WEATHER FORECASTING IN NEW ZEALAND.

New Zealand Journal of Agriculture, Volume XVIII, Issue 6, 20 June 1919, Page 348

WEATHER FORECASTING IN NEW ZEALAND.

By

B. V. PEMBERTON,

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

Meteorological Office, Wellington.

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

INTRODUCTION.

Although the barometer was invented as long ago as 1643 its use as a means of foretelling the weather was recognized only about/ the beginning of last century, The method then adopted was to take an average of the readings of the barometer and thermometer, and the direction and force of'the wind. These results were then charted to show the means of the elements over the earth. . This method was a 'great advance, and gave am impetus to the scientists of those days to further the study of weather prognostics. • • •

It was found, however, that these general means were certainly not sufficient to give an idea of what weather to expect from day to day, or even of the average climatic conditions from month to month, that could be depended on. For instance, with regard to temperature, the mean monthly temperatures, taking the average of many years, may prove very different in any particular year. Napoleon discovered this to his cost, for he assumed, on the judgment of Laplace, that the cold of Russia would not set in until January, but in that year the great cold came a month earlier, and his army was lost in consequence. , ' . ,

During the Crimean War a violent storm was experienced on the 14th November, 1854, and the French and English fleets in the Black Sea narrowly escaped destruction, while the French man-of-war “ Henri IV ” was lost. It was known that a storm raged some, days previously in western Europe, and this fact influenced the French Minister of War, Valliant, to order an investigation by the astronomer Leverrier. From the data collected it was shown that the Crimean storm was identical with that experienced some days previously to the westward, and this occurrence caused a greater interest to develop in the study of storms and their progression over the earth’s surface, and a revision of the science of forecasting weather-changes generally. •

About the year 1861 the first attempt of forecasting by means of synoptic charts , was undertaken by Admiral Fitzroy, who was afterwards in charge of the London Meteorological Office as its first Director. The whole system of weather forecasting is primarily dependent on telegraphic communication, and the development of radio-telegraphy, by an extension of the observation area, has also been a further practical aid to the forecaster. ■ ■

METHOD OE FORECASTINC

The history of the development of this branch of meteorology having thus been briefly touched on, an attempt can now be made to explain the principles of forecasting’ as adopted by weather bureaux throughout the world ; and, as our own local weather is naturally of more particular

interest, it will be more easily followed if the conditions in New Zealand are used to illustrate these remarks.

-The particulars necessary for the construction'of a weather chart are telegraphed to the central office from places distributed over the country, and also, if available, from places farther afield. - These observations of wind direction and force, barometric pressure, temperature, weather—from coastal stations, state of sea and tideare all taken simultaneously. The most important of these reports is the barometer-reading, and in order that the. observations from’ this instrument may be intercomparable they must be corrected and reduced to sea-level. This is. necessary because the pressure of the atmosphere diminishes' with height, and so the reading of the barometer decreases approximately one-tenth of an inch for every 100 ft. increase in altitude-. The corrected readings are entered by signs and figures on a plain map, and lines are then drawn connecting all those stations having the same barometric pressure. These lines of equal pressure are called isobars, and are drawn for every tenth of an inch : thus a line would join all places representing a pressure of 29-9 in., 30-0 in., 30-1 in., and so on. . . ...

The wind is indicated by arrows, the head of the arrow, pointing in the direction in which the . wind is blowing, and the force being shown by arrows of different types. An arrow with a barb on one side only indicates light wind (see Fig. 1), a barb on each side indicates a breeze, while one., two, and three “ feathers " on one side of the shaft indicate progressively stronger winds, the last representing a gale. There are also numerous signs representing different conditions of weather, state of sea, &c.

Ballot’s Law.

If the different synoptic charts which follow are examined' definite principles will be perceived in regard to the direction of the wind with relation to the isobars. Professor Buys Ballot, of Utrecht, was the first to formulate a general rule with reference to the direction of the wind and its relation to barometric pressure. This was in 1850, the law in simple language being : Stand with your back to 'the wind and the barometer will be lower on your right hand than on your left.” This is applicable in the Southern Hemisphere, while in the' Northern left must be substituted for right, and right for left. For example, suppose at Wellington a northerly wind is blowing, then if we face south our right will be the side where the barometer is lower ; if the wind be westerly, then the barometer should be lower to the south and higher to the north of us. The wind does not assume a direction strictly parallel with the isobars, but it is nearly always found that the direction is inclined at an angle of about 35° to the isobar and towards the position of low barometric pressure.

Mountain-ranges, steep coast-lines, or other topographical features. often cause a marked deflection in the wind-direction, in some cases to such an extent that the very opposite wind may be experienced to what would be anticipated according to the isobaric trend.

Baric Gradient and Wind-velocity.

From the consideration of the direction we now pass on to that of the velocity or rate of the wind, which is likewise related to the distribution of atmospheric pressure. If we take a line at right angles joining two isobars, the measure of this line is called a “gradient.” The shorter the

distance between the isobars the greater or steeper is the baric gradient, just. in the same way that the varying distances between contours on a military map denote varying steepness of the hills. It is the. practice to compute the gradient in hundredths of an inch per 15 nautical miles, or the quarter of a geographical- degree ; but when one becomes accustomed to the drawing of isobars over a: certain defined area one very quickly becomes experienced in estimating the amount of gradient approximately enough for practical purposes. The barometric gradient enables one to anticipate the velocity of the wind, the law applying to this being that the strength of the wind depends upon the amount of the baric gradient : the steeper the gradient the greater will be the velocity of the wind.

The air being extremely mobile, it follows that when there is an inequality .. of level between different places the tendency is for a flow to set in from the higher level to the lower, in an endeavour to restore equilibrium. Consequently the greater the difference in level, as shown by the closeness of the isobars, the more rapid will be the flow.

Tables have sometimes been set out for different places showing the gradient and the corresponding mean wind-velocities, but these can be taken as only approximate. There are many factors, such as local configuration and conditions, which also govern the wind-velocity, and therefore the highest winds are not always found where the gradient is steepest. For instance, when the gradients are steeper either over the North Island or the South than they are about Cook'Strait the strongest winds are invariably experienced in the Strait, and this is naturally accounted for by the indraft through the Strait. As the widest portion of the Strait is to the north-west, with a westerly wind the volume of air becomes more confined when it reaches'the narrower limits about Wellington, and consequently the horizontal pressure exerted must be greater and the velocity increased. Thus it is found that a steep gradient over the South Island causes a stronger wind in Wellington than would a similar gradient over the North Island. - • •

It will therefore be seen that the actual reading of the barometer at a single station can give one but little indication- as to the direction and force of the wind, or the kind of weather to expect. For the purpose of anticipating the character of the weather the larger one's field of vision the greater is the probability of the forecast being verified. On this account it is evident that large tracts of country, such as continents, should present less difficulty in forecasting weather than does, for instance, New Zealand, whose greatest width from the west to the east coast is only .280 miles in the North Island, and 180 miles in the South.

Cyclones and Anticyclones.

When a set of weather maps is examined an endless variety of isobaric shapes may be discovered. In fact, it would be almost impossible to find two maps presenting identical characteristics in this respect. There are, however, really only seven fundamental types of pressure represented by these isobars, and of these the two principal ones are the cyclone, and the anticyclone. .

Both these systems are approximately circular or oval. In the cyclone the central or inner isobar represents the lowest reading of the barometer. Around the centre the wind rotates in a similar direction to the hands of a clock, so that, should the cyclone be moving in an easterly direction, in

front of the centre the winds would be northerly, and in the rear southerly ; directly north of the centre , the winds would be westerly ; while to the south, easterly winds would prevail. These directions hold good everywhere in the Southern Hemisphere, while north of the Equator exactly opposite directions are experienced, a law which is governed by the effect of the rotation of the earth on all moving bodies on its surface.

Usually the- isobars in a cyclone are very numerous, and this accounts for the strong winds experienced. Besides- the cyclone there are other types of pressure which cause high winds, and heavy rains, and it is a mistake to .refer to every storm of exceptional severity as a cyclonic storm, without having the knowledge that the conditions were really due to a cyclone.

An anticyclone, as its name suggests, is the reverse of a cyclone, for the pressure is highest in the centre, and gradually diminishes outwards. The winds are ..more variable, particularly in the central region, while in the Southern Hemisphere their general direction is opposite to the; motion of the hands of a clock, the front of the centre, when it is moving due east, having southerly winds, and the rear northerly. The winds generally have but little force, since the isobars are not so closely bunched together as.in a cyclone. . ,

The most important conditions ■ relating to these two systems are the association of unsettled weather with cyclones, or low Tarometric pressure, and fair weather with anticyclones, or high barometric pressure.

Areas of high and low pressure entirely control our weather, and the changes due to both are brought about by their translation across the Dominion, for the whole system of isobars moves forward from west to east, and when any distinct type in the system is passing over the country the winds and weather usually associated with such a type are experienced. Our knowledge of the direction and rate of their movements makes the forecasting of weather possible.

Circumpolar Pressures,

There are two theories with regard to the arrangement of the Antarctic circumpolar system, of isobars. The one which seems the more likely in the light of the observations made by explorers in recent years is attributed to Lockyer, and is as follows -: The polar, region is covered by. relatively high pressure, while on its outer or northern perimeter are a succession of cyclones, which are constantly .travelling from west to east at the rate of about 400 miles per day. Such a distribution of pressure would account for the strong westerly winds between the 40th and 50th degrees of latitude ; from the 50th to the 60th we then expect to find easterly and southerly winds, and beyond the 70th degree there would be a continual outflowing of southerly, win from the Pole. •

The other theory is that the whole of . the polar area is covered by an immense cyclone which is centred at the Pole. If this were correct we should invariably expect westerly winds in the higher latitudes ; but Sir Douglas Mawson at Adelie Land experienced mostly southerly and south-easterly gales. The lowest pressure, also, would nearly always be found nearer the polar region ; but according to both Sir Douglas Mawson’s and Captain Amundsen’s observations this is not so, for pressure is invariably lowest farther north.

. The Macquarie Island* reports have been extremely valuable in helping to elucidate this matter, and from these it would seem that the centres of the cyclones above referred to pass usually somewhat to the south of that island. .

The mean number of intense “ lows ” passing Macquarie Island during the three years 1913 to 1915 was as follows : January, 6-3 ; February, 5-7 ; March, 3-6 ; April, 4-0 ; May, 5-0 ; June, 4-3-; July, 5-7 ; August, 4-3 ; September, 6-0 ; October, 9-3 ; November, 6-5 ; December, 4-0.

There are therefore sixty-four cyclones passing during the year, and, taking the average rate at 400 miles per day, there would appear to be at least eight or nine primary cyclones circling the subantarctic regions at the same time.

Westerly Areas of Low Press

Farther north than the latitudes in possession of the cyclonic systems we come- to the region of anticyclones. .The latter move somewhat irregularly from west to east, accompanied on their southern extremities by the before-mentioned Antarctic cyclones. ' Between adjacent anticyclones the northern portions of these cyclones tend to wedge themselves, with the result that in each case a -shaped depression is formed. This is the commonest of all the “ lows ” which pass over the Dominion, and is frequently referred to in the weather reports as a westerly area of low pressure, particularly when., the isobars assume a wider sweep, than in an actual Antarctic A*

TYPICAL EXAMPLES.

The above descriptions distinguish briefly the chief weather controls, and show the principles on which the forecasts are based. It will make the matter more explicit if the statements made are now illustrated by means of examples of atmospheric systems that have actually existed.

A Typical Anticyclone.

Fig. 1 shows a typical anticyclone or high-pressure system which overspread the Dominion on the 13th August, 1912. Its centre, or inner isobar, circled the whole extent of New Zealand, and when such is the case fine and clear weather results, and in winter, late autumn, and early spring sharp frosts occur in places subject to them. The large arrows denote the approximate direction, of the wind in relation to the isobaric trend.

The actual direction of the wind at each reporting station is denoted by an arrow, and a glance will show how variable it is in the central area, which is about Cook Strait. At■ Wellington the wind is north-east, while at Farewell Spit it is south-west. North of the path of the centre the tendency is all for easterly winds, while in the south the predominating direction is westerly. Along the east coast of the North —that is, in front of the central isobar of 30-3 in.—southerlies prevail.

As the whole < system moves forward to the east the winds. over the North Island will “ back ”* (or change in an opposite direction to the motion of the pointers of a — i.e., from easterly to northerly ; while in the South they will “ veer ” (or change in the same direction to the motion of the pointers of a clock) — i.e., from west to northerly. Such, an anticyclone, in summer, would , account for high temperature in the daytime,. and the nights would be cool ; frosts might even occur at some of the high inland places. ...

west and east over New Zealand, and favourable to strong westerly winds, especially over the South Island and in Cook Strait. With this distribution of pressure showery weather is experienced on the western coast, extending as far north as Cape Egmont at least, and also often passing through Cook Strait.

This is a type of pressure which is rather prevalent in. the spring, and to which the continued dry weather so often experienced at this season of the year in the eastern provinces is accountable.

The normal track of our anticyclones varies according to the seasons, being located in higher latitudes in the summer than in the winter., In the spring months the position, then, would be midway, as shown in Fig. 2, and this would therefore appear to be the critical period for the east-coast districts, especially as precipitation then is more urgently required for growing crops.

Anticyclone centred South of .New Zealand.

Fig. 3, the chart of 23rd June, .1913, proves an exception to the above rule, for instead of being in lower latitudes in this case the “ high ” centre is considerably to the south of the Dominion, while relatively low pressure exists to the northward. This synoptic chart, as will be readily seen, is the reverse of the preceding one (Fig. 2) ; as the latter was representative of westerly type of weather, Fig. 3 may be taken as a high-pressure system in which easterly and southeasterly winds are dominant. It was responsible for a spell of intensely cold weather in the east-coast, districts, with heavy passing showers, particularly northward of Kaikoura. In the western districts clear skies ruled, with frosty nights. -

The centre, shown in the chart south of New Zealand, was only one of several existing in this anticyclone, which proved to be an exceptionally extensive one and reached over almost the whole of Australia. After the front had moved farther eastward conditions improved in the eastern districts, the centre following taking apparently a more northerly track over the Dominion.

Variation in Annual- Rainfall.

When referring to Fig. 2 it was pointed out that the normal position of the east-west axis of anticyclones. is different in summer and winter. In summer the mean track is a little south of Auckland, while in winter it is along the latitude of the Kermadec Islands.. In some years, however, the movement of these “ highs ” is much farther north or south of the normal path, and this extreme migration has an important bearing on the. climate of certain portions of New Zealand, especially with regard to the annual amount of rainfall. The effect is. particularly noticeable in the case of the North Auckland district, and the annual rainfall records of the City of Auckland present some interesting features in years when the anticyclonic track has been abnormal. For instance., in 1914, which was a year when the high-pressure belt was in more northern latitudes than usual, the fall was only 28-42 in. (— 35 per cent, of mean), while in 1916 and 1917, when the belt had receded south of its normal track, the total falls were 66-36 in. (-f-52 per cent, of mean) and 74-15 in. (+70 per cent, of mean) respectively.

The District of Canterbury is affected chiefly by the more northern position of the belt, and the rainfall does not show any marked effect from the southern extension. In 1914 the total year’s fall at Christchurch was

19-90 in. (—21 per cent, of mean). In 1916 and 1917, however, the rainfall was about the average, but slightly above it in the latter year. The rainfall returns for Dunedin, which may be taken as representative of the southern districts, in some cases show a fall below the average when that at Auckland in the same year of abnormal anticyclonic movement is above it ; but this rule is by no means consistent in the records, and so it would appear that the extreme south is not greatly affected by the variation from the normal of the anticyclonic track in any year. The excessive rainfall in the northern districts when the high-pressure systems move in higher latitudes is' brought about by the greater number of extra-tropical cyclones, which are then able to approach within effective range of the northernmost portion of New Zealand. On the other hand, when the high-pressure areas are in lower latitudes, Antarctic or westerly depressions extend farther north, and the west-coast and southern districts of the-South Island receive heavy rains. The conclusion to be drawn is that in years when extra-tropical cyclones are more numerous Antarctic or westerly areas of low pressure are less in evidence, and vice versa, and this the records have generally proved. Since the annual rainfall at Auckland shows a marked relationship to the abnormal positions of these anticyclonic belts, it might be expected to show evidences of any cyclical movement if any such existed, but from records which extend back to 1864 no 'definite periods in the intervals of maximum or minimum swings of the anticyclonic belts can be deducted. The periods between the years of maximum rainfall, and also between the years of minimum rainfall, appear to be altogether irregular, ranging fiom one to ten years for the former and one to eight years for the latter. The mean period in both cases is 4-4 years.

(To be continued.)

* The Macquarie Island wireless station was established in connection with Sir Douglas Mawson’s Australian Antarctic Expedition. It was closed down about the beginning of December, 1915. Discussing the matter publicly at the time, Mr. D. C. Bates, Director of the Dominion Meteorological Office, said, “ It is a distinct loss to science. . . . The importance of Macquarie Island is very great, since it is so far south and yet has relationships so closely affecting our conditions.”

„ ; The terms veering ” an( “ backing ” were originally used to express the change oi wind-direction with and against the sun’s apparent movement. The International Meteorological Committee, however, in 1905, adopted the following resolution • Meteorologists in either hemisphere are requested to denote by the word ‘ backing ’ a change of wind-direction at the place of observation (or on board ship) in the direction west-south-east-north (counter-clockwise)—irrespective of accompanying changes of weather—and to use the word ‘ veering ’ to denote a change in the opposite direction—-west-north-east-south (clockwise).” (Rep. Internal. Met. Conf, at Innsbruck, Sept., 1905, p. 41, item 47.) In the Southern Hemisphere the terms are, therefore, now used in the reverse sense to that in which they were originally employed.