[Read before the Wellington Philosophical Society, 11th October, 1879.]
The subject treated of in this paper is one which has for some years past received great attention in other countries, and their publications upon Weather Forecast give us ample information as to the method by which their predictions are arrived at, and the measure of success which has attended their efforts. It is now proposed to give some description of the manner in which the Weather Forecast of this colony is carried out. The subject is one which may be considered of general interest, and the method of procedure differs in some respects from that practised in other countries; but as it has already stood the test of some remarkably changeable seasons, with a large percentage of success, it seems now desirable to invite further investigation in the matter.
In the first efforts to arrive at some conclusions as to probable changes in the weather, the principal difficulty which came under consideration was the deflections which the mountainous nature of the country seemed to have power to create, not only in the direction of the wind, but also in the distribution of pressure. For instance, an isobar will generally run very fairly from Russell to Grahamstown, Tauranga, Gisborne, and Napier; but instead of continuing toward Castlepoint and Lyttelton, it will be found to curve inland and westward toward New Plymouth, then to turn southward between the latter place and Wanganui, and return eastwards towards Castlepoint which it leaves on its right. Now, it will be found upon investigation that pressure is nearly always higher at Westport and Hokitika than at Kaikoura, which necessitates this isobar being again drawn westward, i.e., back through Cook Strait, and passing Cape Farewell turn toward south to Westport and Hokitika, from which point it returns eastward, passing out between Blenheim and Kaikoura. A second isobar should have commenced at Auckland and run toward Taupo, Wanganui and Blenheim; and a third might be reasonably expected to pass over Westport, Hokitika, Queenstown, and Balclutha; but it is evident that, according to meteorological law, they must follow the first line of equal pressure upon that side which will show that they define the position of a lower pressure, and this necessitates their following it through all its sinuosities, but westward of it.
Such distributions of pressure were found to be by far the most prevalent state of affairs, and in many instances the curves were much more distorted than shown on the diagram (Pl. I., fig. 1), but it seemed probable that other disturbing influences must also be in existence, and tending to
exaggerate whatever was due to irregularity of the land surface; and it was only by closely watching the changes accompanying the passage of every low area, and patiently endeavouring to assimilate these conditions, that it became evident that the complications already alluded to were the result of there frequently being more than one low area in the vicinity of the country at the same time.
One of the principal aids in arriving at this conclusion is derived from the substitution of contour lines for the isobar as generally drawn; these isobaric contours are drawn in the direction which the wind blows to—flow in fact with the wind—and the idea of adopting this plan originated in an endeavour to comprehend local peculiarities, for it was evident after investigating such remarkable distributions of pressure as those already referred to, that, if warnings were sent of an approaching gale, they must make known the limits within which it would be locally experienced, for if the warning were based upon theoretical principles it would frequently be incorrect, and the calculation of gradients under these circumstances seemed rather difficult.
In drawing these isobaric contours, the mobile nature of air has to be taken into consideration. We all know how susceptible it is of expansion or contraction, to changes of temperature and pressure; and this being the case, it is but a step further to allow that a cyclonic wind, whose shape may be assumed as circular while beyond the influence of land, may become much changed in shape when it approaches a mountainous country, and by reference to Piddington and other writers upon circular storms it will be found that this point has already been under notice; but a little further consideration will show that this is not the only difficulty that has to be dealt with, as it will soon become evident that the subject contains some complex features; for while the front or advancing curves of a cyclone, encountering high land, become retarded and deflected, the centre continues to press forward with undiminished speed, and consequently the isobars become packed in the vicinity of the retarding influence; but from these causes the curves will have a tendency to open out at the rear of the cyclone (Pl. I., fig. 2), and as the land has a retarding effect upon the cyclone's advance, so will it also act upon it after the centre has passed, causing the rear curves to be extended; but the advancing curves will be compressed under either condition (Pl. I., fig. 3).
In drawing these isobaric contours, it must be constantly remembered that they must never cross each other, and that the object must be to endeavour to depict a series of concentric rings more or less bent out of their true shape; each tenth of an inch of barometric difference must be thus shown flowing in the direction toward which the wind moves, and each curve in itself
reciprocating the movements of the curves upon either side of it, and every advantage must be taken of river-channels, mountain-passes, and other such routes by which the contours can be shown to make their way across the country; the state of the sea, especially at exposed places, must also be carefully shown by these contours; and by close attention to these principles, the occurrence of gales at points far distant from each other, and the existence of but moderate winds at places close to where such gales exist, can be accounted for, and reason can be shown why the sea-disturbance is subject to similar irregularities, all apparently the effect of purely local causes working independently of each other, but which are in reality part of one system, whose effect is intensified at these points.
The isobaric contour also enables another difficulty to be accounted for; viz., the fact that the barometer moves more rapidly at some places than it does at others; and why, after it has fallen from a high point at all stations, there are frequent instances of its not recovering its original position over a considerable extent of country for a long time; for instance, let it be assumed that pressure ranges between 30·50 in the northern part of this colony, and 30·20 in the south, and that a fall takes place amounting to about seven-tenths of an inch in the latter, and two-tenths in the former districts; now, when a recovery sets in it will be found that the barometer seldom rises to its original position at all places by a difference not unusually amounting to half an inch at the southern stations, and although rapid oscillations may take place in that part of the colony, yet pressure southward of Napier does not return to its original position for a period varying from a few days to several weeks.
This general position of the lower pressure toward the south is in accordance with our meteorological knowledge of this hemisphere, but it does not seem to offer an explanation of why high readings of the barometer in the south are not so unusual as may generally be supposed, and it should render forecast easy, as it gives a gradient showing westerly winds; but experience in storm-warning shows that easterly winds are frequent, and easterly gales must by no means be left out of the estimate of probable weather, especially at places lying south of the 40th degree of latitude, as it almost invariably happens that whenever the wind backs at places southward of Napier it changes into north-east, although it does not always blow a gale from that quarter.
In the attempt to account for some of these points the isobaric contours were of great value, and their use led to the idea of the possibility of the existence of multiple areas lying southward of contours, or lines of higher pressure, which retreat or move northwards as the low areas approach; advancing southwards when they have passed, or if they are passing at a considerable distance
from the country. These are the special principles of New Zealand Weather Forecast; and the diagrams (Pl. II.) accompanying this paper of the pressure within the New Zealand area on 7th, 8th, and 9th July, 1879, are examples drawn in accordance with its rules, but they must not be considered specimens of actual forecast—a subject which will be treated of in another paper.
In these diagrams the wind deflections are eliminated, and they are, in fact, diagrams of the results already arrived at.
In support of the system now in use it may be urged that if according to accepted principles each cyclone, or area of low pressure, is a complete circle, then it follows that from whatever point pressure commenced to diminish it must return to that point again as the low area passes away, unless it be assumed that, instead of pursuing a direct route, the cyclone has moved in a more or less erratic course, but by this principle of multiple areas we can readily perceive that it is possible that gale may succeed gale in rapid succession, and for a considerable period, each depression following its predecessor, and the whole system moving in a more or less curved but well-defined route; and it obviates what has always appeared to me to be an impossibility, viz.: the retrograde movement of any low area.
Secondly: the facility with which the approach of a “backing” wind can be foretold; this “backing” being in reality the advancing curves of a new depression, whose approach will cause pressure to diminish before it has reached the point from which it at first commenced to fall. The proximity of such an area is shown by the extent of the area over which the barometer is shown to be rising, and, together with the further area over which the isobaric contours show that pressure is likely to increase the sea-movement, has also to be taken into consideration.
Thirdly: the advantages afforded by it for reliable forecast of sea-movement, a point of information which is of considerable value to bar-harbours and roadsteads.
This principle of contour lines and multiple areas enables an explanation to be offered as to how such complex movements, as a decrease at the extremes and an increase in the central portions of the country, can take place; and, also, why several successive rapid movements may take place in the south without being nearly so remarkably produced in the north; and it also affords a means of determining the positions of the depressions, although they may lie at a considerable distance to seaward.
It also shows the existence of what may be termed double-centred areas of low pressure; in these the barometer falls rapidly, the wind veering by north and west and blowing a heavy gale, a recovery then sets in, the wind changes southward of west, and the barometer rises rapidly for about half an inch, and immediately that it reaches its highest point it commences to
fall again within a limited area, but continues to rise slowly at places beyond those limits, while within them the wind suddenly “backs” to the northeast, and a second hard northerly gale becomes rapidly developed within this area; this second fall usually reaches the same point as its predecessor, the barometer then makes a second rapid increase which extends to the whole colony, and a heavy gale from the southward is generally experienced, this being the second southerly gale within the limited area already mentioned; the lowest pressure in these gales generally ranges between 28·90 and 28·50, and the total fall at the southern extreme of the colony amounts to about 1·25 inches.
These areas usually travel about east by north, and the general, or I believe it may be termed the normal routes of the gales which approach New Zealand, are between W.S.W. and S.S.W., moving to the opposite quarter; but after a considerable period, generally not less than six months, a depression comes in from the north-west, after whose passage the normal route is resumed, and this change takes place, on some occasions, with such promptitude that it is difficult to issue warnings in advance of it. Comparatively few of these north-west areas have come under investigation since the principles of Forecast now in use have been fully in operation, but there have been several of them, and there is no doubt that they, at times, are of the class here described as double-centred. The depressions which approach from the west of south are systems of multiple areas, some of which are of intricate construction, and during their passage the wind changes from north-east to north and west, veering as we term it in New Zealand, but backing according to meteorological law; and, as each successive area approaches us, the wind moves from west of south to north of west, which we term backing, but which is a veering movement according to meteorological law. From these remarks it will be seen that the wind-change in this colony is the same as in Great Britain, but during the passage of areas from north-west the wind obeys the laws for the Southern Hemisphere.
An interesting subject for investigation is offered by the atmospheric circulation of the temperate zones; for in England and America the same routes seem to hold good as in New Zealand, the depressions travelling from south of west to the north of east, and being at times interrupted by the passage of an area from north-west, and it thus appears that northern countries are mainly supplied from a tropical direction, the balance being restored from a polar quarter, whereas our supply is generally derived from the polar side, and recouped, as it were, from the tropic.
A marked feature of this Forecast are binding isobaric contours, or binding-lines, which enclose the secondary and minor areas, and which are
detected by pressure being lowered by successive steps, none of which are immediately recovered. These binding-lines are an indispensable feature of the work, and to explain them more readily it is necessary to refer to the accompanying diagrams (Pl. III.), the first of which is intended to illustrate the passage of a system whose low areas travel on a route inclined about 67£ from the true North or E.N.E. It extends over a period of eleven days, an interval which has been, for convenience-sake, extended to fully one-third more time than such a system would usually occupy. Each division upon the line of route represents 24 hours, further subdivided into 12-hour spaces; and by moving the diagram on the line of route, making each division coincide with that upon the fixed line, and marking the barometer readings at Hokianga, Wellington, and Bluff upon the usual form of register, an illustration will be obtained of how movements, which do not appear to have much in common, may be shown to be the result of one system of depressions, and that they are in reality reciprocal.*
This diagram shows that on the first day the barometers were 30·55 at Hokianga, wind north-west; 30·47 at Wellington, wind north-north-west; and 30·30 at Bluff, wind north. As we advance the diagram to the right, we find, after an inverval of twelve hours, that it has fallen nearly a tenth at Hokianga, five-hundredths at Wellington, and one-tenth at the Bluff, but without any material change in the wind-direction, though it will have increased in force, and would under these circumstances amount to a strong wind at places in the South Island. By the second day we find pressure still diminishing, and that during the last twelve hours the barometer has fallen to 30·40 at Hokianga, 30·30 at Wellington, and to 30·00 at Bluff. The wind has at each of these places changed more towards west, backing according to meteorological law, but veering according to our views, and a heavy northerly gale is now blowing at places lying southward of the contour of 30·30, there being also a heavy north-west sea at Grey-mouth and Hokitika. A further interval of twelve hours shows that the barometer is still falling; and on the third day it reads 30·25 at Hokianga, wind west; 30·10 at Wellington, wind north-west; and 29·60 at Bluff wind about north-west. The total fall at this station now amounts to seven-tenths of an inch in three days, which would in reality have occurred within one, but it has been extended for the sake of keeping the curves further apart. By continuing the movement of the diagram to the right, we find that within the next twelve hours the barometer at Bluff makes a further downward movement to 29·55, giving a total decrease of seven-and-a-half
[Footnote] [*Note.—For the moving diagram, a chart has been, for convenience sake, substituted, on which the position of New Zealand is depicted in relation to the isobaric contours at successive periods of two days' interval (Pl. III.).]—Ed.
tenths, and that within the same twelve hours it rises to 29·64, the wind veering south of west as pressure increases. A southerly gale is now blowing over the South Island, and a heavy sea accompanies it at Greymouth and Hokitika; but the barometer is still falling at Hokianga and Wellington, and we further find that it rises at Hokianga, where its lowest point is 30·25 before pressure has at all increased at Wellington, where it falls to 29·93. On the fourth day the barometer has risen to 29·90 at Bluff, the sea making moderately on the eastern coast; at Wellington the wind has changed southward, and pressure has decreased to 29·88; and at Hokianga the barometer has risen a little. The wind is now from the southward of west throughout the colony, and the low area which has just passed is now shown to the eastward.
Now, if this cyclonic wind is a true circle, pressure should continue to increase in all parts of the colony until it has returned at all stations to the point from which it commenced to diminish, and the wind should change to eastward of south; but these conditions are frequently delayed for a considerable time, and it generally happens that the barometer commences to fall again in the south long before it has attained the height necessary to ensure complete cyclonic formation, the deficiency being curiously graduated, the approach to the complete form being most nearly attained in the north and becoming markedly less so in the south.
To anticipate this falling movement, which is always accompanied by a backing wind, is one of the difficulties of weather forecast; and, as its occurrence is a sure sign of more bad weather, it is evident that a warning received after it has taken place is deprived of much of its value in practice. The approach of this backing movement is shown by the tendency of the curves to open out, caused by there being but little difference of pressure at adjacent stations. It is more readily detected in the southern than in the northern part of the colony, there being a wider land-area in the former case, and it is also accompanied by a decrease of sea on the western coast of the South Island; unless the new area be of large dimensions, in which case the sea will change northward, even while the barometer is rising; or, if the depressions are passing more to the southward of us than usual, and are at the same time moving on a north-easterly line, then the sea will continue from the south-west, as if the barometer were about to continue rising.
Another point of value is humidity, which will usually be found to have decreased as the barometer rises, but not to the extent that the increased pressure would imply, while, in some instances, it will be found that an increase accompanies an increase of pressure; neither of these movements seems unreasonable, if it can be admitted that the northern winds of one depression can exist in close proximity to the southern winds of the one im-
mediately preceding it; in the space intermediate between the areas the weather is nearly always fine with light winds, whose direction mainly depends upon whether the place is nearest to the approaching or departing depression.
In this example the space between the two is shown to be 29·90, and thus the line of 30·0 becomes an isobarometrical binding-contour or binding-line of one or more such areas as have just passed, and if pressure again diminishes, the re-appearance of all the contours above 30·0 will be delayed, and therefore they are also binding-lines; but as experience shows that pressures above 30·10 are of much less frequent occurrence at the southern stations than at others, it will be more convenient to consider 30·10 as the first binding-line, 30·0 the second, and so on, should the systems be sufficiently complex to necessitate the number of these lines being increased.
Referring again to the diagram, it will be found that while the wind continues from the southward upon the eastern and western coast, it has already backed at the Bluff, and as the diagram moves along it shows pressure diminishing rapidly in the south, and within twelve hours it has fallen to 29·75 at Bluff, wind north, and a second northerly gale has now commenced, accompanied by a heavy sea upon the western coast; but at Wellington the barometer has risen to 29·95, wind still from the southward, while at Hokianga it has risen to 30·28, wind W.S.W. The fifth day shows a further fall to 29·55 at Bluff, and a corresponding movement has evidently taken place over a wide area, resulting in a heavy northerly gale; the barometer has now risen to 30·0 at Wellington, the highest point reached being about 30·02, the wind soon afterwards backing to N.W., while but little change has taken place at Hokianga; within the next twelve hours the barometer falls to 29·45 at Bluff, and it also rises to 29·50, wind as before, veering southward as pressure is increased, and causing a heavy sea on the western coast; at Wellington the barometer falls to 29·86, and although this is only a fall of 16/100 it is accompanied by a heavy northerly gale at this station; at Hokianga the barometer falls to 30·22, wind W.N.W.
On the sixth day a further increase has taken place in the south, the reading at Bluff being 29·72; a hard southerly gale is now experienced throughout the South Island, but pressure at Wellington has diminished to 29·78, and at Hokianga to 30·18, the wind having now changed south of west at both places; twelve hours later the barometer has risen to 29·90 at Bluff, and the gale has now decreased at stations south of Lyttelton, but is blowing hard between that place and Napier, barometer reading 29·82 at Wellington, rising; and 30·15 at Hokianga, where it has fallen slightly, the wind being a gale from the southward of west at both places, and the sea is now heavy between Kaikoura and Castlepoint.
Now, according to the difference in pressure, which has been frequently shown by this diagram as existing between Hokianga and Wellington, it is evident that gales should have been more generally mentioned as occurring at the former station; but in practice this is not found to be the case nearly so often as these differences would admit; and this is mainly caused by there being a spreading out of the contours at the upper part of the North Island, which will be more readily understood by a reference to Diagram No. 1. The second low area is now shown to the eastward of the colony; and by the seventh day pressure is shown to be still increasing. The second binding-line has passed the Bluff, and this is evidently the first occasion upon which the barometer has stood at 30 inches at that station since the commencement of this series; pressure at Wellington has risen to 29·90, and to 30·18 at Hokianga. This seventh day shows the second depression at a long distance from the land, but it is evident by the contours that it continues to influence the weather, as the winds are still all from the southward; but there is now again a widening of the contours in the South, showing the approach of a backing wind; the barometer at Bluff now reads 30·02. A further interval of twelve hours shows that pressure has continued to increase in the North, the readings being 30·0 at Wellington, and 30·20 at Hokianga, but at Bluff it has fallen to 30·0, and the wind has backed into north. It now becomes evident that the first binding-line of 30·10 encloses not only the two areas which have already passed, but that perhaps several more may be approaching. The eighth day shows a fall to 29·85 at Bluff, but it has continued rising at Wellington, where the barometer reads 30·06, the second binding-line having passed, and pressure has also increased at Hokianga to 30·25. The diagram now shows that during the next twelve hours the barometer at Bluff has fallen to 29·75, and has a third time commenced to rise, wind again changing to the south of west; and by the ninth day pressure has increased to 29·90, but it has decreased at Wellington to 30·02, wind backing about four points, while pressure has steadily increased to 30·30 at Hokianga; during the next twenty-four hours, the second and first binding-lines are shown to have passed the southern extreme of the colony, and the barometer has risen at all stations, being now 30·15 at Bluff, 30·08 at Wellington, and 30·30 at Hokianga, all with winds from southward. The third depression is seen to the eastward of the South Island; the barometer continues steadily rising; and by the eleventh day the first binding-line is shown to the eastward of Wellington, and pressure has increased to 30·25 at Bluff, 30·18 at Wellington, and 30·33 at Hokianga.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|New Zealand.||Weather Reports for July 7th, 1879, 9 a.m.||Weather Reports for July 8th, 1879, 9 a.m.||Weather Reports for July 9th, 1879, 9 a.m.|
|Station.||Barometer corrected for height.||Attached Thermometer.||Wind Magnetic.||Force 0 to 12.||State of Sea.||Barometer corrected for height.||Attached Thermometer.||Wind Magnetic.||Force 0 to 12.||State of Sea.||Barometer corrected for height||Attached Thermometer.||Wind Magnetic.||Force 0 to 12.||State of Sea.|
|Wangarei||29·98||53||S.E.||3 to 4||29·57||53||S.W.||3||29·64||56||S.E.||4|
|Auckland||30·05||60||E.N.E.||2 to 3||29·62||58||Calm.||29·57||59||Calm|
|Coromandel||30·01||50||N.E.||2||29·55||47||Calm.||29·54||52||West||3 to 5|
|Grahamstown.||30·06||51||Calm.||29·62||56||N.W.||1 to 3||29·52||51||W.S.W.||6|
|Cambridge||30·00||47||S.E.||2 to 3||29·65||52||N.E.||2 to 3||29·45||48||Calm|
|Tauranga||30·09||50||Calm.||Slight swell.||29·65||56||N.W.||2 to 3||Heavy swell.||29·51||51||West||2 to 3||Smooth.|
|Taupo||30·04||47||North.||2 to 3||29·58||54||North.||2 to 3||29·44||50||S.W.||8 to 9|
|New Plymouth||29·92||55||S.E.||3 to 4||Smooth.||29·52||56||North.||2 to 3||Heavy.||29·43||52||S.W.||3 to 4||Rather heavy.|
|Opotiki||30·08||48||South.||2 to 3||Smooth.||29·66||58||N.W.||5 to 6||Heavy.||29·48||51||N.W.||2 to 3||Moderate.|
|Opunake||29·82||48||North.||1||Moderate.||29·45||54||N.N.W.||1 to 2||Moderate.||29·38||52||S.W.||3 to 4||Rough.|
|Wanganui||29·96||48||N.E.||2 to 3||29·53||57||Calm||29·42||54||West||5 to 6|
|Napier Spit||30·15||50||Calm.||Slight swell.||29·67||54||Calm.||Slight swell.||29·42||59||Calm||Smooth.|
|Wellington||29·95||54||Calm.||29·45||59||Calm.||29·30||58||N.W.||2 to 3|
|Castlepoint||29·98||48||N.E.||3 to 4||Moderate swell.||29·50||52||N.E.||2 to 3||Rough.||29·25||48||S.W.||3 to 4||Moderate swell.|
|Westport||29·85||47||S.E.||2 to 3||Moderate sea.||29·46||52||North.||2 to 3||Considerable sea.||29·44||48||S.E.||2 to 3||Considerable sea.|
|Kekerangu||29·83||42||N.N.W.||4 to 7||Hvy. East'lysw.||29·36||52||N.E.||2 to 4||Easterly swell.||29·23||53||S.S.W.||2 to 3||Slight swell.|
|Kaikoura||29·82||43||Calm.||Slight swell.||29·27||50||N.E.||2||Heavy.||29·24||46||East||1||Slight swell.|
|Hokitika||29·77||49||N.E.||4 to 6||Modte. N.W. sea.||29·35||48||N.E.||2||Moderate.||29·34||43||East||2 to 3||Considerable sea.|
|Bealey||29·17||46||N.W.||3||29·38||38||N.W.||5 to 6||29·38||37||N.W.||2|
|Timaru||29·77||56||North.||2 to 3||Smooth.||29·20||42||Calm.||Considerable sea.||29·14||46||Calm||Slight S.E. swell.|
|Oamaru||29·89||46||West||2 to 3||Smooth.||29·18||47||Calm||Slight swell.||29·16||48||West||3 to 4||Smooth.|
|Roxburgh||29·65||42||N.W.||5 to 6||29·14||45||Calm.||29·18||50||N.W.||2|
|Port Chalmers||29·79||55||N.W.||7 to 8||29·12||52||N.E.||2 to 3||29·14||46||N.W.||3 to 4|
|Queenstown||29·68||35||North.||2 to 3||29·21||38||N.E.||2 to 3||29·23||39||N.W.||2 to 3|
|Balclutha||29·68||34||N.E.||5 to 6||29·12||40||S.W.||2 to 3||River low.||29·11||30||West||2 to 3|
|Blenhein||30·06||50||West||1 to 2||29·41||52||N.E.||2 to 3||River low.||29·29||52||West||3 to 4||River low.|
|Bluff||29·58||39||N.N.E.||1||29·11||44||N.E.||2 to 3||29·12||46||W.N.W.||7|