Climate and Health in Hot Countries and the Outlines of Tropical Climatology / A Popular Treatise on Personal Hygiene in the Hotter Parts of the World, and on the Climates That Will Be Met Within Them.

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

(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:—