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The Pamphlet Collection of Sir Robert Stout: Volume 6

Transactions of the Australasian Association of the Advancement of Science

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Transactions of the Australasian Association of the Advancement of Science

1891
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[Transactions of the Australasian Association for the Advancement of Science, Vol. III.]

On the Characteristics of the Nor-westers of Canterbury, New Zealand.

The general features of a typical nor'-wester, as experienced in Christchurch, have, unfortunately, this season been only too familiar to us all. It is, therefore, not necessary to repeat here the full description of the phenomena with which the author commenced his paper. The thermometrical, barometrical, nephological, and other effects of the nor'-wester having been given, the high temperature of the wind was next explained. It had been customary to account for it as the result of compression on the eastern side of the Southern Alps, for it was well known that dry air in descending gained heat at the rate of 1° Fahr. for every 180ft. of vertical measurement. The pneumatic tinder-box clearly enough showed that compression resulted in heat. But compression on one side of the mountains only restored the temperature lost by expansion on the other side—provided the air on both sides were subject to the same conditions of barometric pressure, temperature, velocity, and humidity. Here, however, came in a point of the greatest importance. The north-west wind, on reaching the western slopes of the Southern Alps, was heavily charged with moisture, and this rendered latent the heat brought by the wind from the lower latitudes. Now, wet air in an ascending current lost heat much more slowly than dry—namely, 1° Fahr. for every 300ft. of altitude; because the expansion during ascent resulted in the precipitation of rain, and thus latent heat was rendered sensible. This difference in the rate of losing and gaining heat on the part of wet and dry air was the chief immediate cause of the uncomfortable warmth of our nor'-westers; for, assuming that the wind in Westland page 594 was at 60° Fahr., on reaching 9,000ft. in elevation it would fall to 30°—i.e., 9000°/300 would be lost—and on reaching Christ-church there would be an increment of 9000°/180 Fahr.—i.e., 50° Fahr.—which would make, with the 30° Fahr. retained at the summit of the Alps, a temperature of 80°. This reasoning was first given by Espy and Maury (in 1861) without special application, and Dr. Hann, Herschell, and others subsequently used it to explain the föhn wind of Switzerland, a quite analogous wind, which even Dove had failed to account for satisfactorily. This reasoning applied to our nor'-wester, which was a true föhn. But even that explanation did not get to the root of the matter, for it only showed how heavy rain in Westland rendered sensible in Canterbury the heat previously latent. Therefore the question still remained, Whence came that heat? It was brought from the tropics. The hot equatorial wind or return trade, also, in crossing warm oceanic currents, gathered up further moisture and heat as it travelled to higher latitudes. Furthermore, the northwest weather was mostly cyclonic, and all meteorologists agree that, whatever the cause might be—and as yet it was unexplained—a peculiar kind of stifling oppressive heat, giving neuralgia, headache, rheumatism, &c., invariably was developed on the front of a cyclone. In the Southern Hemisphere this heat-spot would be found on the left front of the path of the storm, and, granting that the cyclones producing our nor'westers sheered off to the south of New Zealand, being divertd by the Southern Alps, we should be justified in locating the heat-patch over the Canterbury Plains. Cold nor'-westers occurred occasionally, and were sometimes probably only local. If they occurred in winter, when the initial heat of the winds was great, and little precipitation on the mountains accompanied them, they were not difficult to understand; but they occurred at other seasons, and were then not easy to explain. The clouds of dust which made a nor'-wester so unpleasant were thought by Sir J. von Haast—probably erroneously—to explain the loess formation of the Lyttelton hills and elsewhere. The remarkable fall of the barometer on the approach and during the continuance of a nor'-wester accorded with Dove's law of the Southern Hemisphere generally—the steepness of the baric gradient indicating the intensity of the disturbance. The glass usually went down to 29.2in., and once quite recently had even descended to 28.68in., though in twenty-four hours, when south-west intervened, it had risen to 30in. again. Why the glass fell in a nor'-wester was a difficult question. Maury's explanation about aqueous vapour driving out and being lighter than air did not meet the case so well as what Loomis said about light hot air filling the whole space usually occupied by colder and denser air. The north-west arch of clouds, a very page 595 beautiful and distinctive mark of the approach of the equatorial wind, was developed probably by contact and difference of temperature between the north-west wind and the ordinary air lying over the plain. It lay really parallel to the range, but assumed to us the arch form from the operation of a principle of perspective. It altered its position, unless dissipated altogether, as the wind veered round to the exhilarating south-west, whence came the cold squalls which showed the passage of the storm. North-west rains along the foot of the hills, and particularly in certain spots, were then adverted to, and other instances were given of föhn winds, corresponding to our nor'-westers—particularly the chinook of North America. Though very unpleasant, and occasionally disastrous, our nor'westers deserved more than a passing good word. Dry heat was needful for ripening wheat and fruit. It also killed or blew away the germs of disease, melted the snows on the hills, &c. Most of the health resorts of the world were situated under the protection of mountain-chains across which warm winds blew; and among the many circumstances to which Canterbury owed that salubrity which accounted for its excellent grain, its toothsome mutton, and the average fine physique of its sons and daughters, not the least important, perhaps, was the much-abused nor'-wester.