The Solomon Islands and Their Natives Part 46

THE CLIMATE OF THE SOLOMON ISLANDS.

AMONGST the matters to which I devoted some attention in this group of islands, was the annual rainfall. As far as I know, there have been no continuous observations previously made there; and the only record of rain-measurement, which I have been able to find referring to this region, was an observation made on board the Austrian frigate "Novara"

in the middle of October, 1858, whilst to the northward of St.

Christoval, when three inches of rain were registered in five hours.[491] I therefore set myself to work to do what I could in this matter, making rain-gauge stations at Santa Anna and Ugi and keeping a register myself on board. Mr. Fred Howard undertook to make these observations at Ugi, and I supplied him with a rain-gauge for this purpose. His register, which extended over a period of fifteen months from October, 1882, to the end of the following year, was kept with great regularity; and as I was able to compare his observations with my own on board for a few days, I have every confidence in the accuracy of his observations. At Santa Anna, Mr. William Henghan, to whom I had supplied a gauge, undertook at first to keep the record, beginning in the last week of October, 1882; but he left the island two months after, when Mr. Charles Sproul voluntarily undertook to measure the rainfall, which he did with great regularity until the end of the following year.

I regret to learn that Mr. Sproul has recently died at Sydney. He was one of those men who in a quiet inoffensive way have done much towards preparing the way for future settlers in this group. I have the greatest confidence in his observations, since for a few days at Santa Anna we were able to compare our daily measurements.

[491] Scherzer's "Voyage of the 'Novara,'" Eng. edit., 1861.

Before proceeding to consider the results of these rain-measurements, I will endeavour to convey to the mental eye of my readers a general idea of the most striking atmospheric phenomenon in connection with the rainfall of these regions. I refer to the oncoming of the _black squall_.

A clear and serene sky at first gives no token of the sudden change that is to quickly follow; but the stillness of the air and its increased dryness, together with the consequent greater scorching power of the sun's rays and the apparent nearness of surrounding sh.o.r.es, give sufficient warning of the onset of the rain squall to those acquainted with these seas. In a short time a low black arch appears above the horizon, often in an unexpected quarter, and rising rapidly it sweeps majestically with great swiftness until it appears to span the heavens.

Onward it rushes, quicker far than one imagines; and now must the navigator beware. Under yonder advancing arch a white line of foam marks its van. There, away towards one of its corners, a waterspout rises in fantastic shape; sea and cloud meet in mid-air and become intermingled in the whirling column. Lightning plays about beneath the arch and within its black ma.s.s, illuming for the moment its dark recesses and leaving it in the next far blacker than before. Peals of thunder herald on the advance of the black squall.

"Clear lower deck!" "Hands aloft!" "Shorten sail!" Such were the words of command which were almost daily issued during our cruises in these islands. In a few brief minutes, the s.h.i.+p is prepared to meet the squall. The temperature falls very perceptibly, and the officer of the watch gives a slight s.h.i.+ver as he dons his oilskins. The wind is freshening, a few large drops of rain fall, the men crouch under the bulwarks, and now the arch is overhead and we are in the thick of the squall. Down comes a deluge of rain which in less than a minute wets all who are unprotected through and through. The s.h.i.+p heels well over, even with her scanty canvas. There is nothing more to be done. We listen to the whistling of the wind in the rigging and patiently wait until the weather clears. In half-an-hour the arch has swept over us, and is pursuing its rapid course towards the neighbouring mountain-peaks, perhaps of Bougainville or it may be of Guadalcanar. The blue sky begins to show itself; and in less than an hour all is as before. With reefs shaken out and more sail made, the s.h.i.+p proceeds, plunging cheerily on under a fresh breeze as though glad to shake herself clear of the squall. The sea losing its murky colour reflects the bright hue of the sky now serene; and its white-topped waves sparkle in the sun. The wizard of the storm has shaken his wand, and the scene is changed, as though by magic.

All nature seems invigorated by this short battle of the elements and to be indebted to the bounty of the black squall. Whilst everything before was depressed and lowering, all is now bright and cheerful. Nature has in truth had its accustomed shower-bath, and the reaction that ensues does good to all; makes men the happier and the stronger, elicits a loud chorus from the lower creation in which bird, reptile, and insect, before hushed in the depressing gloom, now combine in strange medley; and the inanimate world shares in the bright change which has followed the storm.

If it be night, the increased luminosity of the sea may be the warning of the arched squall. The s.h.i.+p throws off a bright wave of phosph.o.r.escence on either side of the bow, and leaves a luminous track in her wake. Overhead the cloudless star-lit sky conveys _its_ warning; for the stars s.h.i.+ne with increased brilliancy, those of less magnitude usually invisible with the naked eye are now distinctly seen; and if the navigator, who has often tried in vain to count the six stars in the Pleiades, can do so now, let him look out for the black squall. Such are the warnings. Then sweeps along the lowering arched ma.s.s with its rain and its waterspouts, its wind and its thunder and lightning. On it comes, looking all the blacker as it spreads athwart the heavens and turns the star-lit night into a lightless gloom. Overtaken in the night by such a squall, unable to see more than half a cable's length on either side, and perhaps in the vicinity of sunken reefs the position of which is uncertain, a sailor has need of all his wits. On one occasion, when in this situation, we came unexpectedly in soundings, whilst, as we thought, a hundred good fathoms and more lay beneath our keel. The time was anxious, but nothing could be done until the squall was over. When the arch has pa.s.sed, the stars begin to show themselves, and in a short time they s.h.i.+ne out with all their l.u.s.tre.

With this description of the rain-squall, or black-squall, or arched-squall, as it may be also conveniently termed, I return to the consideration of the rainfall of this region; and first with regard to the observations at the east end of the Solomon Group. During 1883, 12503 inches of rain were measured at Santa Anna, a small island lying at the extreme eastern limit of these islands. Two-thirds of the total amount fell in the live months between the beginning of April and the end of August. At Ugi, which lies nearly 60 miles north-east of Santa Anna, 14624 inches of rain were registered during the same year. About one-third of the total rain for the year fell in the two months of April and July. On comparing the totals for each month at these two localities, there will be found to be but little agreement, which is due to the circ.u.mstance that the daily rainfalls of these two places have little relation one with the other, a heavy fall at one island being often only indicated by a slight fall of rain at the other. It is thus evident that locality has a great influence on the rainfall in this part of the group; and probably Ugi owes its greater rainfall to the proximity of the high land of St. Christoval. Here, as in other parts of this group, I often had opportunities of observing how the contiguity of land affected the rainfall in a single shower. I might have been in the interior of an island exposed to a deluge of rain for a couple of hours, and have found, as I did once in the Shortland Islands, that there had been very little rain on board. Another time, when in my Rob Roy canoe on the south side of Treasury harbour and not more than a mile from the s.h.i.+p, a rain-squall pa.s.sed over me leaving scarcely a drop behind; but as it swept over the s.h.i.+p and was approaching the steep slopes of the island, a smart shower of 20/100 of an inch fell on the deck.

I cannot gather from the observations made in this eastern part of the group, that one season of the year has a heavier rainfall than another.

On comparing the two records for 1883 of Ugi and Santa Anna, it might be thought that the closing months of the year would usually prove to be the driest; but on referring to the register kept on board the s.h.i.+p in this locality in the latter part of 1882 (page 365), which is one of the heaviest records we had in the Solomon Group, such an inference would be negatived. Nor do I find from these registers of rainfall that there appears to be any relation between the amount of rain and the prevalence or non-prevalence of the south-easterly trade, which usually becomes well established in May and lasts till the end of November or the beginning of December, when the north-westerly and westerly winds set in. These observations point towards the inference, therefore, that the distribution of rain through the seasons in this part of the group is capricious; and they do not warrant the conclusion that one season is wetter than another.

Perhaps a comparison of the number of rainy days, or days on which not less than 2/100 of an inch of rain were measured, may help us to form a more definite conclusion. It will be seen that at Santa Anna and Ugi there were much the same number of rainy days, 182 in the former island and 178 in the latter, or in round numbers about half the total number of days in the year were rainy.[492] At Santa Anna, during the prevalence of the trade wind, there were on the average 15 rainy days per month, and at Ugi 13 per month; whilst during the months from December to April inclusive, when westerly and variable winds prevailed, there were 18 rainy days per month at Santa Anna, and 19 per month at Ugi; so that we may infer that in this year of 1883 there were fewer rainy days per month during the prevalence of the south-east trade, _i.e._, from May to November, than during the period of westerly and variable winds, _i.e._, from December to April.

[492] From the record of the rainy days during the six months from June to November of the previous year (1882), it appears that at least 110 days were rainy. During the same months of the following year, only 84 days were rainy.

I come now to the subject of the greatest daily fall of rain in this eastern end of the group. On the 13th of June, 1883, 773 inches were registered at Santa Anna; whilst at Ugi on the same day only an inch and a half of rain fell, a circ.u.mstance showing how confined in their areas some heavy rainfalls may be. At Ugi the heaviest daily fall of 575 inches was recorded on the 28th of January of this same year; whilst at Santa Anna only a little more than two inches fell on this day; and here is another proof of the restricted locality of heavy rainfalls. On the 20th of November, 1882, when H.M.S. "Lark" was off the east end of St.

Christoval, 574 inches of rain fell on the s.h.i.+p; whilst only a small amount of rain was measured at Santa Anna and Ugi... ... With reference to the character of the rain in this part of the Solomon Group, I may remark that as in other tropical regions it is very heavy.

A fall of an inch in an hour is very frequent during a rain-squall; but not uncommonly the rain falls far more heavily. Thus, on one occasion on board H.M.S. "Lark," when in this part of the group, 290 inches fell in an hour; and at another time 103 inches fell in 25 minutes, and on another occasion an inch fell in half-an-hour.

But inasmuch as heavy falls of rain are not peculiar to the tropics, since far greater falls than those above named have occurred in temperate Europe, we can only judge of the character of the rainfall in this region by the total annual fall and by the frequency of heavy falls. Thus we find that at Ugi, in 1883, on 56 days the fall exceeded an inch; and that at Santa Anna, more than an inch of rain fell on 41 days. At Ugi, the daily records on eighteen occasions exceeded two inches; at Santa there was a lesser number of falls of over two inches, viz. 11.

If I were to estimate the probable annual rainfall at the _coast_ in this part of the Solomon Group, I should place it at not far under 150 inches. Although only possessing the rain-register for a small portion of 1882, I am of the opinion, from having spent a large part of the year in this eastern end of the group, that the fall for 1882 was heavier than the rainfall actually registered for 1883;[493] although this is but a conjecture, it enables me to estimate the probable annual fall with some confidence at about 150 inches at the _coast_ in this eastern end of the group.

[493] _Vide_ footnote referring to number of rainy days in 1882 on p. 356.

The observations made on board the s.h.i.+p amongst the islands of Bougainville Straits (Treasury, Shortlands, Faro, etc.) during portions of the year 1883 and 1884 now claim our notice. As shown on page 365, 6043 inches of rain fell in the five months from June to October of 1883, this amount being a little under that which fell at Ugi (6570 inches) and at Santa Anna (6772 inches) in the same period, the two regions lying towards the opposite ends of the group. During the same period of the following year, we measured 6766 inches of rain in Bougainville Straits, an amount a little in excess of that of the previous year. During the same periods, _i.e._, from June to October inclusive, in 1883 and 1884, there were the following number of rainy days, 120 in the one year and 118 in the other. At Santa Anna and Ugi, at the opposite end of the group, the total of rainy days for the same period in 1883, numbered only two-thirds of the amount in Bougainville Straits. During these five months in 1883 there were 16 daily records of over an inch of rain in Bougainville Straits; at Santa Anna and Ugi, in the same period, there were 23 and 26 daily records exceeding an inch.

In the same period of 1884, in Bougainville Straits, there were 22 such daily records, but the total fall was about 7 inches greater than in the previous year.

I may now draw some inferences from the above observations. In the first place, it is probable that the annual coast rainfall of Bougainville Straits and that of the eastern end of the Solomon Group are much about the same, viz., about 150 inches: the chief difference between the two regions being, that in the former region, there are a greater number of rainy days and fewer heavy falls. The heavy falls, when they do occur, are not easily forgotten; thus, at Treasury we measured, in July, 1884, 11 inches of rain in 10 successive hours; but the daily record was only 809 inches, since the rain began in the evening of one day and lasted well into the following morning.

During the heavy rainfalls in these regions the streams swell in an astonis.h.i.+ngly quick manner. Rivulets become turbid streams, the whole hill-slope discharges a continuous sheet of water, and the water rushes down the permanent stream-courses with the roar of a mountain-torrent.

Large blocks of stone are swept some distance along the lower courses of the streams; and the trunks of trees are carried by each successive flood further and further towards the mouth of the stream.

It should be now remarked that the average rainfall for the year, which I have estimated from observations made in different parts of the Solomon Group at about 150 inches, only applies to the _coast_. It is probable that this estimate is generally applicable to the coasts of these islands, except on the lee sides of the loftier islands.[494]

[494] By the lee sides, I mean those sheltered from the prevailing S.E. trade.

This brings me to the question of the rainfall in the higher regions.

The rainfall will increase with elevation until a certain height is reached, where the clouds attain their maximum density; at such a level the greatest rainfall will occur. I learn from an interesting paper by Mr. Bateman on this subject,[495] that it may be inferred that in the Lake District of England the greatest rainfall occurs at an elevation of 2,000 feet, which is the level of maximum cloud density. In India, an elevation of 4,500 feet represents the level at which the greatest rainfall occurs. In the Solomon Islands, a greater height will have to be attained before the level of maximum cloud density or that of the greatest rainfall will be attained. Probably I shall not greatly err if I a.s.sume it to be between 5,000 and 6,000 feet. I have already observed that the south-east trade, subject to its usual variations, is the prevailing wind in the eastern part of the group for nearly two-thirds of the year. Coming laden with its watery burden, it first strikes the eastern slopes of St. Christoval; but although the higher regions of this island must cause the rain-clouds to precipitate a large amount of their moisture, the higher peaks do not rise in sufficient ma.s.s to a height that would receive the greatest rainfall, the extreme height being 4,100 feet. The rain-clouds, with the bulk of their moisture, would therefore be driven over the higher regions of this island, and would deposit the greater part of their burden on the higher slopes of the mountainous eastern portion of Guadalcanar. Since this island, in its eastern portion, rises in ma.s.s to a height of some 5,000 feet and attains a maximum elevation of 8,000 feet, it does not seem probable that, during the prevalence of the trade for nearly two-thirds of the year, a considerable quant.i.ty of rain would be deposited on the western side of the island; and, that such is the case, is shown in the fact that the dense forest-growth that clothes the steep eastern and southern slopes of the island gives place, on the lee or west side of the mountains, to a vegetation which gives to the western portion of Guadalcanar, when viewed from seaward, the appearance of a savannah or a prairie.

[495] Journal of the Victoria Inst.i.tute. Vol. XV. No. 59.

The lofty mountain-ma.s.ses of the east end of Guadalcanar, which forms one of the finest specimens of coast-scenery in the world, are usually enveloped in rain-clouds at their summits. But occasionally one of the peaks is visible above the thick cloud-covering, marking by its elevation, as it were, the line of greatest rainfall lying below. In the same manner the high peaks at the east end of Bougainville, which have an elevation of between 7,000 and 8,000 feet, may be seen occasionally to project above the rain-clouds; but there is, probably, a smaller quant.i.ty of rain deposited on the higher slopes of this island than on those of Guadalcanar, because the mountains are more isolated, possess for the most part the tapering volcanic profile, and do not rise "en ma.s.se," as in the case of the high lands of Guadalcanar. The greatest rainfall in the Solomon Group takes place on the steep southern and eastern slopes of this island of Guadalcanar. Huge mountain-ma.s.ses appear to rise directly from the sea to a height of some 5,000 feet, ultimately attaining a height of 8,000 feet. The fall there must be tremendous, especially when, as is frequently the case, the land of St.

Christoval does not interpose itself in the path of the moisture-laden trade-wind. Then, loaded with vapours after its pa.s.sage across a wide expanse of ocean, and with but a thin tract of intervening lowland to rob it of its moisture, the trade strikes at once upon the precipitous mountain-slopes as against some Cyclopean rampart. There is no ravine or breach in the mountain-ma.s.s to ease the tension. There, on those mountain-slopes, a terrific precipitation must occur, which, if the annual rainfall of the coast is 150 inches, will here be three or four times that amount. This is no exaggerated language, but is the opinion I have formed, after having carefully considered the physical geography of these regions.

The subjoined rainfalls of a few localities in other parts of the world may be interesting to compare with that of the Solomon Group:[496]

[496] Somerville's "Physical Geography," 7th edit. pp. 331-334.

England 32 inches.

Singapore 97 Atlantic Doldrums 225 Western Ghats 302 Cherraponjee 10

SOLOMON ISLANDS.

(_a._) at the coast 150 (_b._) on the higher slopes of Guadalcanar 400 to 500 inches probably.

Comparing the rainfall of the Solomon Islands with some results obtained in other parts of the Pacific, I would draw attention to the small rainfall of Port Moresby on the south-east coast of New Guinea, where 3444 inches were registered at the Mission Station in 1875.[497] In Fiji the rainfall appears to vary between 60 and 250 inches per annum, according to the degree of elevation above the sea, and to the position of the station on the lee or weather sides of the islands, the greatest annual falls occurring in the interior of the large islands.[498] In Oahu, one of the Sandwich Islands, during 1873, the rainfall at the coast was 3785 inches; whilst at a distance of 2 miles in the interior, it was 13406 inches, the elevation being only 550 feet above the sea.[499]

[497] Stone's "A Few Months in New Guinea," p. 143.

[498] Rain-gaugers have been numerous in this colony, and their list would extend beyond the limits of a foot-note. (Vide Home's "Year in Fiji," &c., &c.)

[499] Mosely's "Naturalist on the 'Challenger,'" p. 497.

I will now make a few remarks on the barometric pressure, temperature, and other features of the meteorology of this group. They are based on the results of the observations made by Lieutenant Leeper on board the s.h.i.+p, and by Mr. F. Howard at Ugi. (Tables appended.)

As is usual in these regions of the Pacific, the fluctuation of the barometer, whether daily, yearly, or monthly, is very small. Thus, the range during the 22 months we pa.s.sed in the group, was from 2983 to 3018 inches, or about a third of an inch; whilst the average monthly range was rather under a quarter of an inch, and the usual diurnal variation about 04 of an inch.

Whilst endeavouring to compare the temperatures of the different seasons, I have mainly used the Ugi register, since it gives a continuous record for more than a year. At Ugi in 1883, the portion of the year from June to September inclusive was slightly the coolest, but the difference in the means was not 2; and, in truth, taking all the thermometric observations into consideration, the seasons are scarcely distinguished by their temperatures. As Lieutenant Leeper[500] remarks in his report, the temperature varies but little all the year round, the monthly mean varying between 80 and 85. The annual mean temperature may be placed at 82 to 83, and the range from 75 to 95. The daily variation is considerably affected by the exposed or protected position of any locality at the coast. Judging, however, from the data at my disposal, it is usually less than 10, _e.g._, 79 at night, and 88 at mid-day.

[500] _Vide_ Quart. Journ. Roy. Met. Soc. vol. XI., p. 309. The instruments used on board were previously verified at Kew. From want of leisure, Lieut. Leeper was unable to do much more than tabulate his observations. I have therefore extracted from them such general facts and inferences as they sustain.

From the hygrometrical observations, it may be inferred that the climate of these islands is generally very moist. The _relative humidity_, taking 100 as saturation, ranged at Ugi in 1883 from 54 to 100; but the monthly range was usually from 72 to 95, the mean for the year being 83.[501] This mean degree of relative humidity is much greater than that of Levuka in Fiji which would seem to average about 70;[502] but in truth there is little necessity for me to remark further on this well-known feature of the climate of these islands. Yet, I should add that this proportion of aqueous vapour would not necessarily be oppressive in a temperate lat.i.tude. In a tropical climate, however, any influence that r.e.t.a.r.ds the evaporation from the skin of the normally excessive perspiration, is a cause of personal discomfort, such as would not be experienced in a drier locality lying in the same lat.i.tude. The effects of this combination of heat and moisture are to be seen in the rankness of the vegetation, and in the rapid rusting of steel. Although the foregoing remarks may be taken as generally applicable to the group, it should be stated that on the lee side of a mountainous island, such as the western end of Guadalcanar, there is a comparatively dry atmosphere, and the difference is also shown in the character of the vegetation.

[501] There are no observations for January, but since the mean relative humidity varies with the rainfall, I have approximately estimated that for January to be 83.

[502] Lieut. Lake's observations for 1876 and 1877. (Quart. Journ.

Met. Soc.)

The moderate intensity of the sun's rays in these islands is to be ascribed to the presence of aqueous vapour in quant.i.ty in the atmosphere. When, however, a thunderstorm and its accompanying rain-squall are portending, the air is unusually dry, and the sun's rays are very fierce. At such times it often happens that the sky is overcast; and thus it comes about that the unwary traveller, by rashly baring his legs and arms, suffers severe sun-burns when he least expects it. Waterton and other travellers have, through ignorance of this fact, been laid up for several days, and even weeks. I was unable to walk any distance for about ten days, after experiencing a severe sun-burn of the legs as the result of baring them during an overcast day. The affection is peculiarly painful, though it often excites but little sympathy.

My remarks on the meteorology of this group will not be complete without a short reference to the prevailing winds. The South-east Trade Wind and the North-west Monsoon carry on a continual struggle for the mastery in these islands. However, for two-thirds of the year the Trade prevails, viz., from April to November. The appended record of winds, which extends over a considerable period, I have prepared from the observations made on board H.M.S. "Lark" in different parts of the group, and from the registers kept by Mr. Sproul and Mr. Howard at Santa Anna and Ugi. It will be there seen that at the eastern extremity of these islands, viz., in the vicinity of St. Christoval, the Trade announces its onset in April by unsettled weather, and frequent thunderstorms. In May, it becomes established, but, as Lieutenant Leeper remarks, it blows in fits and starts, is interrupted by calms, variable winds, and heavy rain-squalls, and does not blow home as in Fiji and the groups to the eastward. At the opposite end of the group, in Bougainville Straits, the Trade appears a month later, and does not become established until June. In this locality, however, it is more fitful than in the eastern islands, blows lighter, and is less to be depended on by the navigator.

It may be generally stated that the north-west and west winds set in about the end of November or the beginning of December, and prevail until the end of March. Although heavy gales accompany the frequent s.h.i.+ftings of the wind, especially when it is from the S.W., these islands are beyond the sweep of the hurricanes which in this season of the year occasionally devastate the groups to the eastward. The period of the westerly winds in the Solomon Islands is also characterised by calms and variable winds. The exhilarating freshness of the Trade then gives place to the enervating influence of the Monsoon; and, in consequence, the period of westerly winds is the sickly season.