possible the data given comprise the average mean, mean maximum and mean minimum temperatures, the rainfall, number of rainy days, and average relative humidity for each month of the year.

When possible, the figures given are the averages of several years, but in many cases they refer to a single year only, being derived from isolated observations or from series which have not as yet been collated by a meteorological expert. No barometric data have been included, as they have little interest for any but specialists, and a study of those given will be found quite sufficient to enable anyone to judge with what sort of an outfit he should provide himself. It will be, for example, quite obvious that it is quite useless to take a mackintosh coat to Wadi Halfa, if anyone will glance at the table given for that favoured (?) locality. In some cases the maximum and minimum temperatures tabulated are not the mean, but the absolute maxima and minima, and therefore represent only exceptional experiences and not what one may fairly expect. It must be clearly understood, too, that absolute and mean data are in no sense comparable, as the latter will always lie several degrees within the former; but in the present state of meteorological science one has to be thankful for what one can get, and this, in the case of most of the less advanced countries, is remarkably little.

The plan of the following notes is to make a sort of climatic tour round the globe, but it is obvious that it is impossible within the scope of a work like the present to describe more than a few widely distant examples, so that it is not even possible to include all our tropical colonies; but it is hoped that those given will suffice to give a general idea of what may be expected in most parts of the world. With few exceptions all data are given both in the English and in the metric systems.

The Mediterranean Basin.

—None of the countries comprising the European shores of this basin can be said to come well under the heading of “hot countries,” and those on the African shore are of interest rather as winter health resorts than as tropical places of residence. The first of these that requires notice is—

Algeria.

—Situated in lat. 37° N., within a couple of days’ steaming from Marseilles, this pleasant French colony is much the most easily accessible sub-tropical health station for the whole of western Europe, and forms an excellent resort for persons who find themselves unable to withstand the rigours of a northern winter. Thanks to the French talent in municipal organisation, the traveller finds himself at once among the novel sights and sounds of civilisation of the Oriental type, and yet surrounded with all the comforts and amenities of a fine European town. Few health-seekers venture far from the coast, though there must be many places in the interior that would be well suited for early cases of consumption, as the air of the coast is perhaps too “relaxing” for some cases of the sort. For the north coast of Africa, the rainfall is considerable, but in spite of this, on the average, the air is generally dry, as evidenced by the low average relative humidity.

The country may be divided into four zones, which present great differences of climate.

(1) A narrow littoral zone of low ground, often only a few miles wide. Most of the ports face eastward and are well sheltered by the neighbouring hills.

(2) The Tel, composed of plains and elevated mountain masses cut up by deep valleys, at the bottom of which are torrents which are dry for the greater part of the year.

(3) A high plateau of triangular form about 140 miles wide; intensely dry, but with scattered salt marshes which dry up during the summer.

(4) The Sahara, an immense sandy basin absolutely devoid of water-courses except quite to the north.

The following table of mean temperatures will give some idea of the degree of heat that is met with throughout the year in these different regions:—

Monthly Mean Temperature of Algerian Stations Situated in the Four Above-mentioned Zones.

Station E.F. E.M. January February March April May June

F. C. F. C. F. C. F. C. F. C. F. C.

Algiers (on the coast) 68 22 53·8 12·1 54·6 12·6 57·0 13·9 61·4 16·3 66·1 19·0 72·2 22·3

Orleansville (on the low “Tel”) 390 119 48·3 9·1 50·2 10·1 54·2 12·3 61·7 16·3 68·4 20·2 77·9 25·5

Térriet el Haad (on the high “Tel”) 3,700 1,125 41·0 5·0 43·0 6·1 47·5 8·6 52·3 11·2 59·6 15·4 69·8 21·0

Géryville (on the high plateau) 4,300 1,310 37·5 3·1 40·2 4·5 45·5 7·5 52·8 11·6 63·8 17·7 74·0 22·2

Biskra (Sahara) 410 125 50·2 10·1 54·3 12·4 57·0 13·9 66·0 18·9 76·0 24·4 84·4 29·1

Station July August September October November December Year

F. C. F. C. F. C. F. C. F. C. F. C. F. C.

Algiers 76·0 24·4 77·0 25·0 74·2 23·4 67·5 19·7 60·5 15·8 54·9 12·7 64·5 18·1

Orleansville 85·0 29·4 85·7 29·8 77·2 25·1 66·7 19·3 56·7 13·7 49·9 9·9 65·2 18·4

Térriet el Haad 77·4 25·2 78·7 25·9 68·0 20·0 58·3 14·6 49·9 9·9 43·5 6·4 57·3 14·1

Géryville 79·7 26·5 79·7 26·5 68·5 20·3 56·7 13·7 46·5 8·1 39·8 4·3 56·7 13·7

Biskra 90·0 32·2 90·0 32·2 80·2 26·8 68·0 20·0 57·7 14·3 51·5 10·8 68·5 20·3

Monthly Mean Temperature of Algerian Stations Situated in the Four Above-mentioned Zones.

Station E.F. E.M. January February March April May June July August September October November December Year

F. C. F. C. F. C. F. C. F. C. F. C. F. C. F. C. F. C. F. C. F. C. F. C. F. C.

Algiers (on the coast) 68 22 53·8 12·1 54·6 12·6 57·0 13·9 61·4 16·3 66·1 19·0 72·2 22·3 76·0 24·4 77·0 25·0 74·2 23·4 67·5 19·7 60·5 15·8 54·9 12·7 64·5 18·1

Orleansville (on the low “Tel”) 390 119 48·3 9·1 50·2 10·1 54·2 12·3 61·7 16·3 68·4 20·2 77·9 25·5 85·0 29·4 85·7 29·8 77·2 25·1 66·7 19·3 56·7 13·7 49·9 9·9 65·2 18·4

Térriet el Haad (on the high “Tel”) 3,700 1,125 41·0 5·0 43·0 6·1 47·5 8·6 52·3 11·2 59·6 15·4 69·8 21·0 77·4 25·2 78·7 25·9 68·0 20·0 58·3 14·6 49·9 9·9 43·5 6·4 57·3 14·1

Géryville (on the high plateau) 4,300 1,310 37·5 3·1 40·2 4·5 45·5 7·5 52·8 11·6 63·8 17·7 74·0 22·2 79·7 26·5 79·7 26·5 68·5 20·3 56·7 13·7 46·5 8·1 39·8 4·3 56·7 13·7

Biskra (Sahara) 410 125 50·2 10·1 54·3 12·4 57·0 13·9 66·0 18·9 76·0 24·4 84·4 29·1 90·0 32·2 90·0 32·2 80·2 26·8 68·0 20·0 57·7 14·3 51·5 10·8 68·5 20·3

Note.—In this and all following tables, F. stands for degrees Fahrenheit; C. for degrees Centigrade; E.F., elevation above the sea in feet; E.M., the same expressed in metres; Ins., inches, English; Mm., millimetres.

The character of the climate of the capital, which may be considered typical of the coast health resorts, may be gathered from the following table:—

Algiers. Lat. 33° 47′ N.; Long. 0° 44′ E. E.F. 105; E.M. 33·5.

Month Mean Temperature Mean Maximum Temperature Mean Minimum Temperature Relative Humidity Rainfall Number of Rainy Days

F. C. F. C. F. C. Ins. Mm.

January 56·0 13·3 62·7 17·0 50·2 10·1 60 3·25 82·5 16

February 61·3 16·3 70·2 21·2 54·8 12·7 61 3·17 80·6 9

March 57·2 14·0 64·5 18·1 51·5 10·8 62 3·14 79·1 15

April 60·5 15·8 67·8 19·9 54·2 12·3 65 3·46 88·4 10

May 66·7 19·3 74·7 23·7 60·2 15·7 62 3·53 89·2 6

June 72·5 22·5 79·7 26·5 66·7 19·3 71 0·75 19·1 6

July 75·8 24·1 83·0 28·3 68·5 20·3 68 0·70 17·2 2

August 77·0 25·0 84·3 29·1 70·0 21·1 67 0·20 4·9 4

September 75·5 24·2 83·2 28·4 70·3 21·3 73 0·28 7·1 9

October 71·2 21·8 79·2 26·2 66·0 18·9 72 1·52 38·6 11

November 59·8 15·5 67·3 19·6 55·7 13·2 70 9·0 228·3 22

December 56·8 13·8 67·5 19·7 51·7 10·9 69 1·65 42·0 7

Year 65·8 18·8 73·2 22·9 59·8 15·5 66·7 30·5 777·0 117

From this it may be seen that Algiers possesses a very desirable climate all the year round, though a trifle too rainy in late autumn.

The figures are those of the year 1901, as the French Government does not appear to have furnished the library with mean normal results.

Malta.

—Owing to the large number of our officers and men serving there, the climate of this small island, which is delightful during the winter, is of interest to many, but it perhaps barely merits the name of a hot climate. While resembling in some respects the climate of southern Italy, it approximates more closely to that of northern Africa, the rainfall being very scanty.

The following are the principal climatic data:—

Valetta. Lat. 35° 54′ N.; Long. 14° 30′ E.; near Sea Level.

Months Mean Maximum Temperature Mean Minimum Temperature Relative Humidity Rainfall Remarks

F. C. F. C. % Ins. Mm.

January 60·2 15·7 50·1 10·0 78 5·51 139·7 Total Annual Rainfall, 17·1 ins., or 40·3 cm.

February 63·0 17·2 50·3 10·2 79 1·04 26·2

March 63·4 16·8 49·8 9·9 73 1·04 26·2

April 65·6 18·5 52·4 11·3 78 2·18 55·2

May 72·3 22·4 59·1 15·0 78 0·55 14·0

June 80·7 27·0 64·9 18·3 72 0·38 10·0

July 88·0 31·1 70·6 21·3 70 0·00 0·0

August 84·9 29·4 70·7 21·4 77 0·02 0·1

September 83·3 28·5 68·7 20·4 77 0·10 2·5

October 81·7 27·5 67·3 19·8 78 0·60 15·2

November 71·1 21·7 59·0 15·0 83 3·64 92·5

December 62·6 16·8 52·2 11·2 85 1·04 26·2

During the summer, periods of hot dry winds blowing from the burning African deserts are somewhat trying, but these do not, as a rule, last for many consecutive days, and on the whole the climate is not unhealthy.

A peculiar infective fever, commonly known as “Malta fever,” but also met with in other parts of the Mediterranean littoral, as well as in India, and very troublesome on account of its obstinate tendency to relapse, is the most serious drawback in the matter of disease, but under improving modern sanitation the disease is yearly becoming less common.

The climate of the Syrian coast is very similar, but somewhat warmer, with a heavier rainfall; so also is that of Algiers and the North African coast generally, there being, however, mostly a wider range between the hottest and coldest months than in the purely insular Malta. Malaria, dysentery, and other tropical diseases are not uncommon, but seldom either widely spread or particularly virulent. As we leave the coast, the range of temperature, both annual and diurnal, rapidly increases, and is especially marked in the highlands of Asia Minor, as, for example, at Erzerum, where the temperature in January falls as low as -20° F. (-29° C.), and in summer may exceed 90° F. (31° C.).

Cyprus.

—Long. 32° 20′ to 34° 35′ E; lat. 34° 33′ to 35° 41′ N. Our administrative connection with this island, and the circumstance that it has of late been strongly recommended as a fairly stimulating winter health resort for delicate people, and especially for cases of chest diseases and others, make it desirable to include an account of it within our list.

The following are the principal climatic data:—

Nikosia, on Central Plain.

Months Mean Maximum Temperature Mean Minimum Temperature Relative Humidity Rainfall Number of Rainy Days

F. C. F. C. % Ins. Mm.

January 73·4 23·0 32·3 0·2 84 4·0 101 11·8

February 70·7 21·5 31·8 0·1 84 3·70 94 11·8

March 76·5 24·7 36·4 2·4 81 1·23 31 7·4

April 84·5 29·2 38·8 3·8 78 1·14 29 4·5

May 90·4 32·4 46·2 7·9 74 0·63 16 4·3

June 100·0 37·8 52·4 11·3 67 0·39 10 1·4

July 100·7 38·3 55·5 13·1 68 0·13 3 0·3

August 103·0 39·4 57·2 14·0 66 0·8 20 0·5

September 100·5 38·1 54·4 12·3 73 0·04 1 0·6

October 93·5 34·2 47·7 8·7 76 0·36 9 2·3

November 84·5 29·0 39·3 4·1 82 1·97 50 6·5

December 77·5 25·3 34·0 1·1 85 2·31 59 7·8

The climate is somewhat cooler in summer on the coast, and the rainfall slightly higher, but with a smaller number of rainy days.

The island is mountainous; a great mass of hills occupying the greater part of the southern half, and reaching an elevation of over 6,400 feet at Mount Troödos, where a summer sanatorium has been established. A lower range of hills fringes the entire northern shore, and between the two is a central plain, on the highest part of which, at an elevation of about 500 feet, is situated the capital, Nikosia. These hills shelter the central plain from the bitter winds of the Taurus range in Asia Minor during winter, but, on the other hand, cut off the cooling sea-breezes in summer. For eight months in the year the rainfall is inappreciable, and the summer appearance of the plains arid in the extreme, but the advent of the winter rains in October changes the scene to one of the greatest fertility. The hill country, on the other hand, is generally well-clothed with pine-forests, and enjoys, during the summer, a very pleasant climate. At Nikosia the mean temperature of the year is 67·2°, the extreme maximum being 108°, and the minimum 28°, showing the large range of 80°.

At the hill station on Mount Troödos, at an elevation of 5,000 feet, the excessive heat of the plains is entirely avoided. The season there opens in June and closes in October, the temperature never exceeding 85° F. in 1901. The following table shows the great gain in coolness:—

Mean Temperatures, Fahrenheit.

1901 Nikosia Troödos Difference

June 77·2° 61·7° 15·5°

July 83·7° 71·7° 12·0°

August 83·3° 68·2° 15·1°

September 78·8° 61·6° 17·2°

During the greater part of the year the wind is usually from the north-west, but during the coldest part of the winter is usually from the east. It will be noticed that the general characters of the climate are rather those of a Continental than of an insular situation. The air is almost always highly charged with electricity, and there is comparatively little malaria or other specially tropical diseases.

Egypt.

—The climate of this country is, even from the all-the-year-round point of view, one of the finest in the world, and hence its well-deserved popularity as a winter health resort. Apart from the “Khamseen,” which those used to Indian hot weather might esteem a change for the better, its one drawback is what may be termed the co-efficient of rapacity of its hotel-keepers; as for the casual visitor, it is certainly an expensive country.

With endless sights of antiquarian interest, and a gay cosmopolitan society, there is little chance of boredom for either the studious or frivolous, and with the possible exception of California, no climate affords a better combination of warmth and sunlight, with a clean, stimulating atmosphere. It owes this to its peculiar geographical characters, for although it includes on the map a large area, the actually inhabited portion consists of only a narrow strip, a few miles wide, on either side of the Nile; as all the land beyond the reach of the annual overflow of the river, which is at its maximum in September, and lowest in June, is absolute desert, the intense dryness of which is necessarily fatal to all forms of vegetable life, including the organic germs of disease. On this account, even in the middle of the cultivated strip, the air has never been fouled by passing over any considerable extent of habitations and cultivation, with their inevitable emanations, but must always come, almost fresh and germ-free, from the illimitable expanse of sand and rock that immediately succeeds the narrow band of river alluvium.

Dr. Dalrymple, one of the earliest writers on the subject, remarks, “It is scarcely possible to imagine anything more invigorating and life-giving than the air of the desert; there is a dryness and elasticity about it like nothing else, and the sense of renovation when breathing it is, to the languid invalid, like a new lease of life.”

Both he and Dr. Sandwith, from whose excellent “Egypt as a Winter Resort” the following tables are taken, seem agreed as to its suitability for all cases of chest disease that have not gone too far to be amendable to climatic treatment of any sort, and Dr. Sandwith finds it also suitable for such cases of heart and kidney affections as are unfavourably affected by cold; while the sulphur baths of Helouan have been found remarkably useful in the chronic forms of rheumatoid arthritis, rheumatism and gout.

The main characteristic of the climate is its intense dryness. Even on the coast at Alexandria, the rainfall is but trifling, and above Cairo it may almost be neglected; but in spite of this even during the hottest months the climate is quite bearable. “Northern rooms, if closed in good time, need never exceed 83° in the hot weather, or fall below 52° in the cold season, provided the sun-warmed air be allowed free entry.” The prevailing wind is a gentle breeze from the north, but both at Cairo and Alexandria, during the fifty days about Easter-time, a peculiar dust-laden wind, highly charged with electricity, and known as “the Khamseen,” blows at intervals. It is very disagreeable while it lasts, the dust sometimes obscuring the sun almost as completely as a London fog, but it rarely persists more than two days at a time, and does not usually occur more than three or four times in a season. The following tables will give a good general idea of the climate:—

Alexandria. Lat. 31° 13′ N.; Long. 26° 53′ E. E.F., 66 ft.

Month Temperature, F. Relative Humidity % Rain in Inches Clouds, 0-10 Winds

Mean Mean Maxima Mean Minima Direc- tion Force, 0-10

January 58·1 64·0 53·2 67 2·33 4 N. 2·5

February 58·6 64·2 54·0 65 1·43 4 N.W. 2·5

March 61·6 68·0 56·0 65 ·78 3 N.W. 2·7

April 66·0 73·0 60·6 66 ·12 2 N. 2·5

May 70·0 75·4 65·6 70 ·03 2 N. 2·2

June 75·0 79·6 71·2 72 — 1 N. 2·3

July 77·6 81·2 74·8 75 — 1 N.N.W. 2·4

August 79·0 82·4 76·1 73 — 1 N. 2·0

September 77·4 81·2 74·3 69 ·11 2 N. 2·3

October 74·6 79·2 70·6 68 ·33 2 N. 2·1

November 68·2 73·4 64·0 67 1·32 3 N. 2·2

December 62·0 67·8 57·0 67 1·79 4 N. 2·4

Annual 69·0 74·1 64·8 68·6 8·24 2·4 N. 2·3

Cairo. Lat. 30° 4′ N.; Long. 31° 15′ E. E.F., 108.

Month Temperature, F. Relative Humidity % Rain in Inches Clouds, 0-10 Wind

Mean Mean Maxima Mean Minima Direc- tion Force, 0-10

January 53·6 61·4 46·6 69·7 ·19 4·1 S.W. 2·2

February 57·0 65·3 48·8 66·2 ·24 4·2 N. 1·4

March 62·8 73·2 53·0 56·2 ·03 3·4 N. 2·5

April 70·4 81·2 59·9 47·8 ·12 3·4 N. 2·6

May 75·2 86·8 63·4 48·4 ·22 2·3 N. 2·8

June 82·6 94·7 70·2 44·0 ·02 1·0 N. 3·0

July 83·8 93·0 72·2 49·0 — 1·2 N. 4·3

August 82·2 92·9 71·4 55·3 — 1·6 N. 4·1

September 77·8 87·5 68·0 62·1 — 1·8 N. 4·3

October 74·3 84·0 64·8 65·8 — 2·5 N. 3·2

November 64·4 74·2 56·3 67·5 ·21 3·0 N. 2·1

December 58·4 67·7 50·4 69·6 ·19 3·7 N. 2·2

Year 70·2 80·1 60·4 58·46 1·22 2·6 N. 2·9

Luxor. Lat. 25° 40′ N.; Long. 32° 35′ E. E.F., 292. Winter Climate.

Month Temperature, F. Relative Humidity Rain in Inches Clouds, 0-10 Wind

Mean Mean of Maxima Mean of Minima % Direc- tion Force

November — 78·99 62·1 — N i l . — S.W. 1·0

December — 70·0 53·6 — — N.E. 1·8

January 56·7 65·1 41·3 53·2 2·9 N.W. 1·0

February 62·6 70·6 42·4 51·0 1·9 N.W. 1·1

March 66·9 80·1 47·6 45·0 2·1 - N.W. - 0·7

N.E.

Assouan

, 133 miles further up the Nile, has a winter climate which is said to be almost 5° F. higher than Luxor, and to be freer from dust-storms. The building of the great dam and the large artificial lake which has been thus formed can hardly fail to modify the climate, so that it is hardly worth while reproducing Dr. Sandwith’s table.

For the rest, the whole of Egypt up to this latitude may be said to be very healthy for Europeans, there being much less malaria than one would be inclined to expect as a consequence of the annual overflowing of the Nile; a circumstance which may, I believe, be attributed to the generally neat character of the cultivation, and the care with which every square yard of cultivable soil is utilised. Internal worms are, however, extremely common amongst the natives, but care in the matter of drinking water will render the risk run by the European very trifling.

Another terribly common disease amongst the natives is granular ophthalmia, a disease easily acquired by contact, or indirectly through the agency of flies, but Dr. Sandwith finds that it is extremely rarely taken by Europeans, though soreness of the eyes from dust and glare is not uncommon, for which he recommends the daily use of a little boracic solution as a toilet wash. It might, however, be perhaps advisable to protect young children by means of a veil when abroad, especially when entrusted to the care of a native attendant.

The Remainder of the African Continent.

—Our knowledge of African meteorology is necessarily in its infancy, and it is only possible to furnish a few tables of widely distant parts of this immense area, which may give some idea of its climatic characters. The whole of the northern part of the continent is extremely dry, much of it being quite rainless, especially towards its eastern side. Following on this is the equatorial belt, a great portion of which is barely, if at all, explored, but which seems to usually present the general characters of such latitudes and to be usually blessed with an ample rainfall across the entire width of the continent. Southward of the equatorial zone we again meet with immense dry and desert areas, such as the Karoo, but here it is the western side that is the more arid, none of the south-eastern coast being in any sense rainless.

Malaria is extremely rife in almost all parts of the huge peninsula, and in addition to this we have in “Blackwater fever” and sleeping sickness, diseases which seem to be at present its own peculiar privilege, though, fortunately, yellow fever is not as yet included in its list of dangers. The low-lying country along the coast and of the great rivers is notoriously unhealthy, such as the Bight of Benin, where, according to the sailor’s proverb,

But inland there are considerable tracts of elevated country which present climates by no means to be despised, and which will no doubt in time, with the advance of civilisation, become eligible and healthy sites for European occupation, and more than one example of climatic conditions that appear decidedly inviting will be found amongst the tables furnished below.

An example of the climate of the Sahara has already been furnished in the notice of Algeria, and in the northern part of the continent the only other at all well-known climate (apart from Egypt) is the Soudan.

Practically rainless in parts, the climate is intensely hot and dry, the relative humidity showing one of the lowest records in our collection; but as the equatorial zone is approached a moderate rainfall develops, and throughout the region the large daily range of temperature results in the nights being comparatively cool, and therefore less trying than many parts of India. The northern portion is too dry to be very unhealthy, apart from the danger of abdominal chills, but as we ascend the Nile it expands into the immense marshes described by Baker and others, which are necessarily intensely malarious, while round the great lakes the sleeping sickness, previously rare or unknown there, is spreading rapidly.

Commencing with the dry Soudanese region, climatic tables of three stations will be found below, the first and most northern of which, it will be seen, is practically rainless. The figures of the first table are averages of ten years’ observations.

Wadi Halfa. Lat. 21° 55′ N.; Long. 31° 19′ E. E.F., 590; E.M., 128.

Month Mean Temperature Mean Maximum Temperature Mean Minimum Temperature Relative Humidity Rainfall

F. C. F. C. F. C. At 9 h. At 21 h.

January 59·4 15·2 73.7 23·2 47·5 8·6 42 44 Prac- tically nil. Drops re- corded 15 times in 10 years

February 62·6 17·0 77·2 25·1 48·4 9·1 39 36

March 71·5 21·9 87·5 30·8 56·5 13·6 30 30

April 80·5 26·9 95·8 35·5 63·7 17·6 23 24

May 87·8 31·0 104·3 40·2 70·5 21·4 17 19

June 90·7 32·6 106·7 41·6 74·4 23·5 20 22

July 90·5 32·5 105·7 40·9 74·5 23·6 23 30

August 90·0 32·2 103·5 39·7 75·0 23·9 31 34

September 88·7 31·4 99·6 37·6 73·5 23·1 35 38

October 82·5 28·1 97·2 36·2 69·0 20·6 37 10

November 70·7 21·8 85·0 29·4 58·7 14·8 41 43

December 63·0 17·2 76·6 24·7 51·0 10·6 45 45

Year 78·0 25·6 92·9 33·8 63·5 17·8 — —

The two remaining tables are for the year 1901, and are not even complete in places. Kassala is, of course, much nearer the sea than Omdurman, and hence its better rainfall.

Omdurman. Lat. 15° 38′; Long. 32° 29′. E.F., 1,250; E.M., 376.

Month Mean Temperature Mean Maximum Temperature Mean Minimum Temperature Relative Humidity Rainfall

F. C. F. C. F. C. Ins. Mm.

January 72·9 22·7 88·5 31·4 62·6 17·0 24 — —

February 79·9 26·3 94·5 34·7 70·2 21·2 28 — —

March 82·7 28·2 99·7 37·6 — — 19 — —

April 87·5 30·8 103·7 39·8 72·5 22·5 14 — —

May 93·0 33·9 112·0 42·4 79·0 26·1 20 — —

June 89·9 32·1 106·2 41·2 77·4 25·2 46 0·63 16·1

July 90·4 32·4 103·9 39·9 79·3 26·3 48 0·50 12·8

August 87·0 30·6 99·0 37·2 77·7 25·5 58 0·13 3·3

September 90·4 32·4 104·5 40·3 81·0 27·2 40 — —

October 89·9 32·1 103·2 39·5 79·5 26·4 30 0·32 8·0

November 82·4 28·0 96·7 35·9 72·4 22·4 24 — —

December 76·2 24·5 91·2 32·9 65·5 18·6 28 — —

Year 85·2 29·5 99·9 37·7 74·4 23·5 32 1·58 40·2

Kassala. Lat. 15° 28′ N.; Long. 36° 24′ E.

Month Mean Temperature Mean Maximum Temperature Mean Minimum Temperature Relative Humidity Rainfall

F. C. F. C. F. C. Ins. Mm.

January 72·6 22·6 86·4 30·2 58·9 14·9 — Drops

February — — 95·6 35·4 — — — — —

March 82·0 27·8 100·5 38·1 68·1 20·1 51 — —

April 86·4 30·2 103·3 39·6 73·0 22·8 32 0·08 2·0

May 89·6 32·0 106·2 41·2 76·9 24·9 25 0·25 6·4

June 86·0 30·0 101·2 38·4 75·5 24·2 40 2·85 72·4

July 83·7 28·7 96·9 36·0 74·2 23·4 59 1·48 37·4

August 79·8 26·6 92·5 33·6 81·8 27·7 64 3·96 100·6

September — — 99·8 37·7 75·4 24·1 — 1·24 31·3

October — — 101·4 38·5 76·5 24·7 — — —

November — — 99·2 37·3 73·5 23·1 — — —

December — — — — — — — — —

Year 83·0 28·3 98·5 36·9 — — 40 9·84 250·1

Abyssinia.

—The greater part of the Ethiopian empire is high ground, and in many parts of the country the climate may almost be described as temperate, as may be judged from the following statement (derived from a French official source) of the climatic factors of the capital during the year 1901.

Adis-Ababa. Lat. 9° 1′ N.; Long. 38° 43′ E. E.F., about 7,000.

Month Mean Temperature Mean Maximum Temperature Mean Minimum Temperature Relative Humidity Rainfall Number of Rainy Days

F. C. F. C. F. C. Ins. Mm.

January — — — — — — — — — —

February 55·4 18·0 73·4 23·0 47·9 8·8 81 2·15 54·6 8

March 58·0 14·4 75·7 24·3 50·4 10·2 83 5·23 132·7 10

April 58·4 14·6 74·2 23·3 51·4 10·7 86 4·39 111·1 11

May 60·9 16·0 78·5 25·8 52·7 11·5 74 1·36 34·7 4

June 57·5 14·2 72·3 22·4 49·7 9·8 72 8·36 212·3 30

July 56·0 13·3 69·0 20·6 49·7 9·8 80 10·10 256·5 30

August 60·2 15·7 69·0 20·6 50·0 10·0 79 9·46 240·1 28

September 63·0 17·2 73·5 23·1 49·8 9·9 66 5·48 139·2 14

October 61·5 16·4 77·2 25·1 46·5 8·1 37 0·60 15·2 3

November 60·0 15·6 77·0 25·0 44·5 6·9 28 — — —

December 57·2 14·0 74·4 23·5 44·3 6·7 39 0·54 13·5 3

Year 58·4 14·6 72·2 23·6 48·7 9·3 66 49·11 1247·5 141

The Region of the Great Lakes.

—Turning now to the great lakes of Central Africa, the following data from Hann’s “Klimatologie” may give some notion of the conditions prevailing.

In the Victoria Nyanza region (at E.F., 3,900; E.M., 1,200), according to E. G. Rauenstein, the mean annual temperature is 71·2° F. (21·8° C.), March 73·7° F. (23·2° C.), July 67·7° F. (19·8° C.), October 75·2° F. (24·0° C.), December 71·7° F. (22·0° C.). The extreme mean monthly maxima and minima (of January and February), 94° to 54° F. (34·4° to 12·2° C.), and the absolute maximum and minimum, 99·7° and 50·2° F. (37·7° and 10·1° C.).

The average rainfall for nine years was as follows:—

Jan. Feb. Mar. Apl. May June July Aug. Sept. Oct. Nov. Dec. Year

Ins. 2·5 3·3 3·7 4·8 4·2 3·4 2·5 3·3 4·8 4·8 5·3 3·2 40·6

Mm. 63 84 94 122 106 87 63 83 122 122 136 80 1,160

In the north (Uganda) the climate is warm and moist, though without excessive rainfall, but the air is a good deal drier along the southern coast.

In Tanganyika, in lat. 4° S., long. 29° E. (E.F., 2,670, E.M., 813), the mean annual temperature is 76·7° F. (24·8° C.), the mean of the hottest month (of October) being 81·7° F. (27·6° C.), and of the coolest, in December, 74·2° F. (23·4° C.), the extremes being 90·7° and 64·4° F. (32·6° and 18° C.). The rains occur in April and May and November and December, while from June to September is dry, the annual rainfall being 50 ins. (1,270 mm.).

Further south, Zoruba, in Nyassaland, has a total rainfall of 62·23 ins. (1,581 mm.), falling on 144 days in the year, but should otherwise be a pleasant climate, as the relative humidity is usually low.

Zoruba. Lat. 16° S. E.F., 1800. E.M., 548.

Month Temperature at 7 a.m. Mean Monthly Maxima Mean Monthly Minima Relative Humidity at 2 p.m. Rainfall Number of Rainy Days

F. C. F. C. F. C. Ins. Mm.

January 69·8 21 81·2 27·3 65·9 18·9 78 11·62 295 27

February 66·7 19·3 77·6 25·3 63·7 17·6 74 15·42 391 20

March 66·7 19·3 78·9 26·1 64·7 18·2 90 7·60 193 23

April 64·4 18·1 75·4 24·2 61·7 16·5 76 11·74 298·5 14

May 61·1 16·1 76·9 25·0 58·0 14·4 65 0·37 9·0 7

June 57·1 13·9 68·4 20·3 54·4 12·5 66 2·55 64·8 12

July 55·0 12·8 68·8 20·5 52·9 11·7 65 0·87 21·8 6

August 55·4 13·1 71·4 21·9 52·3 11·3 51 0·15 3·8 4

September 62·7 17·0 79·0 26·1 58·1 14·4 48 1·80 45·7 4

October 68·5 20·3 85·4 29·7 62·6 16·9 53 ·95 24·1 4

November 72·0 22·2 88·3 31·3 67·1 19·4 53 ·35 8·9 4

December 73·2 23·0 83·6 28·6 65·6 18·6 66 8·81 223·8 19

The capital, Blantyre, at E.F., 3,280, E.M., 1,000, is naturally even cooler, but has a heavier rainfall, distributed as below:—

Jan. Feb. Mar. Apl. May June July Aug. Sept. Oct. Nov. Dec.

Ins. 19·51 17·60 14·48 13·77 4·05 3·28 3·34 1·56 1·83 4·13 5·54 15·35

Mm. 484·6 447 368 349·5 102·9 83·7 85·1 39·5 46·8 105·3 140 390

The Congo Basin.

—The climate of the basin of the Congo is notoriously unhealthy, especially as the portion as yet opened up seldom extends far beyond the low malarious banks of the great river and its tributaries. The warmest month is February or March, and the coldest July or August.

Hann gives the following table of the temperatures of the following stations:—

Station Latitude Mean Annual Temperature Mean Temperature of Warmest Month Mean Temperature of Coldest Month

F. C. F. C. F. C.

Luluaberg 5·9 ° S. 81·3 27·4 81·7 27·6 80·8 27·1

Congo, mouth of 6·0 ° S. 76·8 24·9 80·5 26·9 70·9 21·6

Vivi 5·7 ° S. 77·2 25·1 80·5 26·9 71·7 22·0

San Salvador 6·3 ° S. 77·9 25·5 81·0 27·2 72·8 22·7

Brazzaville 4·3 ° S. 81·2 27·3 84·7 29·3 75·2 24·0

Bolobo 2·2 ° S. 80·5 26·9 81·5 27·5 79·4 26·3

Equatorville 0·0 79·1 26·2 80·6 27·0 78·2 25·7

Bangala 1·5 ° N. 78·9 26·0 80·8 27·1 77·7 25·4

These are typical equatorial climates, the greatest range between the means of the coldest and hottest months being at most 9·5° F. (5·3° C.), while in one case the range is less than one degree of the Fahrenheit scale, and though the temperatures are in no case excessive, the dampness of the atmosphere makes the heat of a very trying character, especially in certain localities, as at Stanley’s Pool, where during August and September there are certain peculiar night-winds which, not unfrequently, are the cause of cases of heat apoplexy, and this although the highest temperature recorded is but 97°, at Brazzaville.

There are two rainy seasons, in April and November. The dry period falls in June and July, but is not very marked in the interior.

The table on next page will give an idea of the amount and distribution of the rainfall.

On the West Coast

, at Bathurst, the highest temperature occurs in October, during the period of the greater rains, the annual extremes being 98·8° F. (37·1° C.) and 57·4° F. (14·1° C.); the greatest daily variation of temperature being met with during the dry season, January to April, when it amounts to about 20° F. (11·5° C.). The rainfall varied during eleven years from about 32 ins. (813 mm.) to 78 ins. (1,980 mm.). In December, a cool morning breeze known as the Harmattan sets in, and continues till February or March. The rains begin in June and end in September.

Monthly Rainfall in the Congo Basin.

Month Station.

Congo, Mouth of Lower Congo Bolobo

Ins. Mm. Ins. Mm. Ins. Mm.

January 3·70 94 2·93 74 5·0 127

February 3·12 79 3·87 98 6·97 177

March 4·09 104 4·73 120 4·61 117

April 3·86 98 8·87 225 7·17 182

May 2·98 76 2·84 72 5·64 143

June 0·23 6 0·19 5 0·39 10

July 0 0 0 0 0·04 1

August 0·08 2 0 0 2·60 66

September 0·16 4 0 0 3·98 101

October 0·48 12 2·17 55 6·54 166

November 3·95 100 8·31 211 9·58 243

December 2·28 58 4·64 118 10·80 260

Year 24·95 633 39·85 1,008 62·7 1,593

At Sierra Leone

, in spite of its lying north of the equator, the distribution of the monthly temperature resembles that of the Southern Hemisphere, the minimum falling in August, while the hot season lasts from February to May. The mean annual temperature is 77·6° F. (25·4° C.), and the average annual extreme temperatures 97·5° F. (36·4° C.) and 64·8° F. (18·2° C.). No month is absolutely rainless, and the annual rainfall is very heavy, ranging from 100 ins. (2,540 mm.) to 204·5 ins. (5,230 mm.).

Curiously enough, the Cape Verd Islands are very dry, although they agree generally as to temperature with the coast, tempered by their insular position and the influence of the trade wind, having a rainfall of only about 10 ins. (260 mm.), the most rainy month being September.

Gulf of Guinea.

—Along this coast there is a double rainy season; the greater from March to the end of July, and the lesser in October and November; with the dry season in August and September, and the cool Harmattan blowing between November and March. In speaking of the Harmattan as a cool breeze, it must be remembered that one refers only to the sensations produced by it; for as a matter of fact, it has no appreciable effect on the mean temperature, and feels cool only by virtue of the accelerated evaporation from the skin caused by its intense dryness. On the coast it really raises the mid-day temperatures, although it renders the mornings and evenings cooler. In the interior the Harmattan may figure indeed as a hot wind, and may be additionally disagreeable on account of the red dust it carries.

The rainfall is everywhere very heavy, that of the Cameroon district reaching the enormous figure of 350 ins. (8,970 mm.), the second greatest in the world.

The mean annual temperature is about 77° F. (25° C.), with annual extremes of 89·6° F. (32° C.) and 68° F. (20° C).

The following table of the climatic data for the coast of British Nigeria will give a fair idea of the conditions that will be met with in the coast towns of the group of colonies in this region:—

Old Calabar. Lat. 4° 58′ N.; Long. 8° 17′ E.

Month Mean Monthly Temperature Mean Monthly Maxima Mean Monthly Minima Relative Humidity Per cent. Rainfall Number of Rainy Days

F. C. F. C. F. C. Ins. Mm.

January 83·4 28·6 90 32·2 68 20·0 78·8 2·68 68·1 1

February 86·5 30·3 94 34·4 72 22·2 78·1 6·69 170·0 5

March 84·8 29·2 94 34·4 71 21·7 81·6 7·70 195·6 8

April 85·5 29·7 93 33·9 71 21·7 75·8 11·01 279·5 10

May 81·9 27·8 94 34·4 75 23·9 77·6 10·95 279·0 19

June 80·8 27·0 92 33·3 71 21·7 84·4 32·59 827·0 22

July 77·9 25·5 90 32·2 70 21·1 85·8 13·61 345·4 25

August 77·1 25·0 86 30·0 70 21·1 88·2 6·39 162·4 15

September 80·8 27·0 92 33·3 70 21·1 85·7 11·84 300·0 25

October 82·3 28·0 91 32·8 70 21·1 83·6 9·38 238·0 17

November 82·3 28·0 91 32·8 71 21·7 83·9 11·34 288·3 12

December 81·9 27·8 89 31·7 70 21·1 83·6 1·32 33·6 1

Year 82·2 28·0 94 34·4 68 20·0 82·1 119·50 303·6 150

The remarkable uniformity of the temperature data is very striking. In the interior of Nigeria, however, much higher temperatures are experienced, especially at the times when the hot Harmattan is blowing off the northern deserts, the noon temperature at such times being rarely under 100° F. (37·8° C).

As we get farther south the temperature moderates and the rainfall rapidly diminishes; the mean temperature in Angola being no more than 68° F. (20° C.), while in the elevated interior the climate is neither disagreeable nor unhealthy.

Dr. Yale Massey sends me the following information from his Mission station in the Benguela district, at 4,700 feet, lat. circa 12° S.; and long. 17° E. “The distinctly wet months are from October to April inclusive, and this is also the hot season. There are usually a few showers in September, and rarely some in May, and during the dry season there is usually a strong breeze. In the wet season the mid-day temperature ranges from 80° to 100° F., at night from 45° to 60° F.; while during the dry weather the mid-day temperature is from 70° to 90° F., and at night even slight frost may occur. As might be expected from the elevation and climate, this is a generally healthy locality, but there is a certain amount of fever, most of the cases occurring in April and May.”

East Coast.

—The reputation borne by the east coast is scarcely more enviable than that of the western, the accounts of travellers voyaging on the Zambesi being generally alternate wails on attacks of mosquitoes and upsets from hippopotami. The rainfall is, however, much lighter, and at corresponding latitudes the temperatures generally seem somewhat lower.

Owing to the presence of a considerable Arab element, the population have attained, in some places, a larger grade of civilisation than is the case on the west coast, so that the introduction of hygienic measures might be somewhat more practicable, albeit that Arab civilisation, per se, has hardly reached the stage of promoting sanitation.

The two following tables of a tropical and subtropical station, each on this coast, will give some general conception of the conditions prevailing.

Zanzibar Island. Lat. circa 7° 30′ S.

Month Mean Maximum Temperature Mean Minimum Temperature Rainfall

F. C. F. C. Ins. Mm.

January 86·1 30·0 79·6 26·5 3·26 82·6

February 87·0 30·6 80·3 26·8 1·51 38·3

March 86·3 30·2 79·3 26·3 6·24 159·1

April 84·7 29·3 77·6 25·4 11·94 303·3

May 82·4 28·0 75·6 24·3 10·23 250·2

June 81·5 27·5 74·2 23·5 1·36 34·7

July 80·2 26·8 72·7 22·6 2·75 69·8

August 81·8 27·0 72·7 22·6 1·68 43·0

September 82·0 27·8 73·4 23·0 2·10 53·3

October 83·2 28·1 75·3 24·0 3·74 95·1

November 83·7 28·7 77·1 25·1 8·23 209·1

December 85·8 29·8 79·5 26·4 4·18 106·3

Year 83·6 28·7 76·4 24·6 57·25 1,454·2

Natal, Durban. Lat. 29° 50′ S.; near Sea-level. (1902.)

Month Temperature at 9 a.m. Monthly Maxima Monthly Minima Rainfall Number of Rainy Days

F. C. F. C. F. C. Ins. Mm.

January 74·0 23·3 92·1 33·4 56·4 13·6 6·53 166·2 23

February 78·2 25·7 95·2 35·1 60·5 15·8 2·09 53·2 12

March 74·5 23·6 93·8 34·3 59·2 15·1 10·23 256·5 20

April 70·1 21·1 86·2 30·1 56·9 13·8 2·52 64·0 9

May 66·5 19·2 83·5 28·6 52·0 11·1 1·21 30·6 10

June 61·6 16·5 78·3 25·7 47·4 8·6 0·73 18·0 3

July 62·0 16·7 88·0 31·1 49·5 9·7 0·27 7·0 4

August 63·5 17·5 79·0 26·1 48·4 9·2 3·90 99·1 12

September 67·1 19·5 105·6 40·9 51·5 10·8 2·54 64·7 13

October 69·6 20·8 91·2 32·9 51·3 10·7 2·23 56·6 17

November 70·9 21·6 92·3 33·5 57·1 13·9 5·15 130·8 19

December 75·2 24·0 91·4 33·1 58·2 14·5 3·96 100·4 18

Year 69·4 20·8 105·6 40·9 47·4 8·6 41·18 1,047·0 160

Madagascar.

—With the exception of the littoral, which is rather warm and extremely malarious, the greater part of this island is too elevated to present a really hot climate, but the rainfall in the interior is rather heavier than on the mainland, that of the capital, Antananarivo, being 52·4 ins. (1,331 mm.), which is distributed as follows:—

Monthly Rainfall of Antananarivo. E.F., 4,850; E.M., 1,478.

Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.

Ins. 11·54 9·28 7·36 2·00 0·71 0·33 0·20 0·28 3·18 3·51 5·25 11·0

Mm. 294 236 187 51 18 8 5 7 17 89 133 280

The Island of Mauritius.

—Latitude circa 20° 20′ S. The mean annual temperature of Port Louis is 77·2° F. (25·1° C.), the absolute extremes of temperature in nineteen years being 89° and 53·5° F. (31·6° and 11·9° C.), and the mean relative humidity 74 per cent., so that in the matter of heat there is nothing to be feared; but unfortunately malaria, which was quite unknown in the earlier days of its colonisation, is now very rife and of a very obstinate type. The rainfall amounts to 74·5 ins. (1,892 mm.), which is rather heavier than that of Madagascar, and is distributed as below:—

Monthly Rainfall of Port Louis, Mauritius; near Sea-level.

Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.

Ins. 11·55 8·18 11·35 9·14 5·73 4·65 4·25 3·86 2·71 2·64 3·24 6·98

Mm. 293 208 288 232 145 118 108 98 69 67 82 177

The southernmost portions of Africa, Cape Colony, &c., do not belong to the category of hot climates, either in climate or in their diseases, and so need not be considered here.

Red Sea and its Coasts, including Somaliland.

—The horrors of the climate of the Red Sea are too well known to need comment. The whole region is almost rainless, subject to suffocating calms, and the presence of the large, but completely land-locked, sheet of water renders the relative humidity constantly high. Moreover, the whole basin is comparatively shallow, so that it becomes highly warmed even in its depths. At its southern end the temperature of the water at the surface may reach 95° F. (35° C.), and 90° F. (32·2° C.) has been registered at a depth of 5 fathoms. In the Gulf of Suez, pleasantly fresh days may be met with during winter, but in the south the mean temperature of a day seldom falls below 80° F. (26·7° C.), and in July the mean maximum temperature exceeds 108° F. (42° C.) July is the hottest month, but there is little to choose between the discomforts of any of the four months, June to September. The least hot month is January, but the climate is a singularly uniform one, the night bringing comparatively little relief, and when followed by a breeze of about the same speed as the ship, cases have occurred in which steamers have actually been obliged to put about and steam against the wind, in order to prevent the crew from falling victims to heat apoplexy.

North of lat. 19°, the prevailing winds are from the north or north-west, while in the south the predominating winds are from the south and south-east, between the two lying a belt of variable winds. From June to August north-west winds prevail over the whole Red Sea. This is known as the “Kamsin,” or fifty days’ wind, the word being derived from the Arabic root of that numeral, which, originally, intensely hot and dry, rapidly takes up moisture from the water, and is hence particularly insupportable on the Arabian side of the sea; though the fine sand with which it is loaded makes it equally objectionable from another point of view on the African side. Its velocity is often considerable, and under such circumstances may be even dangerous to the lives of those who are so unfortunate as to be exposed to its fury in the open desert. The extremely fine dust penetrates everywhere in spite of closed doors and windows, reaching even ships far out at sea. Fortunately, on the coast it is generally a good deal modified by sea-breezes springing up in the afternoon, but there is also a tendency to fall dead calm at night, under which circumstances the dark hours are even more intolerable than those of the day.

The following table of the principal climatic data of Massawa in the Italian colony of Erythrea, compiled from data contained in a pamphlet by Dr. Giovani Petella, of the Royal Italian Navy, gives a good idea of the character of the climate of the Red Sea littoral.

Massawa. Lat. 16° N.; near Sea-level.

Month Mean Temperature Mean Maximum Temperature Mean Minimum Temperature Relative Humidity % Rainfall Number of Rainy Days

F. C. F. C. F. C. Ins. Mm.

January 78·0 25·6 90·4 32·4 68·0 20·0 75 2·05 52·1 5·2

February 78·9 26·0 92·2 33·4 68·4 20·2 76 0·63 16·2 1·6

March 81·0 27·2 94·8 34·9 70·1 21·2 74 0·68 17·5 1·7

April 84·3 29·0 98·2 36·8 72·8 22·6 69 0·11 2·5 ·2

May 88·5 31·3 101·7 38·7 76·6 24·7 66 0·56 14·1 1·4

June 92·4 33·5 105·9 41·0 80·7 27·0 51 — — —

July 94·7 34·8 108·5 42·5 84·5 29·2 56 0·13 3·3 1·3

August 94·6 34·7 106·7 41·5 83·4 28·5 57 0·26 5·7 1·7

September 92·0 33·3 103·0 39·4 78·2 25·7 60 0·17 4·0 1·0

October 89·0 31·7 98·7 37·0 77·2 25·1 60 0·35 9·0 1·0

November 84·3 29·0 95·2 35·1 75·0 23·8 65 0·78 20·0 2·1

December 80·7 27·0 92·0 33·3 69·5 20·8 70 2·27 57·6 3·7

The total rainfall amounts only to 7·86 ins. (198 mm.), falling on 29·2 days in the year, but the amount and distribution is very capricious, varying greatly in different years. In so far, however, as Massawa can be said to possess a rainy season at all, the wet weather comes in the winter, instead of about August, as is normally the case in the Tropics of the Northern Hemisphere.

Sometimes a whole year may be practically rainless, as for example 1885, in which only 41·2 mm. (about 1¹⁄₂ ins.) was collected, whereas 1891 had the respectable rainfall of 500 mm. (or 19¹⁄₂ ins.). Apparently, however, it never rains in June.

Owing to the antiseptic powers of the intense light and heat, the place is singularly free from zymotic diseases, the cases of fever being usually not malarial, but truly climatic.

For the greater part of the year the skin is kept in a continuous bath of perspiration, and accordingly prickly heat in its most acute form, with the usual sequel of boils, is very common; as also, of course, are heatstroke and less acute forms of nervous prostration. During the continuance of the Kamsin Dr. Petella finds that the temperature of even strong and healthy individuals is raised distinctly above the normal.

The extreme character of the climate of Suakim, the frequent scene of British military activity, may be gathered from the following nearly complete table for portions of the years 1902-1903.

Suakim. Lat. 19° 5′ N. Near Sea-level.

Month Absolute Maximum Temperature Absolute Minimum Temperature Rainfall Clouds Relative Humidity Wind

F. C. F. C. Ins. Mm.

Jan. 78·5 25·8 68·4 20·2 1·17 29·7 4·5 71 N.N.W., occasionally N. and E. in afternoon

Feb. 79·0 26·7 70·4 21·3 0·68 17·0 4·8 78 N.W., shifting to N.E. or N.

Mar. 82·5 28·1 71·9 22·1 0 0 2·1 90 Ditto, ditto

April Wanting

May 95·0 35·0 76·1 24·5 0 0 0·5 78 Ditto, ditto

June 100·2 37·9 78·0 25·6 0·13 3·0 1·2 69 Variable, but generally N.E. in afternoon

July 107·8 42·1 82·0 27·8 0·36 9·3 1·5 48 S.W. to S., shifting to E. or N.E. in afternoon

Aug. 112·0 44·4 84·4 29·1 0 0 1·3 68 Ditto, ditto

Sept. Wanting

Oct. 92·0 33·3 78·0 25·6 4·80 122·0 3·5 75 N.W. to W., shifting to N.E. in afternoon

Nov. 86·5 30·3 75·9 24·3 6·10 154·9 5·6 78 Ditto, ditto

Dec. 81·0 27·2 69·5 20·8 2·02 51·5 4·5 78 Ditto, ditto

No European constitution could, however, endure such climates for any considerable time with impunity without periods of relief in a more moderate climate, and it is therefore fortunate that, owing to configuration of the Colony of Erythrea, the inland portion of which for the most part consists of elevated plateaux and mountains; these extreme conditions of heat and moisture are limited to a comparatively narrow belt of country, consisting of plains formed of slightly elevated coral formation, and the foothills which gradually rise to elevations at which the climate is necessarily temperate, some of the peaks reaching over 7,000 feet above the sea.