Art. X.—A Wellington Weather Prognostic.
[Read before the Wellington Philosophical Society, 18th December, 1895.]
The weather is a subject which interests us all, and any help towards guessing correctly what sort of weather we may expect within the next twelve hours or so is valuable. I say “guessing” because no weather forecasts are infallible, even when aided by all that science and long observation have enabled us as yet to attain to. Observations of the fluctuations of the barometer, and of the winds and weather experienced at a number of distant points, collected at a central office by means of the telegraph, do enable a competent person to predict with a great measure of certainty the character of weather to be expected in the immediate future; but the chain of causes influencing weather is so complicated and so far-reaching that in the existing state of our knowledge certain prediction cannot be insured—only great probability; we know how great the probability is by comparing the daily forecasts made by Captain Edwin with the actual weather which follows; and I think we must all acknowledge that his forecasts are very generally right, although not always right.
What I wish to bring before this meeting is a prognostic which every one can observe, and which, since I first observed the sign, about two years ago, has hardly ever failed to be followed very shortly by a northerly blow and rain. I mean a peculiar form of clouds. I call them “fish” clouds; but probably “mushroom-shaped” or “lenticular” clouds would
be more correct, as, although these clouds, as seen generally rather low down in the eastern sky, seem like fishes with smooth hog backs, yet no doubt they would present somewhat the same appearance if viewed sideways from any other point.
The peculiarity of these clouds is that they are very smooth and regular in their upper curved surfaces, and comparatively flat below—quite different from the bubbling surface of a cumulus cloud, or from the straightly-drawn-out forms of stratus clouds, and also widely differing from the delicate feathery forms of the high cirrus clouds.
The fish clouds belong to the class cirrostratus generally, and I should estimate that their normal level is at least 10,000ft. above the sea. Generally, the barometer is high when they begin to appear. It then begins to fall, and a northerly blow follows, sometimes within six hours, more generally after about twelve hours; but occasionally it is delayed for twenty-four hours, or even more.
The appearance of these fish clouds may be that of small, delicately-shaped, scattered fishes, in which case the following north wind is generally not strong, and there is little or no rain. If the fish-clouds be more massive, or if, as often happens, they are joined together so as to form undulating, eel-shaped clouds, with the characteristic smooth, hard, curved outline above, then probably the northerly blow will be strong, with rain. Sometimes the fish clouds are superimposed one on another, so as to form, as it were, a pile or piles of fishes. This form is not so common, but I think it also is followed by bad weather.
So far I have merely dealt with observed facts, which I hope others will also observe, if they have not done so already; and it will be specially valuable to have instances when northerly blows with rain have not been preceded by fish clouds, or when fish clouds have not been followed by the wind or rain. But, admitting that my observations are correct, and that this form of cloud usually is seen before a strong northerly wind, can we in any way account for it? We know that the great system of circulation in our atmosphere, produced by the joint action of the sun's heat and the daily rotation of the earth, gives rise to vast eddies in the air, known as cyclones and anticyclones—the cyclones, or “lows,” if viewed from above, being like great saucers, rotating in this hemisphere as the hands of a watch; the anticyclones, or “highs,” like inverted saucers, rotating the other way. But it is with the cyclones, or “lows,” we are now concerned, as they give us our strong winds and storms. The motion of the air in a cyclone is very complicated: it is drawn inwards below, it is poured outwards above, it ascends in a spiral course,
and the whole system, extending, it may be, over one thousand or even two thousand miles of the earth's surface every way, is moving rapidly to the eastward. The rate of eastward progress averages about five hundred miles in the twenty-four hours. This causes the variations in the direction of the wind and in the height of the barometer. But the force of the wind varies with the rotary motion. The wind at the front of a cyclone approaching us from the west must be from the northward or north-westward if, as is almost always the case, the centre of the storm is not to the north of us. Very generally it is south of New Zealand altogether. Very rarely it is north of Wellington, and in these rare cases, when the storm would begin with a north-east wind, changing by east to south, probably the characteristic fish clouds would not appear.
For I imagine that their history is somewhat like this: As the cyclone advances from the west, warm, moist air is drawn in below from the north on that side of the eddy; it is whirled onward, upward, and southward until it reaches a cold level, where its water-vapour is condensed into clouds, and the dry air pouring over them smooths down their upper surfaces into the fish-back forms which we observe.
This seems to me a probable explanation of the way in which these clouds are formed on the north-eastern edge of an advancing cyclone here, and of the reason why their appearance should be a usual precursor of a north-westerly blow with a falling barometer, to be succeeded by a southerly blow with a rising barometer, as usually happens. The cause I have assigned is, of course, conjectural, but it seems to me reasonable; and, if it be true, the same weather prognostic ought, I think, to be true all up the west coast of this Island, and probably as far as Westport on the west coast of the South Island, or even farther south. On the east coast, or inland, probably this form of cloud would not be so usual or characteristic, as the advancing cyclone circulation is, as we know, much broken up by the great mountain barrier running nearly north and south through these Islands, and the indraught of air would be modified by the land-surface over which it must pass. The break in this barrier at Cook Strait and the direction of our coast-line here are undoubtedly the causes of the prevailing northerly or southerly winds experienced here, the westerly winds being deflected north or south, and easterly winds very rarely occurring, because, as before observed, the centres of the cyclones usually are to the south of us.