Types of Australian weather
By Henry A. Hunt,
Second Meteorological Assistant, Sydney Observatory.[With Forty Diagrams.]
By Henry A. Hunt,
Second Meteorological Assistant, Sydney Observatory.[With Forty Diagrams.]
In continuation of the valuable work on Australian Meteorology which the Hon. Ralph Abercromby initiated several years since, by offering a money prize for the best essay on Southerly Bursters; he has recently selected the phases of Australian Weather which are treated in the following twenty studies of "Types of Australian Weather." Many of these appear to be peculiar to Australia, and at the same time connected with Equatorial and other weather. That they throw much new light upon the source of the greater part of Australian rain, and show how these rain storms develop out of ordinary weather conditions is certain; at the same time they form an important contribution to the study of weather in the Southern Hemisphere generally. The work has been done by Mr. H. A. Hunt, at Mr. Abercromby's expense, and Mr. H. C. Russell, has edited the work.
As a general rule, weather is set fine when anticyclones move rapidly, and in a straight line across Australia, i.e., at a rate exceeding five hundred miles per day. And weather is unsettled when they move slowly, and not in a straight line, i.e., in a zigzag line, especially if they show no appreciable forward motion for a day or two. When anticyclones move in low latitudes the conditions favour dry weather, in high latitudes, wet weather, especially if they rest for a time south of South Australia.
All the examples of weather phases which follow have been selected from the Sydney Weather Charts, and illustrate each type. The originals were carefully traced, and then reduced from 22" x 17" to the size used in this essay by means of the camera.
Gg—Dec. 4, 1895. Ordinary symbols have been used, except the circle half filled for thunderstorms, and the straight line shading; parallel lines indicate the area of rainfall under one inch, crossed shading over one inch.
One of the best marked features of Australian weather is the steady easterly progression of all the types, and the governing type, that in fact about which all the other types seem to congregate, is the anticyclone; it has therefore been placed first in the series, with three charts to show the progress made by a quick moving one in forty-eight hours. The average daily progress of anticyclones is four hundred miles per day, but the speed at times rises to one thousand miles.
Investigation so far leaves no room to doubt that in these latitudes a series of anticyclones surround the globe; the latitude of the average one varies with the season, being farther south in summer than in winter. The normal circulation about an anticyclone brings southerly winds in front of them, and northerly winds in the rear, hence our cold and our hot winds.
Chart No. 1 shows the position, on 15th August, 1893, of the eastern half of an incoming anticyclone; it rests over Western Australia, while the departing one is seen over the Tasman Sea; between these is seen the usual depression, which is of average intensity, and a dormant tropical low pressure to the north. In Chart No. 2 the anticyclone has moved nearly nine hundred miles in the twenty-four hours, the centre being located near Fowler's Bay, north of the Australian Bight; the antarctic depression is well across the Tasman Sea, while the tropical or monsoonal isobar depicted in the previous chart has apparently merged into the high pressure system, a curious and not unusual kink being formed to the north-east of it, following the contour of the Gulfof Carpentaria. On Chart No. 3 the anticyclone is shown to have moved a further seven hundred and fifty miles, or a total of
one thousand six hundred and fifty miles in forty-eight hours; the monsoonal dip is still noticeable south of the Gulf, while the antarctic Λ depression has its centre to the east of New Zealand.
The passage of this anticyclone was an unusually rapid one, and is presented as portraying with a minimum number of charts the easterly motion of anticyclones over Australia.
TYPE II. MONSOONAL RAIN STORM.
This type is undoubtedly the chief rain agent in the Australian Continent. Monsoonal depressions or tongues may occur at any time of the year, but particularly between the months of September and April, and most frequently during January, February and March. The readings of barometers in the depression seldom fall very low, the grade from the surrounding areas to the centre of the tongue ranging from one to three-tenths of an inch generally; the depression may intensify, that is the tongues between high pressures may protrude further south anywhere during their passage across Australia, but show a preference to do so after they have crossed central Australia, a fact which suggests that the heated interior has at least some influence in their development.
When and wherever the tongue is well defined, rain certainly follows in its track, and thunderstorms as a wide spread and simultaneous feature are never experienced without it.
On April 18th and 19th, 1894 (Chart Nos. 4 and 5) occurred one of the finest monsoonal rain storms on record, the area affected being very extensive, embracing the whole of the eastern colonies, Tasmania, and the greater part of South Australia. Many inches—up to five and six—of rain were recorded on the north-east coast of Queensland, and over the eastern half of the rain area the country benefitted to the extent of over two inches generally before the storm was over.
On this Chart No. 4 the monsoonal isobars, after passing the most southern part of the tongue, are shown sweeping round a high pressure of considerable energy, situated over the Great Bight. Following these isobars to the eastward, we find them recurving over another high pressure of greater energy in the Tasman Sea. On Chart No. 5, both anticyclones will be noticed to have worked northward, and while so doing they have lost somewhat in pressure, and the low pressure tongue has extended further south and broadened; at the tip, two cyclonic rain centres will be observed to have formed.
The chart antecedent to 18th April presented the general characteristics of Chart No. 1, except that the Λ antarctic depression was a considerably less active feature, but on the other hand the monsoonal dip was somewhat more pronounced, and possessed two instead of one isobar. Following the 19th, the chart of 20th revealed no sign of the monsoonal tongue, and the continent was covered with a high pressure of very slight energy.
TYPE III.—DEVELOPMENT OF A CYCLONIC STORM IN LOW LATITUDES
FROM A MONSOONAL DEPRESSION.
In Type No. 3 we have the development of a cyclonic storm out of a monsoonal depression. The seasonal peculiarity of this phase of the tropical low pressure is similar to that in Type No. 2. The cyclone seems to develop when the southern extension of the monsoon is out of proportion to its width, and it becomes so narrow at one part of it that the opposing winds which circulate round it interfere, set up the cyclonic circulation, and it then progresses eastward as a rain storm. (See Charts 6 and 7.)
These storms frequently develop in South-east Queensland, and they are generally most severe there; the quantity of rain which sometimes comes with them is very remarkable, as in the case of the phenomenal flood in Brisbane in 1893, which was the result of one of these storms.
The example selected and shown in Charts 6 and 7 took place on September 28th and 29th, 1892; on the 28th (Chart 6), it appears that one of these storms had developed in the previous twenty-four hours, at the southern end of a narrow tropical or monsoonal tongue of low pressure. An anticyclone of good energy lies to the east of it, with its isobars extending well to the north and contracting the width of the tongue in the north; another anticyclone lies to the west, and this also seems to be pressing on the narrow monsoonal tongue and helping to contract its diameter, at the same time the energy in both high pressures is adding force to the circulation of the wind, and so aiding in the development of the cyclonic circulation. A modified Λ depression exists to the south-east of the Australian Bight, and there is another over New Zealand.
On September 29th, (Chart 7), the small cyclone has extended its area and energy on its way to the coast, but its motion of rotation has been rapid, and this probably accounts in a measure for the comparatively small area over which rain has fallen, although, in this instance, over an inch of rain fell in the central and eastern parts of New South Wales. The winds circulating about the western isobars of this storm are rather stronger than usual in such cases. In the twenty-four hours both anticyclones have lost a considerable portion of their energy, while that of the depression in New Zealand is about the same as on the previous day.
TYPE IV.—DEVELOPMENT OF A CYCLONIC STORM IN HIGH LATITUDES
FROM A MONSOONAL DEPRESSION.
These are somewhat similar to Type 3, but, the rainfall is usually not so heavy, and the wind much more violent.
Chart No. 8 shows the development of one of these cyclones on April 15th, 1889; in this case the monsoonal depression had extended across Australia into the Australian Bight. The development of one of these storms is heralded by the strong easterly gales on the south-east coast of South Australia and south coast of Victoria. The energy of wind circulation increases over South Australia, and as the whole system moves bodily eastward from the Australian Bight to the mainland, the circulation is seen to be that of a fully developed cyclone of small area, with a diameter three to five hundred miles, formed out of the southern part of the extensive monsoonal depression.
In Chart 8 a high pressure is shown over the south-western part of Australia, and evidently encroaching on the moonsoonal about central Australia.
In Chart 9, the western anticyclone has extended northwards and has seriously contracted the diameter of the monsoonal low pressure, thereby facilitating or helping to cut off and start the cyclonic storm in the south; which has now developed a much steeper barometric grade and an energetic wind circulation, and is moving eastward without extending the rain area.
Chart No. 10 shows comparatively little motion in the cyclone, but it is considerably distorted, especially to the west, where the anticyclone is compressing the isobars by its easterly progress and in its endeavours to maintain its rate has overridden the cyclone in the north. As will be seen by a glance at these three charts, the rain resulting from this storm was most extensive and beneficial, and the winds under its influences were strong.
TYPE V.—CONDITIONS FAVORABLE FOR THUNDERSTORMS.
Upon comparison of charts setting forth this type with those of cyclonic thunderstorms, they will be found very similar; the main difference being the absence in this set of the cyclonic area at the end of the monsoonal tongue. The chief feature indicating thunderstorms is the narrowness of the col and somewhat congested state of isobars in the high pressures west and east of it, resulting in opposing winds. Those of the tropical tongues are hot and charged with moisture, while those of the Λ depressions are strong and dry. This type is met with during the monsoonal season. The rains resulting from this feature are not generally heavy, and though thunderstorms may be experienced over extensive areas of Queensland, New South Wales, and Northern Territory, a number of them may occur without any rain falling.
Chart No. 11, January 17th, 1893, shows an extended and narrow tongue lying between two relatively high pressures, a Λ depression similarly situated also exists in the south, this col or area of low pressure separating these two depressions is very small, and opposing currents of wind are noted there as blowing within close limits. Thunderstorms were occurring or had occurred in Northern Territory generally, over a great part of Queensland and in central parts of New South Wales, the rains upon this occasion, as is often the case, were not heavy, though they fell over an extensive and generally unfavoured area.
On Chart No. 12, the following day, though a few storms were recorded, shows a great diminution in number. The col has widened and the monsoonal tongue has lost to some extent its thundery characteristics, having widened at the end. The accompanying barometric systems show no motion since the previous day, but the high pressures have intensified.
TYPE VI.—CYCLONIC THUNDERSTORMS.
This also, like the preceding one, is allied to the tropical low pressures, but in this case a defined cyclonic circulation develops in the lower extension of the tongue without the usual intensification of grades. From this source the thunderstorms radiate in easterly and southerly directions, and at times, as in the instance presented, a vast area is affected.
Chart No. 13, December 12th, 1893. As on the previous set the monsoonal tongue lies over much the same country, though with its axis more east and west; the high pressure to the west is very small, and the systems are somewhat fragmentary.
On the following day, Chart No. 14, a cyclonic circulation has formed in the end of the tongue, while the Λ depression over Sydney on the 12th has moved eastward and nearly filled up, and the high pressures have followed and intensified. The thunderstorms and rains were not so general as on the thunderstorm type, but curiously, were recorded in an almost parallel area of country, nearly three hundred miles wide by twelve hundred long. Though great barometric changes have taken place, the pressures remain about the same as on Chart No. 13.
TYPE VII. A RAIN STORM WITH VERTICAL AND NEARLY STRAIGHT ISOBARS.
This type is one of the best defined and reliable of the series for forecasting purposes, because with them, good general rains almost invariably come. They are the rear isobars of a departing anticyclone, and the wind circulation from north and north-east brings into the interior winds laden with tropical moisture to meet in the west southerly winds laden with antarctic cold, and therefore precipitating power. The sequence of rain is rendered even more certain if rain be recorded in the north-east before the isobars straighten, or in the depression to the south.
The actual height of the barometers is not material, but the greater the number of isobars in a given area, the more extensive will be the rainfall; the rain usually lasts three days. The rain begins to fall north-west of New South Wales, spreads southwards, then eastward, and finally northwards, crossing the mountains near the Queensland boundary. A fine example of this type occurred on 15th, 16th, and 17th October, 1894. (Charts 16, 17.)
On the 15th, Chart 16, a departing anticyclone rests over Tasman Sea, and its rear isobars are shown running north and south over central Australia; another anticyclone is shown over Western Australia and a depression east of the Australian Bight; a trough of low pressure rests over Central Australia from north to south. On this day the only indications of the pending rain were found in the cloudy skies generally over South Australia, western parts of New South Wales and Queensland, and a small area of rain in South Australia.In Chart 16, the straight isobars of Chart 15 have entirely disappeared, but the rain has come over Central and South
Australia. Excepting the Gulf country and its central area, all Queensland, all New South Wales, Victoria and Tasmania had rain. (See shading on Chart 16). For other instances of straight isobars see Charts 5, 27, and 28.
TYPE VIII.—CYCLONES FROM NORTH-WEST.
From time to time fully developed cyclonic storms appear on the north-west and west coasts of Australia, and in the Australian Bight, but the absence of observing stations in the unoccupied country which lies between the overland telegraph line and the west coast of Australia, makes it impossible to trace them over that part of the continent, but cyclones are well known on the northern coast of Western Australia, and their formation in the tropics equally well known. There can therefore be no doubt that when we find a cyclone on the western coast of Australia or in the Australian Bight, that it is one which has come from the north-west, and is in fact recurving to the east and south-east as they do on the east coast.
The one selected for Type 8 was picked up on the west coast of Australia in Latitude 28° on July 4th, 1892, (See Chart No. 17). The winds were light, but the rain heavy along the coast; an inert anticyclone rested over South Australia, Victoria, and New South Wales, where it was moving to the east and making room for the approaching storm.
Chart 18, July 5th, shows that the whole system has moved rapidly, that the cyclone now with an elongated centre lying north-west to south-east now rests over South Australia, strong winds are rapidly developing, and the barometric grade about the
centre is much steeper, and rain has fallen over the south coast generally and extended northwards almost to Central Australia.
On July 6th at 9 a.m. the weather chart presented the features shown in Chart 19. The cyclone has intensified all round, and has moved rapidly to the east, its centre is just entering Bass' Straits; very heavy gales from south-west are blowing in the rear of the centre; heavy rain is falling over Victoria and extends over the greater part of New South Wales. All the winds controlled by this storm were very heavy, and during the 6th July, as the storm passed through Bass' Straits, extremely heavy weather was experienced there. On July 7th it had filled up.
These occur during the summer months, and are most frequent in the western plains; they are developed in hot weather and in the low pressure known as a "Col" between two high pressures, when there is not enough grade to control the winds and the heating power of the sun is great; if to these there is added the presence of moisture from recent showers, we have all the conditions for the formation of a tornado. The force of wind is often sufficient to break off growing trees, two and even three feet in diameter, and the reason there is so little damage to life and property is not the want of power, but the sparse population and the very small number of towns.
Chart 20, March 20th, 1894. An extensive anticyclone lies south of Australia, giving way in its central parts to an extensive monsoonal dip. The isobars are generally uniform and of even gradients, though a suspicious interval exists to the west of New South Wales between the 30•0 and 29•9 curves. This is undoubtedly the area in which the secondary developed. Light rains were recorded in New South Wales and Victoria, but in Central Australia temperatures were high.Chart 21, March 21st. A marvellous change has taken place. The area of high pressure on the previous day over the Tasman Sea has lost two-tenths in pressure. The monsoonal dip so pronounced
pronounced then has retreated, and is now only represented by one isobar, and where the western portion of the anticyclone existed on the 20th, an antarctic depression has protruded itself, and lastly an anticyclone is entering Western Australia. Thus a most favourable and extensive col area exists between these four systems for the generation of these storms. The winds are generally blowing any way, and excepting those of the eastern high pressure have apparently no circulating power.
These were the conditions at 9 a.m., but as the day advanced the col area advanced with its arid heat, and this acting upon the precipitated moisture of the previous two days, resulted in the tornado which we are about to describe, and which occured at Bourke on the morning of March 21st, being one of the most terrific ever witnessed in that district. It struck the town at 10 o'clock, but could be seen approaching for some time from a north-westerly direction. It only lasted six minutes, but during that period thirty-nine points of rain fell, and several hundred pounds' worth of damage was done to houses. Chimneys, verandahs, trees, etc., suffered and general consternation prevailed. Many narrow escapes occurred, as the cyclone came across the common from the direction of Fort Bourke. The theatre was unroofed and a quantity of beams and iron was deposited in an adjoining yard. Pleasure boats on the river were sunk, and a steam-boat was considerably damaged by the falling of a large gum tree.
A comparatively small number of these storms reach the coast of Australia, and owing to the almost complete absence of observing stations, New Caledonia excepted, and the small number of vessels passing their tracks, it is usually impossible to trace their course before they reach Australia, but there seems to be no reason to doubt that they are more or less spent tropical cyclones, which reach Australia in the act of recurving. The majority reach the coast of Queensland between latitudes 20° and 26°; some farther north and south; only one has been traced from the coast inland, and then recurving there to south-east. It reached the coast in the neighbourhood of Brisbane in January 1893, passed inland over the mountains, gradually curving to south past Mudgee and Dubbo, thence curving easterly it left the mainland about latitude 35° S. Its course was marked by violent cyclonic wind and rain. Picton reported 4•76" rain. It has not been possible to present this as an example, because one of the days was a Sunday when very few observations are recorded.
As a rule these north-east cyclones recurve at the coast line. Charts 22 and 23 show the weather conditions on March 10th and 11th, 1891.
On March 10th there was a sudden fall of pressure on the coast of Queensland, about latitude 24° (Chart 22), and an accession
of wind and sea which clearly heralded the coming storm. This was intensified on March 11th, (Chart 23) by a further fall of three-tenths of an inch, and three more isobars of the cyclone were marked on the coast south to south-east; gales prevailed in its neighbourhood with very heavy rains.
On March 12th, (Chart 24) the anticyclone had retreated to New Zealand, and the cyclone travelled to the south along the coast intensifying as it came. The barometer curve at Brisbane was exactly of the cyclone type and dropped to 29•5. In the southern part of Queensland and northern of New South Wales heavy southerly to easterly gales were experienced. On the 13th the storm had disappeared to the eastward.
TYPE XI. SOUTH-EAST GALES.
The south-east gale here referred to is peculiar to the east coast of Australia, and it has been responsible for the most memorable wrecks on this coast. For the most part these gales appear to be partially spent cyclones, which come in from north-east or east, and travel down the coast until they begin to recurve to the eastward.
The warning of their coming is usually very short; it consists of a sudden increase in the sea on some northern part of the coast with wind from east to south, and falling barometers, while the high pressure over Victoria and South Australia becomes intensified and progresses into the Tasman Sea. The south-east circulation about this anticyclone increases in force with the increasing barometric grade, and also by the wind circulation about the cyclone, and the effect of the two causes acting together is to produce a most serious gale. Rarely, these storms originate in a monsoonal depression somewhere over South Australia, which travelling eastward intensifies on the east coast. Heavy rain is a marked feature of these storms, but it is confined to the coast, and rarely if ever extends inland.
The storm selected to illustrate this type was a very severe one, and began on September 23rd, 1892, about 6 p.m. The barometric conditions antecedent to it are shown in Chart 25; the main
features being the bend in the high pressure isobars, and the dormant low pressure off the coast of Queensland.
At 9 a.m. on this day there was nothing in the local weather conditions which would lead one to anticipate the gale that eventuated, the winds being generally light, and at Sydney only a light breeze was blowing from the South-south-west; but at 3 p.m. an unusual fall took place in the barometers to the north-east of New South Wales, and the winds there were freshening generally. On preparing a chart at this hour we found the depression was intensifying and had a cyclonic tendency; at 6 p.m. in Sydney the wind, which had been blowing from the south-east and gradually increasing in velocity, reached the force of a gale, a thick driving rain began to fall, which continued with little intermission until daybreak next day, when over 2" were registered, and an extensive area around the metropolis benefitted to the extent of an inch and upwards; the barometer at this hour also began to fall rapidly and steadily, until at 5 a.m. on the 24th it read 29•203, and the wind had reached in one squall, lasting only a few seconds, the extraordinary rate of one hundred and twenty miles per hour, the mean rate of the gale being thirty-two miles per hour.
At 9 a.m. on the 24th, (Chart 26) the cyclone was still in an active state, but it had passed to the south of Sydney and was receding from the coast; the barometers were rising rapidly on shore, and before noon a light northerly wind was blowing. On the 25th September, the day following the gale, the weather everywhere was generally fine, while all that remained of the energetic high and low pressure systems were parallel isobars lying over the southern areas of Australia with very shallow gradients. This gale was very destructive and did much damage to property in Sydney; in some instances houses were unroofed, and the wind and sea on the coast were very heavy.
The distinction between a Λ depression and a monsoonal low pressure is not by any means well defined, and it is possible that this should be taken as a variation of Type 3; there are however, marked differences, not only in the shape of the isobars, but also in the wind; and the most decided distinction is perhaps the easterly wind circulating round the southern part of the monsoonal low pressure and the northerly and southerly winds about the Λ; but in some cases, as in the one selected, the wind circulation is mixed, northerly, southerly and easterly winds being present, and these from their want of energy tend to throw the forecaster off his guard. The season is, however, some guide, as these storms are most frequent from September to April.
Their sphere of influence is very extensive, as may be noted in Chart 28, which shows rain over half Australia as the apparent result of this storm. Most of these storms take a direct easterly course over New South Wales and Victoria or through Bass' Straits. At times they move to north-east, the Polar winds being more energetic, and this feature intensifies all the storm and rain conditions. The winds in all these storms are violent, and in some very destructive.
The one selected for illustration appeared first on 27th May, 1893, (Chart 27). At 9 a.m. on that day, a dormant and irregular Λ depression existed over the Australian Bight, the winds were moderate northerly and southerly, as usual in such conditions, there were a few light easterly winds. Isobars were close over eastern Victoria, but there was nothing which seemed to indicate the violent cyclone that developed during that day over southern parts of South Australia and south-west of New South Wales. (See Charts 27 and 28.) This storm formed in the rear of a very substantial anticyclone, then over the Tasman Sea, and it should be noted that it did not act as a secondary and travel round the southern and eastern parts of the high pressure, but it moved towards the northern side of it and against its circulation, thus proving its own Polar impulse and energy, and giving rise to very strong gales and steep barometric grades with great fall of temperatures; these conditions produced extremely heavy and wide spread rains, not only within the storm isobars, but over the whole of the eastern half of Australia. We have no means of tracing the rain to the west of the overland telegraph line, because there are no observing stations there.
May 28th was unfortunately a Sunday, and we have no observations for that day, but on the 29th the cyclone is seen in full
strength covering the southern colonies of South Australia and Victoria, and the greater part of New South Wales; its isobars are unusually symmetrical and its rain influence the most extensive we have on record. During the 29th and 30th rain continued to fall over considerable areas, although on the 30th the depression filled up and the storm was displaced by the high pressure coming on from the west.
TYPE XIII.—WESTERLY WINDS.
The Winter anticyclone (See Chart 36) is much more extensive than the Summer one, and its grades are steeper and circulation stronger, while its latitude is further north, often up to 30° S.; hence the circulation on its southern side, unlike the Summer one, affects the mainland of Australia and gives us our westerly winds. And just as the trade wind intensifies its northern circulation by adding force thereto, so the brave west winds of the southern ocean follow the general move of the weather systems northwards, and thus add force to the westerly circulation of the anticyclone, and its greater dimensions increase the size of the Λ depression, so much that it no longer has the sharply defined change from to southerly winds, but from north-west to south-west winds; all these conditions combine to strengthen the westerly circulation, until at times they seem to be like the brave west winds of the roaring forties.
On the east coast they extend further north than they do inland, and at times include Brisbane within their influence. When very strong the anticyclone is elongated and the southern isobars are flattened. (See Chart 30). They are very cold and dry, having all the raw feeling of the easterly winds of England, and have been known to last for several weeks without intermission; they are the most persistent winds during our winter. In heavy westerly winds the cold is very severe in Bass' Straits, between the south coast of Victoria and Tasmania, rain, hail, and sleet being very common. It is almost needless to say that on land they bring very little rain, and they rapidly dry up the soil. The severe drought of 1895 was largely due to the persistent westerly winds which rapidly dried up the occasional rains; this feature becomes intensified if instead of coming from due west they blow from north-west.
Turning now to the charts of the westerly gale selected for illustration, it will be seen in Chart 29, that this storm began on September 3rd, 1895; on that day an elongated anticyclone lay over Western Australia, a flattened and extensive Λ over New South Wales and Tasman Sea, and the winds generally displayed great energy, as might be expected from the close isobars, and unusually low barometers over Tasmania; light rain was falling on the coasts of South Australia and Victoria.
On September 4th, Chart 30, the anticyclone is more elongated and the Λ flattened until its isobars are nearly horizontal, and heavy westerly gales swept all the south-eastern part of Australia and all Tasmania. On the 3rd the wind at Sydney at noon for a short time reached a velocity of seventy-eight miles per hour. On the 4th the wind was less gusty, but its average velocity was quite as strong as it was on the 3rd.
TYPE XIV.—SOUTHERLY BURSTERS.
The southerly burster is a well known feature or type of Australian weather, so well marked in character indeed that it requires no special training in meteorology to recognise it; its characteristics are so obtrusive that they cannot be overlooked, and they are welcomed as the most pleasant relief after the high temperatures and oppressive northerly winds which precede them. They come in the late spring, all summer, and part of autumn, but as a rule, in order to get a strong southerly an antecedent excessively hot day or days must be experienced. The duration of a southerly burster may be anything from two hours to ten days, but it is not to be understood that the term burster is applied to the whole period or duration of the southerly wind. What is called a burster is the squall or sudden and violent change of wind direction, and the violent rush or "burst" which marks the advent of this wind. We need not go into all the characteristics; these will be found in the Abercromby Prize Essay on this subject. It is there explained that the south wind comes in front of an approaching anticyclone, and that it is felt from West to Eastern Australia, but it is only on the eastern coast, where, aided by the smaller friction of the ocean and the shelter which the mountains afford from other winds, that the southerly becomes more vigorous and rushes northwards in a squall, which happens so suddenly and with such force, that at times ships drag their anchors in Sydney harbour.It is not definitely made out yet that these storms are ever "line storms" in the sense that the change of wind comes as the dip in the isobars passes over each place in succession, but there are many facts which suggest that such is the fact in some instances. Our present purpose is to describe a "burster" as a type of Australian weather. The essential feature of it is a sharp Λ in Chart 31 such a depression is shewn existing over Victoria and Tasmania, with its axis lying from north-north-west to south-south-east. An anticyclone of good energy for this time of the year exists to the west, and hot northerly winds occupy northern Australia; these are the elements for the good burster that followed. As a general rule, the position and character of two
such systems as shown would bring the burster to the coast of New South Wales within twenty-four hours; in this case it took thirteen hours.
November 16th, light to fresh north-east winds were blowing on the coast at 9 a.m., while in the front of the high pressure strong south-west winds were blowing. In Chart 32, the southerly
is shown in full force in front of the anticyclone, which by the way has lost none of its energy since the previous day; the depression is well off the coast and on its way to New Zealand. This burster reached a velocity of forty-nine miles per hour, and lasted thirty-five hours.
TYPE XV.—THE BLACK NORTH-EASTER.
This is a somewhat uncommon but nevertheless well known type of weather on the coast about Sydney. Its characteristics are a very strong and persistent north-east gale, continuing day and night for two or more days; it has been known to last five days and nights, and it ends with the advent of a southerly burster. Its cause is found in an extensive col, the rear of one anticyclone being at rest over this coast, while another lies over the Australian Bight. If the grade is rather steep and the system at rest for several days, then the north-east wind persists with force proportionated to the grade, until the whole system moves forward; the southerly winds in the front of the approaching high pressure then displace the north-easter and the storm is over.
There have been no good examples of this type since weather charts have been printed here. Chart 33 shows the necessary forms of isobars, but the grade is not steep enough for a gale.
TYPE XVI.—WIND BLOWING CONTRARY TO ISOBARS.
In this type the wind blows with considerable force in a direction directly opposed to that which the isobars would lead us to expect. For instance in No. 34, it will be seen that an extensive high pressure rests over the east coast, and the isobars are comparatively close together. The normal circulation with these isobars would be fresh north to north-west winds, when, as a matter of fact, strong southerly winds were blowing as far as Sydney with a velocity of thirty miles per hour. Such conditions are rather troublesome in forecasting; fortunately they do not come often, and the fact is not confined to southerly winds. The general direction of the coast line is northerly bearing east a little, a range of mountains from two to four thousand feet high runs nearlyto it, and this local formation has a very important effect on the circulation of the wind; as in Chart 34 it seems to have more effect than the isobars, and probably the grade was rapidly intensifying and had not been long enough in existence to fully control the winds. It seems probable, so far as this type has been studied, that we should find that when the wind blows contrary to isobars, it does so because of some impulse given to it before the new grade had time to control the circulation.
TYPE XVII.—SUMMER ANTICYCLONES.
One of the most marked features of weather in Australia is the regular easterly motion of it in all seasons of the year, added to this the anticyclones—the controlling element in our weather—change with the sun, so that the latitude they follow in their easterly progress is further south in summer than in winter. The summer latitude is about 40° south. They are less extensive in summer than in winter, and do not so completely control our weather as they do in winter, for their southern position leaves room for the southerly extension of monsoonal low pressures, which make a great deal of our weather; but the anticyclone makes the change from hot northerly to cool southerly winds, the bursters of the east coast.
TYPE XVIII.—WINTER ANTICYCLONES.
During the winter months the marks their centres, and the rains come chiefly from the low pressures between each pair of anticyclones, and the strong westerly gales are but part of the circulation about these high pressures. (See Charts 29 and 30.)are much larger than they are in summer, and their latitude about 30° S.; very commonly their area is equal to Australia, and their control of the weather more complete than it is in summer. Fine weather
The one selected for illustration occupied the whole of Australia on June 4th, 1895, Chart 36. Its form was remarkably symmetrical, and in the central area the barometers read 30•6, which is somewhat unusual, hence the circulation is active, and in northern Australia where the trade wind adds force, it is very strong. Under the central influence of these great anticyclones the whole of Australia enjoys fine weather.
TYPE XIX.—SQUARE HEADED Λ DEPRESSION.
This type is a variation of the usual Λ depression, but is sufficiently characteristic to be placed by itself as a type or rather a sub-type. Its isobaric peculiarity, as may be seen by reference to Charts 37 and 38, is that there is a flat top or square head to the isobars of the Λ, the usual form of which is a sharp and regular curve, and the marked feature of this is that under it the weather is remarkably squally and charged with thunder and hail storms, and deluges of rain. This type is not peculiar to any season of the year, and the country they affect is usually south of Lat. 30°. The particular square headed Λ depression under discussion occurred in July, 1891. Its effects were widespread and its life persistent, and its peculiarity in a more or less distinctive form was maintained during its passage over six degrees of longitude, and its effects most severe in Victoria.
In Chart 37, July 8th, 1891, the square headed Λ is shown when over South Australia. The gradients about it excepting those on its eastern side were moderate; winds were from fresh to strong. Following it is a high pressure of decided energy, and preceding it to the east an anticyclone of little or no character; on the previous day rain fell over greater part of the southern seaboard and in Tasmania.
Chart 38, July 9th, the depression has become more extensive, but has lost energy. The relative powers of the anticyclone remains the same, winds were generally lighter, but the rain covered a much wider area than on the previous day. As this storm moved to the east its characteristic weather was maintained.
TYPE XX.—THE ADVENT OF AN ANTARCTIC STORM.
This is a type of weather that does not often visit Australia but its severity makes it noteworthy for the winds and weather which come with it are very destructive, and the cold severe, it might almost be called a southern blizzard; yet the warning is short and often difficult to read, for it comes from the Antarctic where we have no out stations.
The storm under consideration began to affect the south coast of Australia, making the winds fresh to strong on June 21st, 1892, (Chart 39), yet the season (winter) and the general conditions pointed to westerly winds, and said nothing definite of the storm which was telegraphed on the morning of June 22nd, (Chart 40). Barometers were then seen to be four-tenths lower in Tasmania than they were on the 21st, and the wind had increased to a furious gale along the south coast of South Australia and Victoria, and the isobars indicate a very steep grade commensurate with the wind. On shore also the wind rose in places to hurricane force; at Ballarat, in western Victoria, buildings were unroofed and trees blown down. So severe was it there that the storm is recorded as the Ballarat storm.
In South Australia and New South Wales the wind did a great deal of damage without reaching the intensity it had in Victoria. The isobars show a retreat of the anticyclone and a tilting of the major axis, and a very remarkable increase in the number of isobars on the south coasts of South Australia, Victoria, and
Tasmania. It looks as if the low pressure had retreated and its western parts forced their way north. What probably did take place was that a storm centre south of the Australian Bight and indicated by the northerly winds in Chart 39, had in the interval surged northwards on to the west coast of Victoria, bringing with it all its antarctic energy and severe cold. This view is supported by the fact that there was in the twenty-four hours but little change in the New Zealand isobars, and further by the upward tilting of the eastern part of the anticyclone caused by the northing of the antarctic storm, and lastly by the blizzard-like cold which was so marked a feature of this storm.
LIST OF TYPES OF AUSTRALIAN WEATHER.
I.—Moving Anticyclones, Charts 1, 2, 3.
II.—Monsoonal Rain Storm, Charts 4, 5.
III.—Development of a Cyclonic Storm in Low Latitudes from a Monsoonal Depression, Charts 6, 7.
IV.—Development of a Cyclonic Storm in High Latitudes from a Monsoonal Depression, Charts 8, 9, 10.
V. Conditions favourable for Thunderstorms, Charts 11, 12.
VI.—Cyclonic Thunderstorms, Charts 13, 14.
VII.—Vertical and nearly straight Isobars, Charts 15, 16.
VIII.—Cyclones from North-West, Charts 17, 18, 19.
IX.—Cyclones from North-East, Charts 20, 21, 22.
X.—Tornadoes, Charts 23, 24.
XI.—South-East Gales, Charts 25, 26.
XII.—Development of Cyclones from a Λ Depression, Charts 27, 28.
XIII.—Westerly Winds, Charts 29, 30.
XIV.—Southerly Bursters, Charts 31, 32.
XV.—Black North-Easter, Chart 33.
XVI.—Winds Blowing Against Isobars, Chart 34.
XVII.—Summer Anticyclone, Chart 35.
XVIII.—Winter Anticyclone, Chart 36.
XIX.—Square Headed Λ Depression, Charts 37, 38.
XX.—Advent of an Antarctic Storm, Charts 39, 40.
- Russell - Quarterly Journal R. M. S., Vol. xix., No. 85.
- For explanation of this term see p. 18.
- Journal Royal Society, N. S. Wales, Vol. xxviii.
- Moving Anticyclones—Quarterly Journal Roy. Met. Soc., Vol. xix.
- Moving Anticyclones—Quart. Journ. R.M.S., No. 85, Jan. 1893.