Southeast Australian foehn

Foehn occurrence in southeastern Australia is generally linked with the passage of a deep low pressure system or westerly cold fronts across the Great Australian Bight and southeastern Australia that cause strong winds to reorient virtually perpendicular to some parts of the Great Dividing Range, predominantly between late autumn into winter and spring, but with more frequency from late winter to spring.[lower-alpha 2] 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.[1]

Averaging between 60 km/h (37 mph) to 70 km/h (43 mph), sometimes they may be brought on by a large polar air mass from the southwest of the continent in the Southern Ocean which advances northwards across Victoria towards the east coast.[lower-alpha 3][4] Moreover, temperatures on the lee of the Great Dividing Range tend to rise substantially (due to a katabatic effect)[5] when westerly cold fronts passing over southern Australia 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).[1][6]

As such, the foehn effect is one the few reasons why Sydney, among other places in the coastal plain, registers high temperatures in the warm season but seldom attains cold maximum temperatures in the winter.[7][8][9] Furthermore, when the warm season northwesterly winds strike, 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. Much lower relative humidity figures would also observed in these leeward stations.[10]

Föhn wind illustration (Left: NSW/VIC Inland Slopes, Right: NSW/VIC Coastal Slopes).

As the moist air rises over the windward side of the ranges, it cools and it would condense, thereby creating precipitation on the upwind slopes. The precipitation then gets rid of the moisture from the air mass on the lee side of the ranges, and the condensation raises the air temperature as it descends the lee slopes towards the coastal plains because of the adiabatic compression.[11]

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.[1]

The existence of topographically induced atmospheric waves in connection with foehn occurrence has been indicated, which develop with the descent of upper-level air above of the ridgetop and pass into the lee of the ranges as broad-scale, vertically supporting gravity waves. The wind shears and the strength of the downslope motion manifested in the model examination also point that the onslaught of foehn conditions results in increased turbulence near the surface, evident in the gusty conditions observed at the lee stations.[1]

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.[1]

The Great Dividing Range (yellow) is the cause of foehn winds in southeastern Australia, in addition to blocking cold fronts and thereby creating a climatic difference between the inland and the coastal slopes.[1]

The Great Dividing foehn is primarily observed in the southeast of New South Wales in the Hunter Valley, the Sydney metropolitan area, the Southern Highlands, the Illawarra, Canberra, the Monaro region, and the South Coast. In many instances, it is observed in the eastern Gippsland and Melbourne region in Victoria[lower-alpha 4] as well as the eastern portion of Tasmania to the south. It can also occur in the Central Coast and the Mid North Coast to the north. Foehn winds may also impact other parts of Australia, such as the east of the Flinders Ranges in South Australia, east of the Great Dividing Range in southeast Queensland and northern New South Wales.[1]

Where they occur, foehn winds can be particularly damaging to homes and would affect flights, in addition to being uncomfortable, as the wind chill factor would usually make the temperatures feel cooler than what they are.[12][13][14] Much like the Santa Ana winds in California, they may elevate fire danger in the warmer months due to their dry, gusty nature.[15]

Leeward and windward areas

With leeward areas, precipitation is predominantly derived from the Tasman Sea to the east, since the Great Dividing Range blocks westerly cold fronts from the Southern Ocean (which tend to arrive between winter and early spring). Therefore, winters in leeward zones are drier (due to the foehn effect) with the summers being relatively wet. When it comes to windward areas, little or no precipitation is received from the Tasman Sea to the east[lower-alpha 5] due to the large distances and the Great Dividing Range; instead, precipitation derives from the South Indian Ocean and the Southern Ocean, advancing northwards across Victoria.[1] Winters in windward areas are generally wet and summers tend to be (relatively) dry.

Leeward

Typical bleak conditions resulting from uplift on the western slopes. (Billapaloola State Forest, near Wee Jasper)

Gusty foehn conditions occurring simultaneously in the coastal plains because the frigid air is compressed as it descends the Ranges' leeward side, melting snow rapidly. (Sydney CBD)

From north to south, the most prominent leeward areas include those that lie to the east of the Great Dividing Range; Taree, Newcastle, Singleton, Sydney, Wollongong, Nowra, Moruya, Batemans Bay, Narooma, Tathra, Bega, Merimbula and Eden, which are cities and towns on the coastal plains of New South Wales, and would therefore feature relatively dry winters and springs. In the Blue Mountains, Mount Boyce, Wollemi National Park, Lithgow and Katoomba are on the leeside, including those to the south in the Southern Highlands, such as Bowral and Moss Vale.[1]

Towards the east coast of Victoria, Omeo, Sale, Orbost and Bairnsdale are somewhat on the leeward side, hence their warmer winters, but may be exposed to the more vigorous southwesterly fronts. In Tasmania, areas such as Hobart, Launceston, Ouse, St Helens and Bothwell in the Midlands are downwind of the Central Highlands (thereby usually receiving foehn winds, particularly in the warm season).

Areas that tend to be sheltered from both easterly flows and western frontal systems due to their sandwiched positions, and therefore fall in a rain shadow, include Bathurst, Goulburn, Braidwood, Canberra, Cooma, Bombala and Nimmitabel, which happen to be relatively dry all year round.

Transitional

Areas which lay in the transitional zone between upwind and downwind include Taralga, Yass, Eucumbene, Aberfeldy, Delegate, Melbourne, as well as Flinders Island, Dover, Liawenee and Devonport in Tasmania. Moreover, Oberon, Shooters Hill and Sunny Corner are on the crest of the ranges and thus exposed from all directions, hence their evenly spread rainfall.[lower-alpha 6]

The main ski resorts in New South Wales, such as Thredbo Village, Perisher and Charlotte Pass, lie transitionally between the leeward and windward side (the former town being more leeward and the latter more windward). Although they receive substantial precipitation from over the crest of the ranges, they lack the persistent cloud cover which characterises truly windward locations on the western face. Because they are transitional, such areas can be exposed to both westerly fronts and moist easterly flows.

Windward

Farther inland towards the Central West of NSW, Orange, Blayney, Millthorpe, and Grenfell are on the windward side and feature relatively cooler, wetter and cloudier winters—and consequently a much wider seasonal range. Farther south in inland New South Wales, examples of areas on the windward side include Crookwell, Young, Burrinjuck, Tumut, Gundagai, Batlow, Tumbarumba, Khancoban and Albury, all of which feature distinctly cloudy winters. On the western face of the alps, Cabramurra, Kiandra, Mount Buller, Falls Creek, Mount Hotham and Mount Buffalo are windward.

In Victoria, Koetong, Beechworth, Bright, Strathbogie, Bendigo, Ballarat, Trentham, Rubicon, Portland, Warrnambool, Hamilton, Ararat and the isolated peaks of Mount William and Mount Cole, to the west of the state, are windward, as marked by their very cloudy and damp winters. The interior regions of New South Wales and Victoria are windward (i.e., Griffith, Wagga Wagga, Echuca and Horsham), although their winter rainfall is not significant; meaning westerly rainbands are not as powerful in such regions, although their winters are still cloudy compared to the leeward areas. It is worth noting that South Australia is entirely windward; Adelaide, Mount Gambier and Kingston SE are exposed to westerly fronts, hence their classic dry-summer, wet-winter Mediterranean climate; the same is true for southwestern Western Australia as exemplified by Perth, Albany and Esperance, among other cities.

In Tasmania, King Island, Deal Island, Queenstown, Strahan, Zeehan and Rosebery in the western half of the state are on the upwind side, as manifested by their higher precipitation amount and cooler summers. On the highlands themselves, upwind areas include Mount Read, Cradle Valley and Butlers Gorge.

• On 29 September 2000, a remarkable foehn event was discovered in the lee of the Blue Mountains region in Sydney, where maximum temperatures at Penrith, Badgerys Creek, Bankstown Airport, and Sydney Airport were around 10 °C (50 °F) above average. The elevated temperatures again coexisted with the inflow of significantly drier air.[1]

• On 29 May 2007, it was observed that temperature at Sale (leeward side) was around 4–9 °C (7–16 °F) higher than the corresponding temperatures at Melbourne and Wangaratta (which lie on the upwind side). In this foehn event, Sale had a high above 24 °C (75 °F), whereas the latter cities struggled to reach higher than 12 °C (54 °F). Furthermore, the relative humidity was 31% at Sale and as high as 80%–90% at Melbourne and Wangaratta.[1] Unusually warm and dry conditions were also registered at other stations in the downwind side of the ranges – Bairnsdale, Orbost, Latrobe Valley, and Nowa Nowa, which recorded temperatures of 24 °C (75 °F), 24.2 °C (76 °F), 22.9 °C (73 °F), and 22.6 °C (73 °F), respectively, making this location in the lee of the ranges consistent with the position of the foehn gap and foehn arch.[1]

• On 27 October 2008, foehn wind dynamics were observed over the Gippsland region to the southeast of the Australian Capital Territory on the lee of the ranges, associated with northwesterly winds over southern New South Wales. These downwind regions experienced lower humidity levels and higher than average temperatures. The temperature at Orbost reached 32 °C (90 °F); the temperature at Mount Nowa Nowa rose to 26 °C (79 °F); Bega reached 36 °C (97 °F), which is approximately 14 °C (57 °F) above the average maximum temperature for October–November. At Moruya, the temperature rose to a maximum of 35.4 °C (96 °F). Similar effects were also observed at Green Cape, Narooma, Merimbula, Braidwood, Canberra, Bombala, and Cooma. In contrast, Albury, which is on the windward side of the ranges, only reached a maximum of 27.7 °C (82 °F).[1]

• On 23 August 2012, a foehn effect caused Sydney to record its 3rd warmest August day on record where it reached 29.0 °C (84 °F) at the CBD and 30.0 °C (86 °F) at Sydney Airport.[16]

• On 18 July 2016, Mallacoota reached an unseasonable high of 23.5 °C (74 °F) due to the foehn effect, a record warm winter day in the state of Victoria.[17]

• Brickfielder
• Santa Ana winds
• Zonda wind
• Diablo wind
• Nor'west arch