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.

by George Michael James Giles

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.

[5] This is simply a seaman’s corruption of the Arabic word for season.

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.