How Weather Forecasting is Carried Out

Radio Record, Volume II, Issue 12, 16 November 1928, Page 2

How Weather Forecasting is Carried Out

By

B. V.

Pemberton

Assistant-Meteorologist

QNE of the impressive features of the ‘Southern Cross" flight was the revelation afforded of the value of weather forecasts and the accuracy with which that forecasting of the course of events was carried out. The fact that on both journeys the aviators suspended the take-off till the "‘meteorological man’’ was able to indicate the probability of satisfactory conditions carried its own lesson of the high degree of accuracy now attainable in such forecasting. Fore-knowledge of weather conditions is of the utmost value to many sections of the community-notably farmers, fruitgrowers, seamen, and many branches of commerce. In the following "talk," given from 2YA, Mr. B. V. Pemberton, assistant meteorologist, outlines the procedure which is followed to prepare the nightly forecast.

mn) UCCESSFUL weather forecasting is dependent almost entirely on the means at | the disposal of the Meteor6siogist for constructing a Ae # \vcather map showing the [Weesuis. systems over a wide area on either side of the country he has to forecast for. In the first place, it must be understood that a meteorological organisation has a widespread network of weather-reporting stations from which, at certain arranged times, observations are transmitted by telegraph to the central office, which, for New Zealand, is in Wellington. . There are about fifty observers in New Zealand living as far apart as Cape Maria Van Diemen in the extreme north and Puysegur Point in the south. Reports are received also from Chatham Island, Norfolk Island, from various places in Australia, and from ships within a radius of one thousand miles of New Zealand. Wireless telegraphy has been a very notable aid to the forecaster, as it has enabled him to chart the adjacent oceans, and, in @ narrow country like New Zealand, this is especially necessary. The reports, which are dispatched twice daily, at 9 am. and 4 p.m., to Wellington, consists of: (1) Barometer reading, reduced to sea level. (2) Temperature. (8) Wind direction and force.

(4) State of weather; and in the case of coastal stations (5) State of sea. All the information is plotted on a map, Which, when completed, is called a "Synoptic Chart." Maps Are Essential. No meteorologist in these days would ° attempt to make a forecast of weather without reference to such a map. No other practical means has been found of making it possible to secure a mental grasp of the rarintion of the weather over a large area. It is possible for an intelligent observer frequently to get some idea of the probable run of the weather from observations of local condition: alone but the success which attends forecasting of this kind must be very limited because the general conditions can seldom be correctly estimated. The official meteorologist has to forecast for a wide area. Some of the places to which his forecast goes are far distant from him; some of them he has never visited. Ile has, therefore, to form a mental picture of what is going on over the region and to base his estimate for the future on established general principles. There is no doubt that an observant person can amplify the official forecast from his own local knowledge, but he must get his ideas as to general situation froin the official message. He has to guess what the weath.r map looks like.

it is important that the synoptic chart should cover a very wide area, und the one prepared every day in Wellington not only includes the New acaland area but takes in the whole of Australia as well. So as to make it possible to take in at a glance the prevailing weather, the latter is indicated on the map by letters or symbols: which were originally introduced by Admiral Beaufort. For instance, b represents blue sky, ¢ cloudy, 0 overcast, a black dot rain, ete. Winds are drawn in by means of arrows pointing in the direction to which the wind is blowing and their force is indicated by the number of feathers in the arrow, one for each number on the Beaufort Scale. An arrow with one feather, then, would indicate a light air, three feathers a gentle breeze, five feathers a fresh breeze, and so on up to eight or nine feathers which represent gale and strong gale respectively. A calm is shown by means of a smill circle. The Basis of Forecasting. JFLORECASTS are based principally on the distribution of air pressure or the way in which the barometer reading varies from one place to another on the chart. To show this pressure distribution, the person who prepares the chart draws "isobars" or lines of equal pressure. It is usual) here to draw isobars for every tenth of an inch. There is an isobar for 30.0 inches, ohe for 29.9 inches, one for 29.8 inches, and so on. The object of the chart-maker in drawing the isobar for 80.0 inches, for instauce, is to make it pass through all places on the map at which the pressure is 80.0 inches. He has to estimate the run of the isobar from ‘the’ actual barometer readings reported from the observing stations. At all places on one side of the isobar for 30.0 inches the pressure will be above 30.0 inches and on the other side below it. Isobars may, therefore, be likened to the contour lines on an ordinary map. Jt is most important that the nature and use of isobars should be understood. Otherwise it will be impos: sible either to make a forecast for oneself or to appreciate fully the official forecast. | Isobars, then, are lines of equal pressure. It is this pressure distribution which determines the direction and force of the winds which we' ex-

perience, and to the distribution of winds are related, to a great extent, the changes of temperature and weather, | What the Isobars Show. HAVING drawn the isobars, it will be found that the lines assume certain shapes corresponding with well-defined pressure types or Ssystems, and of these the two principal ones ate the cyclone and the anticyclone. Both these systems are approximately circular or oval in shape. In the cyclone, the central or inner isobar marks the lowest pressure, while in the anti-cyelone pressure is highest in the centre and the outside rings or isobars represent lower pressures or barometer readings. A region where the barometers are low is ¢cplled a depression or colloqtially a "low," the.

corresponding high pressure region being called a "high." The cyclone is not the commonest s@@€pe of "low"? which affects New Zea7 land. More: frequently the "low" is shaped like an inverted "V" with the isobars wider to the southwards. Such d¢pressions are wedged in from the sdycth between adjacent anti-cyclones. Shévzid the isobars take-on the form of a wide inverted "V" so that they are almost straight lines, they produce the westerly type of weather. . There are several other forms or groups of isobars which appear on our weather charts, some of the more. important types of ‘which may be mentioned. In the rear of a cyclone there will sometimes develop a smaller ‘depression which is termed a "secondary." It may be a further centre or merely a loop or sinuosity in one or more of the isobars. Though usually short-lived the rain is often heavy in a secondary and atmospheric conditions are often very disturbed. Hail, thunder, snow or a cold snap may be experienced. Then we have wedge-shaped isobars, a projecting area of high-pressure wedged between two "lows." The wedge most frequently points to the south. It is a fair-weather type of pressure distribution but the fine weather is usually of brief duration because of the "low" which follows. It is a case when the saying: "It cleared too, quickly to last" might be applied, Another system is the Col, which is a ridge of relatively high pressures lying between two adjacent depressions and joining two anti-cyclones. Here we get variable winds or calms_ with usually fine and sometimes brilliant weather, but occasionally dull, gloomy conditions possibly with thunderstorms. Lastly we may mention straight isobars. They are called straight from the fact that they have no curvature over a large‘area. Usually the low-pressure isobars are on the polar side with high pressure to the northwards, so that the winds are from some westerly direc: tion in conformity with the isobaric trend. «With this type there is associated great. diversity of weather with somewhat unsettled conditions. where the pressure is low and fair or fine weather where it is high. The Influence of Winds. : UPon ‘inspecting the different types of weather charts certain facts will be noted with regard to winds. The latter will be observed to blow with fairly marked uniformity of direction and velocity. They will nearly always blow along the isobars, and if you stand with your back to the wind, baro. meters will be lower on your: right hand than on your left. The wind does not assume a direction stsictly parallel with the isobars but is inclined at an angle across the isobar from high towards low pressure. Thus we have the wind rotating round the centre of a cyclone in a similar direction to the hands of a watch .pszt. spirally inwards so that with Yn cyclone moving in an easterly direction, in front of the centre the winds are northerly, and, in.the rear, southerly; directly north of the centre they are westerly, while to the south, easterlies prevail. . On the other hand, the winds in an anti-cyclone move round the centre in a direction contrary to the hands of:a watch, and at the same time spirally

outwards, the front of the centre having southerly and the rear northerly winds, Next we must consider the force of the wind, which is related to the rate at which the pressure changes as you move from high towards low pressure. If we draw a line joining two adjacent isobars and at right angles to them, the measure of this line is called a "rradient.’ The shorter this distance

is the greater or steeper is the gradient, just in the same way that the punching of contours on an ordinary map denotes steepness of hills. The strength of the wind, then, depends on the distance between the isobars, the force increasing as the isobars lie closer together. Usually the isobars in a cyclone are closer together than in an anti-cyclone, and this. accounts for the — stronger winds experienced with the former system. Cyclones and Anti-Cyclones. OME people apparently are apprehensive at the mere mention of the term "cyclone." It should, however, be understood that the term applies to the particular type of pressure system in which we have isobars enclosing a low-pressure centre, and that not only do cyclones vary greatly in intensity but their characteristics are different in different latitudes. In the tropics they are usually small and intense, and the heavy rain, hurricanes, and sometimes tidal waves they produce do great damage over small areas; but by the time such a cyclone has’ moved to temperate latitudes it has extended and covers a much wider area, and the isobars are not so close together, and though there may be gales and heavy rain in parts of the cyclone they are seldom violent or destructive. .

" It is equally possible for gales and heavy rain to occur in the southern V-shaped depressions, although as a rule, the ‘two .types produce the most severe effects in different parts of the Dominion. Our severest southerly and south-easterly weather usually occurs in the rear of cyclones, whereas in V-shaped depressions, winds are usually from some westerly direction, and it is districts with a

westerly aspect that suffer most. It is for such reasons that it is necessary to distinguish between cyclones and other forms of depression. It is to be remembered, however, that it is just one of our storm types, and need usually be no cause for apprehension. usually be no cause for apprehension. Not only is there confusion between the temperate and the tropical types of cyclone, but the tornado is often called by the newspapers a "cyclone." Tornadoes are. atmospheric whirls of 2 diameter varying from a few yards to a quarter of a mile, and, in America, even more. They are usually associated with wind-shift lines, or violent thunderstorms. They move across the

x, country in the prevailing winds, and where they reach the ground, are usually destructive. In New Zealand, they are rare, and usually small, and of no great violence. The removal of a roof or two, the shifting of small sheds, or the uprooting of trees is usually the extent of the damage they cause. The weather in a depression, or an anti-cyclone differs materially according to the situation of their centres. It will be generally noted, however, that unsettled conditions, with strong winds and rain, are associated with "lows," while quiet and fair, to fine weather is experienced with anticyclones or "highs." Summing up the Data. [= is the business of the meteorologist: to find out in what direction the depressions and anti-cyclones within the area covered by his map, are going to move, and how they are going to change, and accordingly to anticipate the changes of wind and weather incidental to their movements. — If a series of synoptic charts are ex~ amined together, it will be discovered that the areas of high and low pressure follow one another in succession from west*to east at intervals, and it is this important fact which enables us to give notice of changes likely to take place. Having briefly considered some of the main weather controls and _ the principles on which forecasts are based, we may now examine some of the actual charts themselves. It is difficult without resource to such examples to explain pressure systems, their movements and the wind and weather associated with them, Iollowing, therefore, are given some small-scale reproductions of isobaric charts prepared and used by the Meteorological Office in issuing weather forecasts. In Figure 1, which shows the chart for Monday, August 6, 1928, there are two anticyclones. One having recently crossed the northern Tasman Sea is now centred north-east of Hast Cape, while the other, an extensive one, stretches from the South Island over the Tasman Sea and the. whole of Australia. The anticyclone exhibits the eastward advance character: istic of pressure systems and, barometers are rising over the greater ‘part. of New Zealand but in the extreme north a slight fall is taking place on account of a cyclone north of Norfolk Island, pushing its way in a southeast direction. From the South Island extending eastward to the Chatham Islands. there is a moderi..e depression of the westerly or southern type. © The latter crossed the South Island during the previous day’ and caused moderate 1 AT — FS

Beaufort Scale of Wind Force with Equivalent Velocities at 16ft. above Ground.-0 Calm, 0-1 m.p.h.; 1 Light air, 2-3; 2 Slight breeze, 4-7; 3 Gentle breeze, 8-11; 4 Moderate breeze, 12-16; 5 Fresh breeze, 17-21; 6 Strong breeze, 22-27; 7 High wind, 28-33; Gale, 34-40; 9 Strong gale, 41-48; 10 Whole gale, 49-56; 11 Storm, 57-65; 12 Hurricane. Weather.-(b) Blue sky, cloud amount 0-3 tenths; (bc) Blue sky and detached clouds, cloud amount 4-6 tenths;

(e) Cloudy, cloud amount 7-8 tenths; (0) Overcasst, cloud amount 9-10 tenths; (g) Gloomy, dull; (u) Ugly, threatening; (r) Continuous or steady rain; (s) Snow; (d) Drizzle; (p) Passing showers; (h) Hail; (q) Squally; (1) Lightning; (t) Thunder; (f) Fog; (m) Mist; (z) Haze. State of Sea and Swell.-O No swell, 1 Mod. swell, 2 Heavy swell, calm or slight sea; 3 No swell, 4 Mod. swell, 5 Heavy swell, Mod. sea; 6 Rather rough sea; 7 Rough sea; 8 Very rough sea; 9 Mountainous sea.

fo fresh north-west winds in and south 6f Cook Strait with scattered rain in Westland and Southern Otago, the winds backing to southerlies in the rear of the depression and in front of e@ advancing anticyclone. The development of the northern cyclone, however, and the position of the "high" which is departing east of the North Island have complicated matters and ' the winds are rather variable. Higure 2, the chart for the next day, August 7, shows that since the -previous day the northern cyclone has passed rapidly to the south-east and is now apparently centred in the neighpourhood of the Kermadec Islands. Though still relatively low in the far North and about East Cape, pressure is rising everywhere over New Zealand as the large anticyclone to the eastward is gradually advancing. Winds are now more generally southerly as the North Island is being affected by the rear of the cyclone and New Zealand generally by the front of the advancing "high." North of Auckland there had been rain on _ account of the proximity of the cyclone, but it is now rapidly clearing in those parts and fair to fine weather rules over the Dominion, though the freshening southerly winds in front of the "High" have been accompanied by some scat-

tered showers on the Hast Coast south from Hast Cape. Figure 2 also shows that since the previous day a new depression has advanced towards Western Australia. Figure 8, August 8-The northern cyclone has passed entirely beyond the area covered by the map, while the anti-cyclone has iravelled farther east and is now centred over the central Tasman Sea. New Zealand, deing still in front of the centre, is experiencing southerly winds and mainly fair weather, but on account of the steep gradient to the east the southerlies have been strong, reaching gale force at places on the east coast between Akaroa and Hast Cape, and during the previous night there were some heavy rain squalls.on the latter coast. The depression which originally was situated west of Australia is now in the South Australian Bight, and a new anti-cyclone is appearing further westward. In Figure 4, August 12, the intense anti-cyclone which on the 8th was centred over the Tasman Sea is now shown east of the North Island. During its passage over New Zealand from the Sth to the 12th it was accompanied by very fine, pleasant weather with light and variable winds. ‘The clear, calm atmosphere gave free play to the effects of radiation, and many districts experienced fairly severe frosts.

With winter anti-cyclones, also, there is a tendency for the development of fogs, and morning fogs were prevalent on the several days while this anticyclone dominated the situation. During the 1ith the winds tended northerly. and by the ‘2th northerlies were general over New Zealand in the rear of the "High" and were freshening with the advance of a depression over the southern Tasman Sea. The latter is identical with the one shown in Figures 2 and 3, and which was then situated south of Western Australia. It now lies between the departing anticyclone and one covering Australia, the centre of the latter having moved from west of Australia since the 8th of August. This series of charts, Figures 1 to 4, is interesting as showing the paths of the different "Highs" and "Lows" from west to east. There is, however, 2 seasonal variation in the movement of anticyclones which has a marked effect on the weather of the Dominion. Anticyclones, for instance, follow a more southerly track in summer than in winter and spring, and, as 4 result, in the latter seasons, southern or V-shap-ed depressions are able to extend farther north. It is this effect which is one of the chief causes of the more marked prevalence of strong, northwesterly winds in the spring season.

Types of Depressions. By Vigure 4 there is shown a moderate "Y%’ depression. Occasionally these . depressions develop considerable intensity, and Figure 5, August 31, shows one with this characteristic. It is situated east of New Zealand, its trough having crossed the Dominicn during the night of the 30th. There were strong northerly winds generally during the 80th, reaching gale force at many places, The passage of the trough or line of minimum pressures, as is generally the case, was marked by a sudden backing of winds to a southerly direction. The change was accompanied by very cold, squally conditions, rain in most districts and snow in many parts of the South Island and on some vf the ranges in the southern districts of the North Island. This type of depression is usually followed by one or more secondaries, and when this happens the winds will veer to west and north as each of the lowpressure waves approaches New . land, and they will back again to selthwest after it has passed, in conformity with the trend of the isobars. A eharacteristic effect of this type of depression is the occurrence of the" warm, dry north-west winds in the Hast Coast districts in front of the trough, more especially in the Canterbury Plains. This is a similar wind to

the "Fohn" wind experienced in the Alpine valleys of. Switzerland. Both dryness and high temperature effects are caused by the compression of the air in its change from high to low level, when passing over the mountains ‘to the west of the plains where it has parted with its moisture on the windward slopes. . In figures 6 and 7 we have a typical example of*°a cyclone. The development of this cyclone took place in the northern portion of a depression of the westerly type, and, on the afternoon of August 14, a definite centre was indicated on the weather chart north-west ‘of Auckland. From then on it increased rapidly in intensity, and by the idth the centre had moved southwards, and, as shown by the chart, fig. 6, was situated north of Cape Egmont. The usual direction of the winds round a cyclone centre, i.e, clockwise, is weli shown in this instance. At East Cape, which is in front of the centre, it is north-easterly, at Cape Maria van Diemen westerly, at Cape Egmont southeasterly, and farther south chiefly easterly or southern winds prevail. By the morning of August 16 (figure 7) the centre had crossed the North Island, and was then east of Napier: winds on the 16th were mainly westerly north from Cape Egmont and Napier, and southerly thence southwards.

On account of the steep gradient in this cyclone, gales were fairly widespread during both August 15 and 16, and the southerlies were particularly strong in Cook Strait. It was this particular storm, on account of the heavy seas it caused, which damaged the railway lines on the harbour front between Kaiwarra and Petone. The North Island experienced general rains on both days, with some heavy falls, the greatest occurring in the East Coast region. Over four inches for 24 hours was measured at seme places. { ‘Although the paths followed by the centres of cyclones are occasionally _ very erratic, they are usually fairly definite, the most common route taken being from north-west of New Zealand across or north of the North Island to the south-east. They may, however, originate northeast of New Zealand, and advance from

the direction of the Kermadec Islands towards Bast Cape, off which point they finally re-curve and pass to the south-east. This type of cyclone will sometimes remain stationery off Bast Cape for several days, while an anti-. eyclone is situated over the South Island, and may be accompanded ‘by severe south-east gales on the East Coast north from Kaikoura. The movement of "V" or southern depressions from west to east varies

so considerably that it would be hardly possible to.give an average rate, Jt may be as low as 400 miles or as high as 1000 miles ner day. Unlike cyclones, however, the paths of these depressions are well defined, and they can nearly always be followed from western Australia across southern Australia, Tasmania, the Tasman Sea and New Zealand to beyond the Chatham Islands. On the other hand, just as the path of cyclones is more erratic, more yariable still is the rate at which they move. A cyclone centre has been known to move from near Hobart, where it was situated at 4 p.m. one day, to the vicinity of Foveaux Strait by 9 o’clock the following morning, which is equal to a rate of about 64 miles an hour,

Anticyclones are more leisurely in their movements taking, most frequently, about six days to come from western Australia to New Zealand, but sometimes longer. They also have a habit of remaining stationary for several days. Value of Wide Sources. WING to the charts herein having to be produced on such a small

scale, it was impossible to fill in the meteorological information conveyel in the weather reports as is done in the originals. To draw isobars in such detail as is shown in Figures 1 to 6, one requires to have reports from a very large number of stations, but quite a serviceable chart might be constructed from a reduced number as Figure 7 will show. It is based on the information. received from the stations following. These stations are the ones whose weather reports it is intended to broadcast, as mentioned later. Wireless weather reports from three ships are included in the table. At the ent of the table there is given the Beaufort Scale of wind force with equivalent velocities, the Beaufort weather notation and notation for state of sea and swell.

Incessant Change. [tz will be obvious to those who read this brief sketch and examine closely the weather charts reproduced herein that pressure systems are constantly changing in form, position, and rate of movement, and that, as a result, there are always new developments taking place in the meteorological situation. In order, therefore, not to lose the sequence of meteorological events, synoptic charts are prepared twice each day at Wellington, at 9 a.m. and 4 p.m, and forecasts, based on these charts, are issued each morning and evening. In addition to the forecast which has hitherto been broadcast there will henceforth be added reports from various places in New Zealand, from Norfolk and Chatham Islands and from Sydney and Hobart, which will give the actual pressure, wind, weather, ete., ruling at 4 p.m. Besides the precise information which these reporis will give of conditions prevailing at the places concerned, they will enable the listener-in to draw his own weather chart. This is a practice being followed very largely by mariners in Kuropean and American waters, and is one which will, no doubt, be increas-. ingly adopted by those sailing in these regions. But there is no reason why intelligent farmers, school teachers, students, and others interested in the weather should not do the same. By drawing his own weather chart an individual will gain much information which it is: impossible to include in the forecast wthout making it too lengthy. Particularly will he be able to obtain a longer view of the weather changes and of their probable course for several days to come. FoR the benefit of shipping, the information will be issued in morse from the Wellington Radio Station: at 9 p.m. N.Z.S.T., as well as being broads cast by the Radio Broadcasting Come pany between 7 and 8 p.m. Forms are being prepared which will include space for entering the reports and forecasts as sent from Wellington, and also a map of the New Zealand and surrounding areas for use in plotting the. isobars. These forms will be issued freely to such ships as supply the Meteorological Office with weather reports, and at cost price to others re quiring them. (Continued on page 9.)

Weather Forecasting

(Continued from page 5.) Everyone, to a greater or less extent, is interested in the weather, and although most people may not have the time or the desire to acquire a sufficient knowledge of meteorology to enable them to apply it to their own use, there are others, such as farmers and those connected with shipping to whom the subject is especially important. It is essential, therefore, for them to acquire a general knowledge of weather science which will help them to make intelligent use of such information as the Meteorological Office. from its widespread organisation, can supply, and which it is anxious that they should haye.

Station. Norfolk Awanui Auckland Fast Cape Cape IXgmont .... Napier Farewell Spit ...... Stephen Island ... Wellington Cape Campbell .... Greymouth Nugget Point ..... Puysegur Point .. Chatham Island ... Sy dney Iiobart U.S.S. Maunganul: Lat. 37 Ss. Long. 178 W. .... U.S.S. Niagara: Lat. 34 S. Long. 166 E. U.S.S. Kaiwarra: Lat. 43 S. Long. 191 E. ...... Island .... eeeseeve Akaroa Lighthouse Barometer. Temp.. Direcin. deg. F. tion. 30.04 — WwW. 29.66 58 S.W. 29.49 dd W. 29.28 54 | WNW. 29.59 48 S.W. 29,27 53 S. 29.43 50 W. 29.43 d3 Ss. 29.44 44 S.S.W. Interrupted. 29.58 EB. 29.67 44 S.W. 29.89 39 HK. 29.89 42 S.E. 29.31 4) E.S.E. 80.09 _- WwW. 29.69 -~ 29,28 --- N.W. 29.88 oo S.W. ° 80.00 -% S.W. Wind. Force. Beaufort seale, 6 Wisw GQ" BEiMsts cw O-les Beaufort notation. be be c c Mod. r Mod. b Mod. e Mod. e Rough 0 0 Mod. ofr Heavy 0 Mod. be Mod. oO Smooth f c Rough be Rough od Mod. Weather Sea, ‘Notes. Heavy rain in night Rain 114pts. Mod. rain in night. Heavy rain in night. Rain, 11 pts. Rain, 112pts. Rain, 23pts. Heavy rain in night. Light rain in night. °. Heavy W.S8.W. swell Mod. southerly swell,

WEATHER REPORT: THURSDAY, AUGUST 16, 1928.