Southeast Australian foehn

moves northwards. This foehn effect on the east coast of Australia is linked with the passage of a deep low pressure system across the Great Australian Bight and Bass Strait that cause strong winds to reorient virtually perpendicular to some parts of the Great Dividing Range, predominantly between late autumn into winter and spring (being most common during a negative AAO phase). Their occurrence is owed to the incomplete orographic blocking of comparatively moist low-level air and the subsidence of drier upper-level air in the lee of the mountains. Moreover, temperatures on the lee of the Great Dividing Range tend to rise substantially (due to a katabatic effect) when cold fronts push warm and dry air from the desert across the country's eastern states and over the Range (this is generally followed by a southerly buster). As such, the Great Dividing foehn is one the few reasons why Sydney, among other places on the coastal plain, registers high temperatures in the warm season but seldom attains cold maximum temperatures in the winter. Furthermore, when the warm season north-westerly winds strike (such as the Brickfielder), the hottest and driest areas of southeastern Australia will generally be located along the southern coastal region of NSW in the lee of the Great Dividing range and coastal escarpment due to the foehn effect. Much lower relative humidity figures would also observed in these leeward stations. ==Formation==

File:Foehn wind illustration.svg|thumb|300px|Föhn wind illustration (Left [West]: windward side, Right [East]: leeward side). The southeast Australian foehn is distinguished by three criteria; surface winds which blow from the mountains' direction, a sharp rise in air temperature in the leeward side of the mountains, and an accompanying diminution in atmospheric moisture. During these conditions, an orographic cloud band, or the Föhn wall, builds up along the ridgelines of the southeastern highlands due to condensation of moisture as the air ascends the windward slopes. Meanwhile, the Föhn arch, with its broad layer of altostratus cloud, shapes downwind of the mountains in the ascending component of a standing lee mountain wave. In weather maps, a band of clear air called the Föhn gap, which is over the downwind of the Great Dividing region, can be seen between the wall and arched cloud cover. This foehn wind can be referred to as thermodynamically driven. A vertically propagating gravity wave over the affected region exists. The descending motion over the coastal escarpment is stronger than that over the primary range and is connected with more powerful shear. The downslope winds tend to be strong, particularly near the lee's surface of the coastal escarpment. Smaller-scale, trapped lee waves over the affected region exist, and their incidence, together with the strong wind shears, signal significant turbulence throughout the boundary layer, which is concordant with the heavy gusty surface winds registered on the leeside. At nighttime, the foehn effect subsides due to a mountain breeze – This is when denser cool air flows down the mountain slopes to settle on the downwind side, giving way to significant nighttime cooling and likewise a high diurnal range. ==Occurrence==

(notice the cloud streets forming on the slopes). The Great Dividing foehn is primarily observed in the southeast of New South Wales, east of the Great Dividing Range, in places such as the Sydney metropolitan area (Cumberland Plain), the Illawarra, some areas of the Southern Highlands, parts of the Monaro region, and the South Coast. It can also occur in the Central Coast, Hunter Valley and the Mid North Coast to the north. In many instances, it is observed in the East Gippsland region in Victoria as well as the eastern portion of Tasmania to the south. Foehn winds may also impact other parts of Australia, such as east of the Great Dividing Range in southeast Queensland and northern New South Wales. Areas that lie to the west of the Great Dividing Range are windward and therefore never experience a foehn effect under a westerly stream, with persistent cloud cover. Conversely, the Great Dividing Range also blocks frontal systems originating in the southern Tasman as well as the eastern Bass Strait; as such, when south/south-easterly systems lift over the ranges, the western side of the range would experience foehn-like conditions. Leeward zones • From north to south, the westerly foehn strongly affects areas that lie to the east of the Great Dividing Range (the southeast coastal plains or the eastern seaboard) such as, Newcastle, Gosford, Sydney, Wollongong, Nowra, Ulladulla, Moruya, Batemans Bay, Narooma, Bega, and Merimbula. ;Transitional zones • The western portion of the Blue Mountains is transitional (Leura, Katoomba and westwards). Further inland in New South Wales, Mount Boyce, Lithgow, Bathurst, Goulburn, Bowral, Taralga, Braidwood and Canberra in the Australian Capital Territory, occasionally receive foehn winds, though are at times exposed to isolated frontal showers off the west-southwest. When these areas observe foehn winds, they are inclined to have more cloud cover (including wave clouds) than those on the coastal plain to the east. • In the East Gippsland region of Victoria, transitional areas include Omeo, Bendoc, Bairnsdale, Orbost, Mallacoota and Sale, as these are highly susceptible to south-westerly systems and would even experience notable cloud cover from true westerlies in some instances. Due to their south-facing location and western longitude, cloud cover is significantly greater than in their New South Wales counterparts. • In Tasmania, Hobart, New Norfolk, Scamander, Swansea and St Helens on the east coast; as well as Oatlands, Ouse and Bothwell in the Midlands, are downwind of the Central Highlands and thus prone to foehn winds (particularly in the warm season, though sporadically throughout the year). However, due to their south-facing location, they are all susceptible to south-westerly systems and may occasionally experience some cloud cover from westerlies. ==Effects==

The Great Dividing wind can be particularly damaging to homes and would affect flights, in addition to being uncomfortable, as the wind chill factor can paradoxically make the temperatures feel cooler than what they are. The Australian foehn has also impacted international sporting events and as well as recreational aviation, such as in 2007, when a light aircraft crashed in the Central Highlands due to severe winds on a region that is prone to mountain-wind waves. Foehn winds in general have been linked to headaches, depression and as well as suicide contemplation, although this study has not been proven. Though recent studies regarding migraine attacks during Chinook winds suggest there may be some truth in it. ==Notable observations==

) can be contrasted from the dry landscape on the leeward (right, Greater Western Sydney). • 28 May 2000 was a striking example of the 'divided' weather between the western and eastern faces of the range. On the western face, Hunters Hill in Victoria registered a maximum temperature of just , whereas Cooma Airport on the eastern face reached . These stations are at altitudes of and respectively. Furthermore, Thredbo Village reached a maximum of ; this is warmer than that recorded at Hunters Hill, despite being over higher in altitude; whereas Cabramurra at a more similar altitude only topped at . • On 29 September 2000, a remarkable foehn event was recorded in the lee of the Blue Mountains region in Sydney, where maximum temperatures at Penrith, Badgerys Creek, Bankstown Airport, and Sydney Airport were around above average. The elevated temperatures again coexisted with the inflow of significantly drier air. Simultaneously, the leeward stations in the southern New South Wales coast showed a sharp increase in temperature (9°C in 2 hours) and a decrease in relative humidity. Similar warming and drying were also observed further inland at Cooma, Braidwood, Canberra, and Bombala. • On 18 July 2016, Mallacoota reached an unseasonable high of due to the foehn effect, a record warm winter day for that region in Victoria. • On 20 September 2023, during a heatwave in southeast Australia, Gabo Island, Ulladulla and Montague Island recorded highs of , and , respectively, due to strong foehn winds on the leeward side of the mountains. Western Sydney surpassed , and Sydney Airport recorded its highest September temperature at . • On 30 August 2024, hot foehn winds on the eastern seaboard caused Sydney Airport to reach a winter record of . • On 27 November 2024, due to the foehn effect, Sydney Airport reached at 12:15pm, which made it the hottest place in the world at that time. ==See also==