Art. LVI.—On the Gyration of the Wind in New Zealand, with its Characteristics in the Various Quarters. (Author's Abstract.)
[Read before the Wellington Philosophical Society, August 20, 1870.]
The extreme changeableness of the weather in the Southern Hemisphere is a well known characteristic of its climate. In this respect New Zealand is no exception from other southern lands. This is caused, in these regions, by the very regular rotation of the wind through the different quarters, together with its strongly marked characteristics while in these quarters. The striking regularity of these gyrations of the wind is one of the first indications of a change of hemisphere which is encountered by the voyager from Europe, where they are less regular from the greater mass of land producing local winds and disturbing the general atmospheric currents of the world. The moment the Cape is passed, these gyrations of the wind through N. to W., S., and E., with their strongly marked characteristics of warm and dry for the N., stormy and wet for the W., cold and damp for the S., and calm for the E. winds are experienced, up to at least the 50th degree of latitude.
Although the primary cause of these gyrations is now understood to be due to the earth's axial motion, modifying the direction of the great polar and equatorial currents of the atmosphere, still there are many points not yet positively settled in the theory of these winds. Some of these are the period of rotation, whether constant for different latitudes, different seasons of the year, and for different hemispheres,—whether it coincides in any way with the ephemeris of the moon,—at what latitude the cycle is equal sided, that is when the winds pass through the various quarters in about equal times,—at what
latitude these gyrations cease to be felt S. and N.,—and also, how the winds from the various quarters displace each other. On these various points I have but a limited amount of facts to adduce from my own observation, but though the quota is small it may call attention to these interesting points, and accumulate by eliciting the experience of others.
The period of complete gyration seems to be, as nearly as possible, seven days. In Victoria, the hot winds generally keep the same days of the week for months together, as also do the wet days in winter, both of which are coincident with—one the N., the other the W. wind. In New Zealand, these gyrations are more irregular than in Victoria, arising, no doubt, from the more irregular form of the land, and its greater elevation causing greater disturbance to the normal winds. Still, these rotations are well marked and tolerably regular throughout the entire length of the country. In Victoria, the wind rotates through the several quarters in nearly equal times, with perhaps a little skip through N.W.; in New Zealand, it often lingers in one quarter for a longer period, sometimes for an entire rotation, or even several. When it does so, however, the period is generally marked by a calm of longer or shorter duration; often for only a few hours, when the wind begins to blow with renewed vigour from the old quarter for another seven days. This, however, is seldom the case; in general, the wind rotates very regularly. In Southland, it is fond of lingering in the S. beyond its due time, and then backing through W. to N.W., where it would have been had it gone regularly round; when it does so, it blows with great violence, from N.W. for some time, often for two days, then passes in a stormy mood through W. to its home in the S. In bad weather, it thus oscillates through an arc from S.W. to N.W., each succeeding oscillation being more stormy, and the arc more contracted. In the north of the island, this is reversed, where the N.W. is the prevailing wind, and the N.E. gales the severest.
In the middle of the islands, the cycle is more equal sided, so that it would appear that between latitude 38† and 41† is about the middle of this zone of gyrating winds in New Zealand. By careful observation, the period, though more obscured by local and other disturbances than in Australia, can be made out, and appears to be about seven days throughout, so that it appears to be independent of latitude. The seasons disturb the regular course of the wind greatly. In spring, the wind holds longer in the N., and in late autumn, longer in the S., than during summer and winter, when it is generally more regular. During spring and autumn, it is often difficult to make out the complete revolution, but it is observable that when a well-marked crisis occurs, it is generally at the time when it would have been had the wind made its regular rounds, though a long period of disturbance may have intervened. This would argue that the true period is also independent of season, though these may obscure it for a considerable time.
Since the N.W. passage of the wind is the critical time for rain, a knowledge of the wind's days enables us to forecast the days which will be precarious in this respect. In bad weather, there are often two revolutions in one true period, and then there are two critical days in the week—the secondary often being greater than the primary. At other times, the wind remains steady at one point for a whole period, or even two, the crisis being marked by a few hours of calm.
The exact determination of this period over long intervals is a problem of great interest, as it would help to throw light on the causes which determine the period, whether it has any connection with the phases of the moon, as has often been conjectured, or is the result of a certain equilibrium between the strength of the polar and equatorial currents and the earth's axial motion, or other causes.
The characteristics of the wind in the different quarters are well marked and constant. The warm N., the stormy and ++rainy N.W. and W., the cold damp S., and the calm dry E., are invariable. The barometer also invariably rises with a S. and E., and falls with a N. and W. wind. The manner in which the various winds displace each other is also a point of scientific interest. Professor Dové found that the S. wind was blowing in the upper regions before it descended to the surface in the Northern Hemisphere; or, in other words, that the N. wind there displaces the S. wind first in a thin stratum at the surface, acquiring depth as it advances. He found that at the Peak of Teneriffe it is often blowing a S.W. wind when the N.E. wind is blowing at the base. The corresponding case seems to be true for the Southern Hemisphere, since at elevated stations, such as Bealey, the wind is often N.W., while the S. wind has set in at lower levels. The barometer also gives similar evidence, as it rises only when the S. wind actually sets it, while it falls for some time previous to the advent of the N. wind at the surface.
The occurrence of our white frosts before a N. wind and rain, would thus receive an explanation, as being the result of the warm dry N. wind in the higher regions bearing down towards the surface on the strata of cold damp S. wind, and by its evaporating power changing the vapour from a grosser to a finer state of particle, and hence causing cold in the regions below from a well known law, the intensity of the cold becoming a measure of the rapidity of evaporation and the approach of complete saturation or rain.
Another interesting point is the bearings of the line of displacement. This could only be determined with any accuracy by a combined system of simultaneous observations at distant stations free from much local disturbance. The form of the advancing area within which the S. wind is blowing, and the velocity with which it travels, are also points of great importance as indicating the probable path of cyclonic disturbance and the rate of their advance, both of which are found to be totally independent of the direction and force of the
wind within the storm itself. Any observations I have had the opportunity of making are totally inadequate to determine any of these elements, and can only leave the subject in the hope that others having better opportunities will work it up.