Art. VII.—Observations regarding the Hot Winds of Canterbury and Hawke Bay.
[Read before the Wellington Philosophical Society, 25th July, 1874.]
During the conversation which took place at the last meeting, after the President's very suggestive address, doubts were expressed as to the hot winds of Australia having any influence upon the climate of New Zealand. The subject is an interesting one, and I take the opportunity of noticing some facts connected with it, which may be worth the consideration of those who have given attention to the meteorology of this portion of the southern hemisphere.
My first visit to New Zealand was in the year 1858. A fierce hot wind which had blown for some days carried us out of Sydney harbour, and filled our sails for a few hours after leaving the coast, when it suddenly dropped, and we encountered one of those severe southerly gales, locally called “brickfielders,” which was, however, as they are generally, of short duration. As the dense black storm-drift rolled up along the sea, the red-edged clouds borne on the north-wester could be observed rapidly passing on above it. During the remainder of the voyage we had strong southerly and south-easterly breezes, with cloudy weather and heavy rain as we entered Cook Strait.
Never having heard of hot winds in New Zealand I was consequently the more surprised to find, shortly after my arrival at Christchurch, a strong north-west gale come on to blow, which continued for some days, and seemed to possess a large share of the disagreeable characteristics of the sirocco of Malta and Southern Europe, or the desert winds of Australia, whilst the sky presented the same hard features, the same almost stationary ominous clouds in the upper atmosphere, so familiar to travellers in these countries.
I have used the term “desert winds” to indicate that those are alluded to which blow over the great levels of the continent, although the term is scarcely applicable, as the extent of country actually deserving the appellation of desert is much less than generally supposed.
On inquiry I found that these winds had been very frequent during the previous months, as they had also been in Australia, and came to the conclusion, as stated by me in a paper after my return, “That the hot winds which blow over the southern regions of Australia rise above the lower currents, gathering additional moisture as they pass over the intervening sea, and impinging upon the mountain wall of the cordillera of New Zealand, rapidly discharge their burthen upon its western slopes, and rush dry and blighting over the Canterbury Plains.”
These winds are then really dry when they leave the Australian coast, as may be proved by exposing a tumbler of ice to the current, when the moisture contained even in their lower strata is quickly condensed. They are not so. Peculiar electrical conditions may produce the enormous evaporating power they possess (so injurious to vegetation and disagreeable to the sensations), which those only can well believe who have watched daily with anxious eyes the deep lagoons upon which their safety depends disappearing, not by inches but by feet.
All the observations and inquiries which I have had an opportunity of making since tend to strengthen my conviction. The theory however does not find favour with several persons whose conclusions are entitled to much weight, as those are which have been arrived at by so diligent and accomplished an observer as Mr. Travers. How he and others who entertain a different opinion account for the western coasts of these islands being covered with dense forests, and receiving a rainfall of more than four times the amount annually discharged upon the treeless plains on the eastern side of the main range, I am not aware. Certainly an analogous, indeed almost an exactly similar condition of things obtains in Patagonia, where glaciers descend also through the forests of beech and fuchsia to the sea, as they do nearly in the south island, with the fronds of tree ferns waving over the ice. But these Patagonian forests
are all the year round under the influence of the rain-bearing westerly winds. On the coasts of New Zealand there cannot be said to be any prevailing one; and if it owes nothing to those passing over the adjacent continent of Australia, some other cause must be shown for the westerly ones which reach its shores being so much moister than the equally strong easterly gales.
In summer, during the quiet beautiful mornings, under the powerful rays of the sun, the radiated heat over the dry plains of Canterbury raises the temperature to an extent scarcely to be expected in that latitude, and the lower strata of air become quickly rarified; about mid-day red clouds of dust are seen filling the gorges of the great rivers, by which they debouch from the mountains, and the current rushes down over the plains to fill the vacuum, acquiring by its rapid motion somewhat the character of a hot wind. But these are merely local blasts, of short duration, and as evening approaches all again is calm and still, and no rain has fallen on the mountains.
It is very different, however, on the occasions when the north-westers blow intermittently for several days and nights over the Canterbury levels, and in a lesser degree, over those of Hawke Bay, withering the vegetation, crisping the leaves of succulent plants, so that they may be rubbed up like dry tinder, and exercising a most depressing effect upon the energies of animals as well as those of human beings.
Then torrents of rain are being discharged upon the forest-clad mountains of Westland, and floods in the rivers running to the East Coast announce the melting of the snows in the higher regions. When telegraphic communication is established between the two countries we shall be able to ascertain the rate at which these winds travel over the intervening sea.
Meantime I have ascertained for certain that when hot winds have prevailed to any extent in the southern parts of the Australian continent, their influence has generally been extended to the shores of New Zealand, and that it is not merely upon the occasion of such a hurricane as swept the dust and ashes of the conflagrations which occurred on that terrific day known as “Black Thursday” in Victoria across the ocean, and darkened the air in Otago, that the denizens of these islands are affected by the same influences as the dwellers on the banks of the Murray and Darling rivers.
What are properly called hot winds do not generally blow up to the tropic in either hemisphere. On the eastern coasts of Australia they do not extend their influence much beyond the 28th parallel of latitude. In Queensland I can, from many years' personal experience, state that they are almost unknown, even on its southern borders at Darling Downs.
If we take the next eight or ten degrees as the zone of their greatest strength, and extend W.N.W. parallels (which is nearly the course in which
they blow) we shall find that New Zealand from Mokau, south, will fall within their influence. So that the objection raised by Mr. Travers that so small a portion of Australia is beyond the 34th parallel is not of consequence as affecting the question.
The eminent astronomer, Sir Thomas Brisbane, once Governor of Australia, warned me when I first went to that colony, to be on my guard against floods as well as the dreaded droughts, as periodical cycles of wet seasons were certain to occur, in consequence of the southerly extension of the south-east trades. During the last few years unusually heavy rains have fallen all over the territory of New South Wales, as well as in the northern tropical regions of Queensland, extending back to the great levels beyond the Darling. When a river, flowing through a gorge as the Burdekin, an eastern water, does, a mile wide, is raised 100 feet in perpendicular height, it will not be a matter of surprise that these usually arid regions have been temporarily covered with water, as they were when Mr. Oxley, the explorer, first penetrated to what he supposed to be an inland sea.
Now it is a very interesting fact that during late years the withering effects of the north-westerly winds have at times quite taken people by surprise upon the western seaboard of the North Island. Until the last five or six years winds from that quarter were always sure to bring rain in summer to the expectant farmers at Taranaki, whereas they have now come to be dreaded, as causing their crops and fruit to be blighted. During last summer a W.N.W. wind blew for two days, possessing quite the depressing effect of the sirocco. The sky was cloudless, but round the very summit of the cone of Mount Egmont remained a halo of white cloud, perfectly unchanging in shape and extent; so hot was the blast, at all events so great its absorbing powers, even at that elevation, that no fleecy streams were borne away with it from the mountain top.
I inquired as to the weather on the opposite coast at the time, and found that rains had fallen on the mountains, flooding the rivers which run to Hawke Bay, whilst heated winds were parching the plains around Napier. Mount Egmont at this time was nearly without snow, a little only being visible in the crater, so it will be readily understood that the sharp mountain peak was not sufficient to condense the vapour to such an extent as to produce rain at Taranaki; for it was evident that this particular current, which seemed so devoid of moisture to one's feelings, and no doubt left the Australian coast really a dry wind, having deposited its burthen on the usually thirsty plains of that country, had gathered a considerable amount in its passage over the intervening sea, which for some days previous had been perfectly calm, so the partially loaded westerly wind descended and struck upon the land at the sea level.
It was not so during the summer all over New Zealand. The dry blasts, which left the Australian coast, ascended into a higher zone than usual, and brought no rain to the western coast of the South Island, passing on over its mountains until, saturated at last, they returned possibly as a south-east wind, bringing rain to the eastern coasts in unwonted abundance.
However this may be, the facts mentioned seem to bear very pertinently upon the theory suggested by Dr. Hector, that to the elevation of the inteiior of Australia is to be attributed the shrinking of the New Zealand glaciers.
When the vast levels of that continent were submerged, the returning equatorial winds came loaded with a far greater weight of “that vast stream of molecules of water, each enveloped in its own shroud of electric vapour,” upon the mountain chain of the great land, the back bone of which and little more remains.
The soundings between New Zealand and the Chathams to the east, Norfolk Island and the Kermadec group to the north, are not very profound. So when the cordillera stood at an average higher altitude of from 4,000 to 5,000 feet, as Mr. Travers says, the whole of that great area would be dry land, and we have depicted to our imagination a grand country, intersected by majestic rivers, in which such giant forms of life as the Dinornis can more easily be conceived to have originated, than in the limited islands, where the last of the race remained after all the vast changes of scene and climate during which they survived until the present century.
With such a mighty engine as the equatorial current depositing its load of moisture on the névés of these mountains there is no necessity for seeking for other causes, evidences of which are not to be seen; and to suppose it necessary to assume that the same circumstances obtained in remote epochs in the Australasian regions, as in the northern hemisphere; nor to conclude that New Zealand at that era presented in any degree the desolate appearance Dr. Haast supposes.
The development of the glaciers under the cosmical influences proposed would be quite sufficient to account for all the phenomena now so interesting to geologists in the terraces of the river valleys and the lake basins of the alpine regions, and the materials were then prepared affcerwards to be spread out by the rivers as the land gradually subsided on the Canterbnry Plains—a process still going on—the formation of which has been so graphically described by Dr. Haast. It is impossible for any one who has studied his last report to speculate upon the possibility of these vast accumulations being of marine origin, even although he may not have traversed the paved plains of the Rangitata and Rakaia, and had an opportunity of observing the form of the stones and gravel, nor have searched in vain for any fossil of
marine origin in the sections at the gorges of the rivers, unless below where some remnant of the abraded and buried tertiaries has remained and is exposed; nor seen evidence of the gradual subsidence in the remains of the successive forests which have flourished, and been overwhelmed in their turn by the streams of stones and other fluviatile deposits, which general subsidence, if it is now arrested, has been so but at a comparatively recent date. Even totara, durable as it is, will not remain for geological eras unchanged. Submerged forests of this timber in good preservation are to be seen in the North Island, where the land is broadest, near Wairoa, on the East Coast, and immediately opposite between Urenui and the Waitara rivers.