WEATHER FORECASTING IN NEW ZEALAND.

New Zealand Journal of Agriculture, Volume XIX, Issue 1, 21 July 1919, Page 33

WEATHER FORECASTING IN NEW ZEALAND.

(Continued from June Issue.)

By

B. V. PEMBERTON,

F.R.Met.S"oc., Assistant Meteorologist, Dominion

Meteorological Office, Wellington.

[Reprinted from the New Zealand Journal of Science and Technology.']

Typical Westerly Area of Low Pressure.

Fig. 4 depicts a westerly area of low pressure, and is, no doubt, the northern extension of an Antarctic cyclone wedged between the rear and front of two adjacent anticyclones.

The rate of movement from west to east varies . considerably, and averages about 400 miles per day. The dimensions and forms of these disturbances also vary, from the commonest type with a decided A shape to those having practically straight isobars extending over thousands of miles, and which consequently take many days to traverse the Dominion. The one now referred to may be taken as an intermediate between the above two. Its influences were effective over the Dominion between the 12th and 19th October, 1911.

The trough is that portion of the “ low ” along an imaginary line passing through the apices of the /\, and is therefore the dividing-line between the falling and rising barometer. In this case the trough passed over New Zealand on the 18th, and the winds, which had blown with gale force from the north and west previous to this date, backed to southerlies, still with gale force, but gradually decreasing with the advent of an anticyclone.

A characteristic effect of this type of disturbance is seen in the occurrence of hot north-west winds in the east-coast, districts in front of the trough. The cause of the high temperature' of these winds is as follows : As the moisture-laden westerly winds strike the slopes of the Southern Alps and are forced' to rise, the cooling which results from the consequent expansion of the air condenses the vapour, and heavy rain falls on the westward slopes. Latent heat is thus set free. The air descending on the lee slopes is dynamically heated by compression, and, with the addition of the latent heat previously attained, it reaches the Canterbury Plains as an exceedingly hot and dry wind. The name given to such a wind in Switzerland is the “ Fdhn.” Coming in the winter-time it is a welcome change from the keen temperatures then generally ruling, but in the summer it has a very enervating effect.

The change of wind from north-west to southerly is sometimes very sudden on the east coast, according to the sharpness of the /\, and this sudden change has earned the cognomen “ southerly burster.” A rapid fall of temperature takes place, and heavy rain falls, sometimes accompanying an electrical disturbance. When the A is less pronounced the change of wind is not so rapid, the direction remaining westerly while the trough is passing over the country and working gradually round to south-west.

Complex . Westerly Areas of Low Pressure.

It sometimes happens that these westerly areas of low pressure have two, or even three, distinct depressions or /\s, and the chart for the 5th November, 1915 (Fig. 5), is an example of such. The first depression passed over the Dominion on the 4th, and is shown on the chart some distance to the eastward of New Zealand, while the second depression is situated to the westward, over the Tasman Sea. These two “ lows ” are separated by a tongue or ridge or relatively pressure, which, at the time the chart was drawn, was just passing over the Dominion.

A temporary improvement in weather conditions was experienced at this time owing to the neutralizing effect of the two disturbances. Such an effect is usually so brief that a forecast for a fair interval could not be made in safety when the distribution of pressure shows two such intense storms in close proximity. The actual fair-weather period might last only a fraction of a day.

The succession of winds and weather experienced during the passage of this system may be briefly stated. On the 3rd November New Zealand

was in front of the first “ low,” and north-east to northerly winds began to freshen, with clouds gathering over the country. A' few scattered showers were reported in the west-coast and southernmost portions of the South Island, but otherwise fair weather prevailed. By the evening of the 3rd, however, a considerable decrease in pressure had taken place, and heavy rain fell, with strong north-westerly winds.

At 9 a m. on the 4th the trough of the first depression had passed to the eastward in the South, Island, and strong southerly winds were in evidence southwards of Cook Strait; while over the North Island the trough was then just passing, and westerly gales were widespread. Heavy

rain was then falling in all but the east-coast districts, between Banks Peninsula and East Cape. In Westland the rivers were reported considerably in fresh.

On the morning of the 5th, as before mentioned, both wind and'weather had moderated, but during the night a further fall in the barometer took place, and the weather again assumed a very boisterous character, heavy rain falling, accompanying thunderstorms in various localities.

About Cook Strait particularly electrical disturbances were severe, and these were apparently the result of a secondary centre passing through the strait in the early hours of the morning of the 6th.- On the weather map of the 6th the second depression was shown just eastward of the Dominion, and the winds were southerly, blowing with gale force over most of New Zealand, with cold and showery conditions.

Relation of Westerly Area of Low Pressure to Adjacent Anticyclones.

Fig. 6 is an isobaric chart showing the distribution of pressure over a much wider field, and includes the isobars as charted by the Commonwealth Weather Bureau and those of our own on the 5th June, 1913. It depicts the two typical forms of isobars discussed previously, and shows the position of the /\ or westerly low-pressure area in ’ its relation to two adjacent anticyclones. New Zealand is crossed by an almost constant procession of such anticyclones and westerly “lows,”-with other systems of less frequency which will be mentioned presently.

Between the two “ highs ” to the northward will , also be seen a wedge of relatively low pressure. . This is the southern extension of a subtropical low-pressure system, and is in the region where originate many of the cyclones which affect New Zealand.

Winter Cyclones.

During the winter months the' general high-pressure belt is in more northern latitudes, and there is a tendency for the centres of Antarctic cyclones to pass more frequently in the neighbourhood of the southern portion of New Zealand. Fig. 7 is an instance of this type, and the chart is an example of the benefit of the Macquarie Island wireless reports, without which, in this case, it would have been impossible to say definitely whether or not a centre was situated in the position where it is shown on the map. . ,

: The barometer at Macquarie Island read 29-45 in., while that at the Bluff recorded 29-25 in. .. The wind at the former station was easterly, while at the latter it was north-westerly. These reports gave definite evidence, that a centre lay between the two stations. The path of the centre was along the thick line. There were at least two centres in this system, the first being shown to the eastward. The second moved along, almost the identical track taken by the one preceding, which succeeding “ lows ” have a habit of doing. Except when the centre actually passes over the South Island, the accompanying wind and weather in this class of storm are -similar to that of a westerly low pressure, provided that the barometric gradient is about equal in ..both cases. \ . .. The above disturbance mighb be referred to as a westerly cyclone, because the path is in a direction nearly due west to east, and the winds are mostly from some westerly direction over the major portion of New Zealand, the changes being from north-west to south-west. -

Cyclone with Double Centre

. z. On the 9th August, 1913, the barometer ranged from about 29-70 in. to 29-90 in. over the whole Dominion, and the winds were only moderate in force and variable, but with a westerly tendency in the north and an ; easterly in the southern districts. Some heavy seas 'were, however, reported along the eastern coast. Fogs and mists were much in evidence, and scattered places reported drizzling rain. These conditions are characteristic of the approach of a cyclonic system centred due west of New Zealand. From the weather reports of the 10th and 11th this supposition was readily confirmable. On the latter date the centre was presumed

be' about 800 miles due west .of Farewell Spit, and on the following day some 250 miles west of the west coast of the South Island. On both the 11th and 12th northerly winds were experienced northward of Cook Strait, and strong easterly winds ruled in the southern districts. Rainfall was general, and foggy conditions were also prevalent. ■ In the chart of the 13th August (Fig. 8) the cyclone has wholly overspread the Dominion, but since the preceding day two centres have appeared, one off the east coast and the other off the west coast of the South Island: A range of mountains, such as the Southern'Alps, at right angles to the track

of a “ low ” or “ high ” system will sometimes delay its advance, and it is invariably noticeable that atmospheric pressure will remain relatively low or high, as the case may be, on the west coast when the greater part of the system has passed some time previously to the eastward of the mountainrange. It is possible that this is a factor in the - development of many of our “ secondary lows,” and the second centre of this cyclone may have been thus developed. ....

The effects of the division of the central region of the cyclone are shown in the decreased force of the wind south of a line joining the two centres and in the variability of the wind-direction. The latter, however, is quite in accord with the isobars. For instance, on the east coast of the South Island the direction is southerly in the rear of the first centre, while on the west coast of the same Island, from Farewell Spit to Westport, it is northerly in front of the second centre. Greymouth and Hokitika. have a south-east direction, these stations being on the southern front of the second centre. . ;

At the time the chart refers to (9 a.m.) rain was falling in- all that portion of the Dominion shaded in Fig. 8, and in various districts throughout the country the heavy rains caused floods in the rivers. - *■’ >

By the morning of the 14th August the first centre had moved farther eastward, while the second centre still remained • on the west coast, but with a tendency to move northwards. Eventually the latter took a most unusual path, passing due north until the 15th, and then recurving about ■ the northern extremity of the North Island. Fig. 9 shows the approximate paths of both the centres during the period- they were under observation.

It was not until the 16th that New Zealand was in .the rear of the whole cyclonic system, and on that day high southerly winds were experienced; On the two following days cold and changeable conditions prevailed, with passing showers along the east coast. The actual effects of this disturbance, therefore, lasted from, the 9th to the 18th August, with more or less stormy conditions during the whole of this period.

(To be continued.)

Rabbit Boards’ Rating. — It was brought under the notice of the Board of Agriculture at its last meeting that experience of the working of Part III of the Rabbit Nuisance Act (relating to Rabbit Boards elected by . ratepayers) had shown that the method under which contributions are levied from, farmers whose land is included in the Rabbit Board area required alteration. After discussion the Board of Agriculture decided to recommend to the Government that a better and more equitable means of levying the contribution would be at a rate per acre, instead of on the rateable value paid by the farmer on his land as at present. .

Soft Limestone. — Types of soft limestone from the Wellsford district' North Auckland, examined recently by the Department's Chemist were found to contain from 77 to 83 per cent, of carbonate of lime., . a: