WEATHER FORECASTING IN NEW ZEALAND.

New Zealand Journal of Agriculture, Volume XIX, Issue 2, 20 August 1919, Page 97

WEATHER FORECASTING IN NEW ZEALAND.

(Concluded from July Issue.)

B. V. PEMBERTON,

F.R.Met.Soc., Assistant Meteorologist, Dominion

Meteorological Office, Wellington.

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

Secondary “ Lows.”

In connection with, cyclones and westerly areas of low pressure there often develops a smaller or “ secondary ” disturbance, sometimes merely a loop or sinuosity in one of the isobars, or maybe a further centre. With “ secondaries ” there is nearly always a gentler baric gradient,

and consequently the wind-force is slighter, but they have often been known to produce more rainfall than the “ primary.” They are also apt to cause thunderstorms, and the one shown in Fig. 10 was accountable for some severe thunderstorms in the northern portions of New Zealand. -

Judging from the apparent direction of its path, as shown in the chart, this one may possibly have been an offshoot of a tropical “ low ” of which there was evidence on the preceding day. • Of all the atmospheric disturbances the “ secondary ” is undoubtedly the most difficult one for the forecaster to deal with. Its development may take place practically within local limits, in which case its effects would be felt before the forecaster became acquainted of its presence. There have been cases where heavy local rains have occurred which have utterly

falsified the forecast issued, at least for particular districts. When this happens people, especially those lacking a knowledge of weather science, are often apt to blame the forecaster, when neither the forecaster nor the system is really at fault.

Cyclone originating in Tasman Sea.

Cyclones sometimes develop on the Tasman Sea between New Zealand and Australia, and Figs. 11 and 12 show one of this nature, the centre of which passed through Cook Strait between 6 and 8 o’clock on the night of the 16th December, 1913. The winds and the weather proved more intense on the northern side of the centre. The shaded portion of Fig. 11 shows.where rain was falling at 9 a.m. on the 16th. It is noticeable how certain portions of New Zealand had no rain falling, particularly the east coast of the North Island, although near the storm-centre. This may be taken as a further illustration of how a forecast of rain may fail, and how impossible it is to state definitely those places which, perhaps through local peculiarities, are likely to miss many of the effects of the storm. The lowest reading of the barometer at Wellington on the night of the 16th was 29-00 in., and.this took place at 6.30 p.m. according to the barogram Fig. 13. ' ‘

Usually, with approaching cyclones, in front of the centre there is . a striking increase in temperature,, but in this case this effect was somewhat counteracted by the passage of the rear of a “ low ’’ the previous day, the centre of which is shown in the south-east corner of Fig. 11.

Tropical or North-west Cyclones.

Taking now the case of cyclones reaching these Islands from the tropics, Fig. 14 is an instance of one which has come from the north-west. To distinguish it from other' extra-tropical cyclones having different routes it might be advisable to refer to it as a north-west cyclone.

The first indications of its presence were freshening easterly winds in the far north, with a falling barometer, on the 19th July, 1911. On the 20th its centre was definitely located due north of Cape Maria van Diemen, and on the 21st, as shown in the chart, it had only moved a distance of about. 150 miles in the twenty-four hours. This storm was under observation until the 25th ; and between the 19th and 25th stormy weather and heavy rainfalls were the rule, particularly in.the east-coast districts.

North-east . Cyclones.

Figs. 15 to 19 give an example of an extra-tropical cyclone which may be described as a north-east cyclone. Storms of this type originate to the north-east of New Zealand, and advance from the direction of the Kermadec Islands towards the Bay of Plenty, then recurve oft East Cape, and finally pass to the south-east.

As shown in Fig. 15, an anticyclone is invariably located over the South Island with the approach of a north-east cyclone, and this causes a steepening of the baric gradient, and consequently intensifies the effects of the disturbance. These cyclones are the most dreaded ones as far as the eastcoast districts of the North Island are concerned, for during their passage they are responsible for heavy rains, high tides, and heavy seas —three conditions which, acting together, cause the rivers to flood in the Hawke’s Bay and Gisborne districts. This type of storm often remains stationary for several days, and, on account of the persistent south-east gales in and northward of Cook Strait, shipping has a very trying time along all the eastern coast. The saying “ It is an ill wind that blows nobody any good ” applies to this case, for while "the east coast of the North Island suffers severely from such a visitation, Westland, and often Otago, experience pleasant conditions. . „ ; On the 20th,. although the storm-centre was still located oft East Cape, another, low-pressure area- was centred to the south-west of New Zealand. The proximity , of these two depressions to each other caused a neutralizing effect, and Fig. 17 shows a decrease in the wind-force. Very heavy seas were, however, still-breaking on the coast between East Cape and Kaikoura. The improvement in weather conditions was a very brief one, as will be seen from the chart of the following day (Fig. 18). Here the cyclone and the westerly “flow” have combined, and southerly gales are widespread, accompanied by heavy rain in both the North and South Islands. Fig. 19 is the.weather map for the 22nd May, 1914. By this date all the “ lows ” had passed to the eastward, and an anticyclone was approaching, bringing an improvement in weather conditions. Fig. 20 is a chart combining the isobars of Australia and New Zealand for. the 18th May, 1914, and includes all the atmospheric systems shown separately in Figs. 15 to 19. The centre of each system is marked with a letter—viz., A to D—and the same systems are similarly marked in Figs. 15 to 19. By comparing each of the systems in Fig. 20 with the corresponding ones in the other charts we are. able to follow their respective movements. The whole system moves forward in the same way as it would were we to take a chart of the isobars only and move it from west to east over a fixed map of the country.

There are therefore two movements to account for—first, the circulation of the air in the system itself, and, secondly, the translation of the whole system from west to east. If we can anticipate the latter correctly, then the direction of the wind becomes a comparatively easy matter to decide. Just in the same way as eddies in a stream are carried along by the motion of the stream, so the cyclone and the anticyclone, which are huge atmospheric eddies, are carried bodily forward by the prevailing motion of the air, which in these latitudes is from west to east.

Cyclone-tracks.

Most of the cyclonic systems which originate in the tropics and eventually come within effective distance of these Islands take certain fairly definite routes. Fig. 21 shows the approximate paths of several typical cyclones, and these are the directions usually taken. The centre rarely passes directly over the land, especially where mountain-ranges intervene, and tends to follow the coast-line. They thus frequently pass through Cook Strait, and have often moved down the west coast of both Islands, striking eastward only when they have reached the southern extremity of New Zealand.

CONCLUDING REMARKS.

From what has been written it will be seen how indispensable the weather chart is in forecasting, and how such a chart cannot be quickly and accurately compiled except at a central bureau in direct telegraphic communication with a chain of observers. Yet in nearly all countries individuals may be found who, either for private gain, popularity, or maybe as a hobby, issue forecasts, and unfortunately they are sometimes encouraged to do so by newspapers. They are certainly to be admired for their boldness in undertaking a work which a responsible meteorologist would hardly dare do unless he had at hand all the data available only at an organized weather bureau. ' '

A general knowledge of the types and movements of storms should, however, be helpful to those anxious to form judgments of coming weatherchanges, and with this object in view farmers and shipping men particularly should be encouraged to study more frequently the official weather reports and forecasts issued daily. From the weather map it is possible to make predictions for from thirtysix to forty-eight hours, and occasionally a general idea may be obtained of the weather to be. expected over a period of five or six days. It is, generally speaking, impossible to make a forecast for a longer period, and investigations have shown that under the present development of meteorological science long-range forecasting is impracticable. . '

Winter Feeding of Pigs. During the past winter the pigs at-Ruakura have been largely grazed on a good clean field of oats and. vetches:, with excellent results. Good use has also been made of the stored pumpkin crop. ' . j

Limestones. — Samples of limestones recently analysed at the Chemical Laboratory gave the following percentages of carbonate of lime: Eiffelton, Canterbury, 89 ; Opotiki, 78 ; Carrington (Carterton), 89 ; Whitford (2), 87 and 70.