4. The Origin and Development of the North-west Winds of New Zealand.

By John T. Meeson, B.A.

The nor'westers of Canterbury—though they seem, and in some respects are, so exceptional in character—in point of fact are nothing but the winds common to the whole of New Zealand, and, indeed, common to our latitudes in the Southern Hemisphere, right round the earth—winds blowing towards the area of abnormally low pressure (29° to 29 3°) which prevails generally about lat. 56° S., but modified very materially in character by their passage across the Southern Alps, which, though not much higher than two miles even in their topmost peaks, have power to affect the currents of air shown by cirrus clouds to prevail to the height of five miles. These winds are, in fact, the Return Trades, and would be north winds if they followed the direction which they first take after forming at the Zone of Calms; but, having come from parts of the earth where the rotatory motion is great to places where it is less, they have acquired, for the opposite reason to that which Hadley gives to explain the western tendency of the Trades, an eastern proclivity: and as they proceed to higher latitudes after becoming lower surface winds at the Tropic of Capricorn, they become more and more westerly until they merge into the "roaring forties "with which all voyagers to this part of the world have a somewhat intimate acquaintance. Dove remarks that "all winds are liars," and it must never be forgotten that the apparent horizontal direction of a wind, on account of the shape and aërial motion of the earth, is never or hardly ever the real one.

These roaring westerly winds show frequently unmistakable cyclonic disturbance, which, by applying Buys Ballot's, or, as it should perhaps be called, Galton's law, can be roughly located. That law recognises the fact that the wind blows along isobars with less pressure on the left in the Northern Hemisphere, but on the right in the Southern: in other words, if you stand with your back to the prevailing wind, in the Southern Hemisphere the lowest depression of the barometer will be on your right hand—i.e., in the case of the nor'westers, to the south-west. If we knew in New Zealand, as meteorologists in the Old World do know, the exact measurement of the acute angle between the direction of the wind and the lie of the isobar—that is, the inclination of the wind to the isobar—we might fix the position of the cyclonic centre much more precisely. But the narrowness of New Zealand, the few page 73 observing stations therein established, the waste of waters round about, and the small number of ships navigating it and bringing in their weather reports, must make it extremely difficult to draw isobars of a thoroughly reliable nature in our colony.

Now, the cyclones, formed at various points in the northwest gale like eddies in a mighty stream, sometimes are so large as to absorb the whole current and stretch over vast areas. But, just as Loomis shows (Nature, 11th July, 1878) that American storms in the Northern Hemisphere originate in a district near the Rocky Mountains, two-thirds of them north of lat. 36°, and move on to the east through the general circulation of air in that direction, and as Dove considers that many of the storms of the Temperate Zones are tropical ones diverted from their path at or about lat. 30° north and south, and as the English meteorologists recognise certain fairly-definite storm-tracks across the Atlantic, ramifying frequently from certain definite spots and skirting the North Atlantic anticyclone while journeying usually in a north-east direction, so it perhaps should be possible for us to arrive at some conclusion as to the starting-points and general paths of our New Zealand cyclones, though the means that we have of doing so, for reasons already indicated, are exceedingly limited. Even in Europe, with its army of observers, its delicate appliances, and numerous stations, it is very difficult to trace the connection, by continuously advancing minima, between tropical cyclones and their prolongations into the Temperate Zone. Here, therefore, the work must be considerably harder.

In the North Atlantic there is a great anticyclone stretching over the intertropical area and the more southern part of the North Temperate Zone. Along the northern edge of this is the birthplace of the innumerable cyclones of every size and intensity which incessantly move to some point east or northeast along the shores of and crossing the British Isles—to be worn out in the polar regions north of the Scandinavian peninsula. Similarly, in the Southern Hemisphere and in corresponding latitudes there is formed in winter a more or less continuous anticyclone, producing the fine weather of Australia at that season, interrupted, however, occasionally by bad spells, squalls, and thunderstorms, and thoroughly broken in the summer season, when the vast mass of the Australian Continent, with its atmospherical rarefaction in the Torrid Zone (when the sun is south of the Equator), must exercise large attractive action on all sides. This anticyclone at the latter season contracts very much, and leaves a breadth of low pressure over the Pacific between New Zealand and Queensland, in the vicinity of which a considerable number of the north-west storms which dash against or page 74 cross our Islands are probably generated. Hence the forecasts of Mr. Wragge, the Government Meteorologist at Brisbane, have a special value for New Zealand. On our western coasts these storms meet with a great impediment to their further progress south-eastwards in the masses of the Southern Alps, over which many of them fail to pass, and either spend themselves in dashing against the mountains or, with that affection for skirting mountain-chains and coast-lines for which they are remarkable all over the world, go off to the south and round the Island at Foveaux Strait. To what extent the Southern Alps modify the character of the north-west storm before it reaches the Province of Canterbury, and how this modification is brought about, I have considered in another paper.

Our north-west storms seem to belong to the classes 2 and 3, though occasionally we may have one which is simply a prolongation of a Torrid Zone atmospherical disturbance. Some of our storms are undoubtedly identical with those that dash up against the south side of the Australian anticyclonic area—traceable sometimes so far to the north-west as Mauritius—the region, be it remembered, of Indian Ocean hurricanes—but sometimes apparently coming from points much farther to the south, bringing bad weather to the extreme southerly districts of Victoria and to Tasmania, and then crossing over from Bass Straits to New Zealand in twenty-four hours. Such storms, after leaving Australia, always seem to make for the south-east. Even when at first, after leaving Bass Straits, they hug the east coasts of New South Wales and apparently make for the north-east, they turn at right angles to their course before they go very far, and merge into the Return Trades.

It does not follow that, because much of our north-west weather is cyclonic in character, all of it necessarily is so. Some of it may be produced by broad gales which are defined by Dove as strong winds, blowing in with tolerable steadiness from one point of the compass. The phenomena, as far as we in Canterbury are concerned, would be nearly the same whether this were so or not, for the gyration of the wind on one side of a cyclone is the same as that produced by the ordinary currents of the atmosphere, and the effect of the page 75 mountains would be similar in both cases. It therefore, as far as our present investigation is concerned, does not seem very important whether we adopt the rotary theory of Redfield or the inblowing theory of Espy. Dove certainly seems to speak of gales as something quite distinct from the cyclones of Piddington. But, as has been remarked, "the law of storms is the law of wind everywhere," and the terms "gale," "cyclone," and "tromb "only indicate similar movements of the atmosphere affecting areas of different magnitudes. A gale, in fact, may be assumed to be a portion of a huge cyclone whose true character is concealed from us because we are not able to carry on sufficiently wide simultaneous observations. The true cyclone itself is perhaps popularly misunderstood. It does not necessarily imply the violence of the tempest or hurricane, such violence only occurring when the isobars are very near—that is, when the depression is deep and the baric gradient steep. It sometimes is as much as 0 2, though the normal rate of barometric change is not more than 0.02 to more than 0.05.

Do our violent north-westers evidence the passing of cyclones over our heads to the south-east? Loomis and other meteorologists agree that storms as a rule in our latitude travel to the eastward—to the north-east in the Northern Hemisphere, to the south-east in the Southern. They make their way in that direction because of the rotation of the earth, the rain being drawn eastward, Blasius thinks, by traction of deficient air. That being so, however, I do not think that many of our north-west cyclones pass directly over Canterbury to the south-east, for the following reasons: It is very seldom indeed that the wind goes suddenly round from north-west to south-east, or that the central calm of the cyclone is observable, which would always be the case if we were here in the direct-path of the storm. Bain falls also very rarely, and the fall of the rain determines, or is supposed to determine, to a large extent the path of the disturbance; for it gives out heat and thus causes the air to expand and whirl upwards, making a comparative vacuum which the air behind fills, and so the direction of the whole slanting and inverted aërial whirlpool is determined.

What Loomis says about rain or snow on the west side retarding the progress of a cyclone is instructive. It favours the idea that our north-west storms, through the heavy rain they deposit on the Southern Alps, are stopped, worn out by dashing against or amongst the mountains, or else diverted, as already observed, along the coast-line and the flanks of the range. North-westers are experienced in the Chatham Islands, but there, as on the west coast of New Zealand, they are the rain-bringers and quite different from our dry page 76 föhn. They have similar origin to ours, but an independent one. As to whether we are in the line or path of the cyclone or not—apart from barometrical indications—the winds and clouds, especially cirrus and cirro-stratus, if we could perfectly interpret them, would be an infallible sign.

Some such rules as these—which I have adopted from Ley—will show us how we are situated with regard to the cyclonic centre. The diagnosis of our "nor'westers" almost invariably brings them under rules (2) and (4). But, of course, full and detailed synoptic charts—if we could obtain them in New Zealand—would be a far more certain criterion as to the point in question than clouds and winds. When the anticyclone is not to north-east but to east of us, and depression going south,—or west of us, and depression going north-east,—similar rules will apply; the system of points, however, being moved, in the first case, 4 points back, or, in the second case, 8 points forward.

To understand the development and character of our northwest gales in New Zealand we cannot do better than study closely the nature of the south-west storms of Great Britain, under the guidance of such meteorologists as Ley, Abercrombie, and Scott. The British sou'wester is the counterpart of the New Zealand nor'wester. But the former is accompanied usually, and almost everywhere, by heavy rains, because of the absence of a huge mountain-chain athwart its path; whereas the latter is so accompanied only along the west coast of New Zealand. The requisite changes in the direction of wind, &c., for the Northern Hemisphere being made, we see, page 7 however, the strong resemblance between the two winds. The barometer falls in England as the south-west wind approaches; the thermometer rises in the south-east or right side of the cyclonic path, and, as the storm progresses, the wind gradually veers to the west, and eventually into the bracing north-west. Abercrombie explains these phenomena as being the marks of a V depression formed either along the south prolongation of the trough of a cyclone, or else in the col between two adjacent anticyclones. If this be so, and our north-west, followed by south-west, storms (or some of them) be similar in origin, we experience under their influence the result of a V depression formed along the northern prolongations of the troughs of cyclones passing away to the south-east. When pouring rain comes from the south-west after a north-west wind of long or short continuance, we are, indeed, forcibly reminded of the "southerly bursters" of Australia, which also, it appears, are owing to a V depression passing to the southeastward along the southern coasts of that island, the point of the V being to the north, with the wind north in front, and south-west or south in rear. The phenomena of our northwest weather, although possessing a general similarity, are, it must be remembered, of various types, and some of these types may be explicable in the above fashion. The arch is sometimes absent altogether; the increase of temperature varies considerably; so does the duration of the storm, and the character of the south-west wind which supervenes; and therefore we can readily understand that, with somewhat similar north-west weather, the same fundamental shape of isobar will not always be found.

Mr. Cockburn Hood and Mr. Charles Knight (Trans. N.Z. Inst., vol. vii.) think our north-west winds are simply the hot winds of Australia which mount high into the atmosphere after leaving that island-continent, and gather moisture from the intervening sea. But surely there is a fallacy here. A high upper wind would not lick up moisture, and, if it did, it could not hold it for a journey of 1,200 miles, because of the low temperature it would acquire at its elevation. Sometimes, truly, as on "Black Thursday," the north-west weather of Australia may reach as far as New Zealand, for general weather is occasionally very widespread in its influence, as was evidenced quite recently (9th and 10th September, 1889) when the ship "Otago," two or three thousand miles to the south-east of New Zealand, nearly succumbed to the same succession of north-west followed by south-west cyclones as swept at that time over this colony. There is also such great similarity in some respects between the hot winds of Australia—blowing for two or three days together with excessive heat and violence from the north-west, and followed as they are page 8 by the cold south-west with deluges of rain and a fall of the thermometer often of 30° or 40° in an hour or two—and the nor'westers of Canterbury, that, making allowance for differences arising from local geographical peculiarities, it is impossible to suppose otherwise than that the two have some similarity of origin. The Australian wind blows however over an arid continent, and thus increases rather than diminishes its heat as it travels, and it meets with little or no obstruction from high mountains calculated to drive it up into higher and colder regions. On the other hand, the north-west wind of New Zealand by the time it reaches our shores has crossed a wide ocean and gathered from it both moisture and heat as it travelled, and here it meets at a comparatively high latitude with mountains 10,000ft. high, which are quite capable of relieving it of its watery burden. The distance between Australia and New Zealand is not so great as to lead us to doubt the occasional identity of some of our cyclonic disturbances with those prevailing over the water, and it must be remembered we are directly in the line of travel; and cyclones, though they often fill up or diverge from their path, and otherwise conduct themselves in a most unreliable manner, yet as a rule keep a definite track and travel very far. Notwithstanding all these considerations, I do not argue for any identity, except a very occasional one, between the hot winds of Australia and New Zealand. Our nor'westers for the most part have quite an independent origin.

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

By John T. Meeson, B.A., Christchurch, New Zealand.

1.	Introduction and description of phenomena of nor'westers.
2.	Inquiry into the cause of their excessive heat. Compression, the usual explanation, inadequate. True explanation must take account of—(1) Original heat of equatorial aërial current, (2) heat given out when rain precipitated, (3) different capacities of wet and dry air for holding heat, and Dr. Hann's reasoning about the Swiss föhn, (4) development of heat on the left front of all cyclone; why south-west winds not hot though over high mountains. Original and acquired heat of equatorial winds. Great effect of warm ocean-currents in heating air above them. Conclusion re heat.page 9
3.	Cold nor'westers.
4.	The behaviour of the barometer in a nor'wester.
5.	The north-west arch (of clouds).
6.	The south-west wind and subsequent gyration.
7.	North-west rains east of the Southern Alps.
8.	Other föhn winds than the New Zealand nor'westers.
9.	Their generally beneficial character.