The geographic amplifier: How the Great Dividing Range forges Australia's tempestuous eastern coast

“ The conditions which surround the liquefaction of ice and snow, the evaporation of water, and the condensation of the water vapor of the air are fundamental factors in the science of weather. The distribution of precipitation—that is, rain, snow, hail, and the floating forms of fog and cloud—affect the habitability of the earth and human activities to a very great degree. ”

“ The south-east trade cannot be said to blow home upon that part of the coast of New South Wales, which lies between Breaksea Spit and Port Jackson, except during the summer months when winds from that quarter prevail and often blow very hard; they are then accompanied by heavy rains and very thick weather: generally however from October to April they assume the character of a sea-breeze and, excepting during their suspension by south-easterly or westerly gales, are very regular. In the month of December strong south-easterly gales are not uncommon ”

“ In some cases, the wind flies round suddenly from north-west to south-west; and the rainy, thick weather then continues a longer time.
Such is the usual course of the gales along the South Coast and in Bass' Strait; but on the east side of the strait the winds partake of the nature of those on the East Coast, where the gale often blows hardest between south and south-east, and is accompanied with thick weather, and frequently with heavy rain. ”

“ In Australia—east coast—the shore-line is stretched out in the direction of the trades; in South America—east coast—it is perpendicular to their direction. In Australia they fringe this shore only with their vapour; thus that thirsty land is so stinted with showers that the trees cannot afford to spread their leaves out to the sun, for it evaporates all the moisture from them; their vegetable instincts teach them to turn their edges to his rays. In inter-tropical South America the trade-winds blow perpendicularly upon the shore, penetrating the very heart of the country with their moisture. ”

“ Now it is the interior of the continent which is the most arid, waste, and barren. The coasts are comparatively fertile, for they are watered by showers condensed from an atmosphere which is charged with moisture by the neighbouring sea; and this condensation is greatly facilitated in the south-eastern and eastern parts of the continent by a high range of mountains which here skirts the coast for a long distance, attracting the moisture from the ocean and precipitating it in the form of snow and rain. ”

“ The south-east trade-winds in the Atlantic Ocean first strike the water on the coast of Africa. Travelling to the north-west, they blow obliquely across the ocean till they reach the coast of Brazil. By this time they are heavily laden with vapour, which they continue to bear along across the continent, depositing it as they go, and supplying with it the sources of the Rio de la Plata and the southern tributaries of the Amazon. Finally they reach the snow-capped Andes, and here is wrung from them the last particle of moisture that that very low temperature can extract. ”

“ They should be on the slopes of those mountains which the trade-winds or monsoons first strike after having "blown across an extensive tract of ocean. The more abrupt the elevation, and the shorter the distance between the mountain top and the ocean (§ 298) , the greater the amount of precipitation. If, therefore, we commence at the parallel of about 30° north in the Pacific, where the north-east trade-winds first strike that ocean, and trace them through their circuits till they first meet high land, we ought to find such a place of heavy rains. ”

“ The North-eastern coast of New South Wales, from the latitude of about 28 degrees, has a direction from south-east to north-west; and ranges of mountains are visible from the sea, with little interruption, as far north as Cape Weymouth, between the latitude of 12 and 13 degrees. From within Cape Palmerston, west of the Northumberland Islands, near the point where Captain King began his surveys, a high and rocky range, of very irregular outline, and apparently composed of primitive rocks, is continued for more than one hundred and fifty miles, without any break ”

“ Winds.--During the winter months, the wind is mostly from south to west, blowing with great violence for a week together; afterwards it veers round to the southward and south-east, which brings fine weather for a few days, then it veers to east, north-east, and north-west, blowing in heavy gales, and generally accompanied with violent torrents of rain: after which it shifts to south-west: indeed, I do not remember one instance of the wind coming to the north-east, round by west. The south-east wind blows during the summer with very little variation, and sometimes very strong. ”

“ Even as far as Cape Grafton the wind cannot be said to be steady. To the north of this point, however, the winds are always constant from the southward, and seldom or ever veer to the westward of south, or to the eastward of South-East by East; they generally are from South-South-East: fresh winds cause the weather to be hazy, and sometimes bring rain, which renders the navigation among the reefs in some degree dangerous. In my last voyage up the coast, on approaching Cape York, the weather was so thick that we could not see more than a quarter of a mile ahead ”

“ I know, from long experience of this coast, that the strong winds, which prevail during the winter, are from south-west, west, and north-west, and that when the wind shifts to south it invariably moderates; and being assured, by all the masters of whalers in that district, that the south or south-east winds were only during the summer months, and that the shelter from the Murray beach prevented any fetch into Port Elliot, I feel assured that, after a time, it will be found quite safe to extend the anchorage for large ships further out. ”

“ 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. ”

“ In winter, after the prevalence of easterly winds, if the barometer begins to fall and the thermometer to rise, a gale which commences to blow from the southeast will veer to southwest, while the barometer falls constantly. As soon as the wind passes the southwest point the barometer begins to rise, a heavy shower of rain falls, and a strong west-northwest or northwest wind may follow, after which the sky clears and the weather becomes colder. ”

“ We had very heavy gales of wind from east until the 28th, with violent squalls, attended with rain: the air in general thick and hazy, and a high hollow sea running. At one o'clock on the 28th, we perceived a great alteration in the sea, which was become so smooth, that at four o'clock it was, comparatively speaking, smooth water: at half past five, the man who was stationed at the mast-head, saw breakers in the south-east, which were found to be a shoal, bearing from south-east by east to east-south-east, about seven miles distant: it appeared to trend south-south-east and north-north-west ”

“ In like manner, the south-east trade-winds take up the vapours which make our rivers, and as they prevail to a much greater extent at sea, and have exposed to their action about twice as much ocean as the north-east trade-winds have, we might expect, according to this hypothesis, more rains in the northern—and, consequently, more and larger rivers—than in the southern hemisphere. ”

“ When the north-east and south-east trades meet and produce the equatorial calms (§ 212) , the air, by the time it reaches this calm belt, is heavily laden with moisture, for in each hemisphere it has travelled obliquely over a large space of the ocean. It has no room for escape but in the upward direction (§ 223) . It expands as it ascends, and becomes cooler; a portion of its vapour is thus condensed, and comes down in the shape of rain. Therefore it is that, under these calms, we have a region of constant precipitation. ”

“ Doubtless it is given to the winds, that they may bear it off to distant regions, and make lands fruitful, that but for these sources of supply would be almost rainless, if not entirely arid, waste, and barren. These seas and arms of the ocean now present themselves to the mind as counterpoises in the great hygrometrical machinery of our planet.—As sheets of water placed where they are to balance the land in the trade-wind region of South America and South Africa, they now present themselves. ”

“ The chief feature indicating thunderstorms is the narrowness of the col [2] 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. ”

“ In spring and autumn a sudden fall indicates rain; in the summer, if very hot, it foretells a storm. In the winter, after a period of steady frost, a fall prognosticates a change of wind with rain and hail; whilst a rise announces the approach of snow.
6. Rapid oscillations of the mercury either way are not to be interpreted as indicating either wet or dry weather of any duration; continuance of either fair or foul weather is forecast only by a prolonged and steady rise or fall beforehand. ”

“ The strong winds are accompanied by heavy squalls and thunderstorms, the rainfall being much heavier than during the winter monsoon. As the season advances, squalls and rain become less frequent. In some places the wind becomes a light breeze which is unsteady in direction, or stops altogether, while in other places it continues almost undiminished, with changes in direction or calms being infrequent. The winter monsoon blows from the northeast, usually from October to April. ”

“ Thunder and lightning.—"In the Java Sea, during the month of February, the west monsoon blows strong almost continually; in March it blows intermittently, and with hard squalls; but in April the squalls become less frequent and less severe. Now the changing commences; all at once gusts begin to spring np from the east: they are often followed by calms. The clouds which crowd themselves upon the clear sky give warning of the combat in the upper air which the currents there are about to wage with each other. ”

“ The result is that this western coast, with its many good harbors, and its abounding fertility, furnished from earliest times a favorable home for strong and active peoples. Here grew, mingled in great profusion, the fruits and grains of both the temperate and tropical climes. At many points, with the aid of irrigation, the soil yields four or more crops a year. Throughout most of this garden land the climate is semi-tropical without being enervating. The brilliant sunshine is tempered by the cool breezes from the sea and mountains. ”