The Climate and Weather of Australia/Chapter 4

by Henry Ambrose Hunt, Griffith Taylor, E. T. Quayle

Layout 2

 

IV.—DOMINANT WINDS.

Some interesting particulars of the dominant coastal winds, which are more regular than those on land, have been collected from various publications, including the U.S.A. Pilot Charts.

The seas round Australia may be divided into the following regions:—

1. Northern or Tropical Region.
2. Queensland Coast.
3. New South Wales Coast.
4. South Australian Coast.
5. South-west Coast.
6. North-west Coast (Pearling Coast).

The dominant winds in the various months are given in the following table.

Where, as is often the case, winds from one particular octant are the most numerous, this direction only is given. Where, however, two adjacent octants are nearly equal, they are both given; and they are also grouped together under the intermediate direction. The figures refer to percentages of winds in the whole month.

Dominant Winds on Australian Coasts.

—	1. Northern Coast.	2. Queensland Coast.	3. New South Wales Coast.	4. Southern Coast.	5. South-west Coasts.	6. North-west Coasts.
Cold months.
April	E.S.E. = 65 E. = 33 S.E. = 32	E.S.E. = 70 S.E. = 45 E. = 25	Variable	W.S.W. = 42 W. = 22 S.W. = 20	S.S.E. = 62 S.E. = 33 S. = 27	S.E. = 33 and variable
May	E.S.E. = 61 E. = 33 S.E. = 28	E.S.E. = 72 S.E. = 47 E. = 25	Variable	Variable and W.	Variable and S.E.	S.E. = 48 and variable
June	E.S.E. = 82 S.E. = 42 E. = 40	E.S.E. = 55 S.E. = 33 E. = 22	S.W. and variable.	Variable W. =24 N.W. = 20	Variable	S.E. = 45
July	E.S.E. = 81 S.E. = 43 E. = 38	E.S.E. = 59 S.E. = 32 E. = 27	Variable	Variable	S.W and variable	S.E. = 35
August	S.E. = 42	E.S.E. = 68 S.E. = 35 E. = 33	S.W. and variable	Variable and W.	Variable	S. to E. and variable
September	S.E. = 37	S.E. = 68 E. = 38 S.E. = 30	Variable	Variable	Variable, S.E. through W. to N.W.	S.S.E. = 67 S. = 35 S.E. = 32
Warm months.
October	S.E. = 34	E.S.E. = 65 E. = 35 S.E. = 30	Variable	W. = 21	S.S.W. = 55 S. = 33 S.W. = 22	S.E. = 35
November	S.E. = 40	E.S.E. = 62 E. = 33 S.E. = 29	Variable	W. and variable	S. = 33	S.E. = 38
December	W. and variable	E.S.E. = 67 E. = 36 S.E. = 31	N.E. and variable	S.W. and variable	S. = 35	S.W. = 25
January	Variable	E.S.E. = 76 E. = 46 S.E. = 30	Variable	W.S.W. = 39 S.W. = 21 W. = 18	S.S.E. = 77 S. = 43 S.E. = 34	S.W. = 30
February	W and variable	E.S.E. = 66 E. = 36 S.E. = 30	N.E. = 20 variable	S. to W.	S. = 28	S.W. = 31
March	E.S.E. = 62 E. = 33 S.E. = 29	E.S.E. = 75 S.E. = 46 E. = 29	Variable	W.S.W. = 43 W. = 23 S.W. = 20	S.S.E. = 74 S. = 41 S.E. = 33	Variable

Note.—Figures represent percentages of all winds of month.

In this table the months are grouped as cold and warm respectively. This brings out the paramount importance of the sun's annual swing in connexion with the change in direction of the prevailing winds. Of the six wind regions concerned, three (III, IV, and V) may be classed as variable and three as steady (I, II, and VI).

1. The Northern Coast (from Cambridge Gulf to Cape York) may be discussed first. It is seen that the dominant winds throughout the colder months (and indeed, except in December, January, and February) are confined to the south-east quarter. During this period the monsoonal effect of the Asiatic Massif is controlling Northern Australia. In Midsummer (January) the centre of Australia is heated strongly and the winds on the north coast become much less constant, those from the west predominating. As the sun moves north again the wind changes to an easterly, and in June the south-easter is again dominant. It should be remembered that calms are very prevalent in this region.

2. The Queensland Coast (from Cape York to Brisbane) exhibits somewhat similar changes, but here are the most constant winds in Australia, for even in summer the wind merely veers from south-east to east. In January 40 per cent. of the winds are from the east and 30 per cent. from the south-east. In May 47 per cent. are from the south-east and 25 per cent. from the east.

3. The New South Wales Coast.—Here, as might be expected, the winds are very variable, for the region of the high pressure eddies is reached. Still there is some differentiation apparent. During midwinter (June, July, and August) there is, perhaps, a preponderance of south-west winds, due to the cold elevated area around Mt. Kosciusko. In midsummer (December, January, February) there is a similar local "monsoon" effect, giving rise to a strengthening of the north-east components.

4. The Southern Coasts, from Cape Howe to Cape Leeuwin, are on the border line between the high pressure belt and the region of prevailing westerlies. The centres of the Antarctic depressions moving from west to east—lie considerably south of our coasts; and hence these coasts are controlled by the north component (upper limb) of these depressions or eddies. Here the winds are westerly, and this fact probably accounts for the high aggregate of westerlies recorded in this region, although it is constantly traversed by circulating systems or cyclones.

5. The South-west Coast, from Cape Leeuwin to North-west Cape.—Here the winds throughout the year blow chiefly from south and west. A south-east component is more apparent during midsummer and autumn. The westerly component enters this region in winter, as the sun moves north and the southern "prevailing westerlies" swing northwards in unison and affect Western Australia.

The two varying factors of south-east and south-west respectively are in fact but the result of the swing of the trade wind south in summer and of the westerlies north in winter.

6. The North-west Coast, from North-west Cape to Cambridge Gulf.—This region agrees fairly closely with the northern region. It is dominated by the south-east trade for eight months, but during midsummer the almost permanent "low" covering the hot Pilbara region in Western Australia gives rise to a "monsoon" and the coastal winds swing round from south-east to south-west. The south-east wind is, however, not so constant as that off the Queensland Coast, and in autumn (March, &c.) the winds are very variable. In the north calms are common, and except for the "willy-willies" storms are rare.

The South-East Trade Winds.

The whole coast of Australia from Perth northward and east to Brisbane is at times influenced by the South-east Trade, so that a résumé of the movements of this important air current will be of interest.

In April the coastal winds are practically all S.S.E. or E.S.E. over this immense stretch of coastline, the only marked deviation being the occurrence of some S.W. winds (20 per cent. only) off Kimberley. In May, the belt has moved north to Geraldton, but is otherwise unchanged. In June it does not extend south of North-west Cape, but is stronger off Kimberley. In July, August, September, and October, it is practically continuous from North-west Cape to Rockhampton.

In November, as the sun moves south, the belt extends down to Geraldton.

In December the Australian monsoon causes a break from North-west Cape to the Gulf of Carpentaria (dominated by west winds), and the South-east Trade forms a fringe on each side. This hiatus lasts through January and February, and in the last month the South-east Trade is hardly distinguishable on the West Coast. In March, the intervening region of S.W. winds is confined to the north-west coast, and the trades are again in evidence south to Perth.

In April the whole north-west, north, and north-east coasts are controlled by the trade winds.

Land and Sea Breezes.

Since so large a portion of the population of Australia lives on the seaboard, these winds are of great importance. Data are lacking except at the capitals, but type examples from Perth and Adelaide will serve to indicate the effect of the diurnal change.

In Fig. 36 the winds throughout the day are averaged for each month for Perth. (Data for all years to the end of 1911.)

In the first column the average for the month is given. (Here we tend to get the diurnal extremes neutralizing each other so that the normal wind of the region is shown.)

Regional Winds at Perth—

South wind	In October, November, December, and January	Summer
South-east	In February, March, and April	Autumn
North-east	In May, June, and July	Winter
South-west	In August, September	Spring

In this locality the invigorating sea-breeze—called the "Doctor"—blows from the W.S.W. during the afternoon, while a land wind (E.N.E.) sets in when the land has cooled below sea temperatures in the early morning.

The sea-breeze is most active when the land is much warmer than the sea, i.e., in the summer. The land breeze is most active for similar reasons in winter.

Thus we see that in winter the two factors (the normal north-east wind and the land breeze) are assisting each other. Hence the constant N.E. winds every morning during this season. For the rest of the year the morning winds are from the south-east, the resultant of the southerly regional wind and the N.E. land breeze.

The sea breeze absolutely controls the wind direction at Perth in the afternoon, even in winter, and in the summer not much deflection of the normal south wind is necessary to bring it into line.

Land & Sea Breezes at Perth.

Fig. 36.

Land & sea Breezes at Adelaide.

Fig. 37.

In conclusion it may be noted that there is each day a noteworthy change in wind direction near noon. During the first half of the year (February-July) this occurs just after noon; in the latter half of the year (August-January) just before noon.

In Adelaide, much the same phenomena can be noticed. (Fig. 37.)

The normal winds of this region (deduced from the average throughout the month) are—

South winds	In summer (January, February, March, April).
North winds	In winter (May, June, July, August, September).
South-west winds	In Spring (October, November, December).

Here the coastline has much the same direction as at Perth. So that the land breeze is E.N.E. and the sea breeze W.S.W. as before.

It will be noticed that the land wind is very strong in the months of May and June. It also deflects the normal wind appreciably, but not very greatly in other months. The sea breeze is not strong in winter, but all the rest of the year is very important in the afternoon, exercising a strong deflection in January, February, and March.