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.