The dry air in the center of the anticyclones does not extend all of the way to the surface, leaving room for low level moisture to gather. This moisture is typically found below a temperature inversion that inhibits vertical motion and so it is difficult to mix wet lower and dry higher level air and evaporate the cloudiness. Because they have only a small vertical extent, rain is uncommon from such clouds and an area may be cloudy and yet dry at the same time.
This position of the high-pressure belt to the south of the continent allows the tropical moisture from the ICZ to reach into northern Australia. Typically it is well away from the eclipse track but periodically it moves southward to play a prominent role in the weather over South Australia and southern Queensland. Solar heating is extremely high in northern parts of the continent, and a persistent heat low forms south of Port Hedland in Western Australia in response to the abundant sunshine. This Pilbara low is the Australian counterpart of the Botswana low. Its influence waxes and wanes with the flow of the weather, and occasionally strengthens sufficiently to cast a long low-pressure trough southward across the western continent where it plays a role in the formation of cloud and precipitation.
Roads from Ceduna lead only a short distance inland and there is little opportunity to move if December 4 proves to be cloudy. The Eyre Highway and most local roads tend to run east-west, leaving very little room to maneuver on an eclipse track only 25 km wide. The only route along the center line is from Ceduna to Maltee, a distance of about 20 kilometers.
Farther northeast the eclipse track crosses Lake Gairdner National Park, a vast salt pan famous for the world land speed records that have been attempted and achieved on its flat surface. Nearby is Lake Hart, another blinding-white salt pan set in a red landscape, but this lake boasts the main launching pad for the Woomera Rocket Range on its northeast side, now no longer in use. Eclipse sites in this area, though convenient to reach by way of Highway 87 from Port Augusta to Alice Springs, will have to contend with the heat of the interior and the tireless Australian flies. The centerline lies very close to Koolymilka, though there seems to be little in the way of community any more at this former launching site. It should be pointed out that access to the region north of Highway 87 is limited because it is part of the Australian government's Woomera Prohibited Area. Anyone planning eclipse viewing from the region will need detailed maps showing the boundaries of the prohibited area.
A little farther along the track, is Roxby Downs, a thoroughly modern uranium-mining town (along with gold, silver and copper) with a turbulent history since its construction in 1986. It's close to the north edge of the track but could provide a convenient base. The surrounding area is more settled than the earlier part of the eclipse track, in large part because of the availability of water from artesian wells. In this area, the eclipsed sun will hang little more than 5 degrees above the horizon with a duration barely over a half-minute.
Beyond Roxby Downs, the eclipse track skirts the Flinders Ranges. Flinders Ranges National Park, south of the track, is one of the most majestic parks in Australia an old folded landscape of gorges and craggy red mountains. Local Dreamtime stories tell of the serpent that guards the water holes and formed the Flinders' contours by wriggling north to drink dry the salt lakes. Continuing northward, the shadow path moves past the Flinders Ranges and into the Sturt Desert. Touching the junction of three states, the track makes a brief passage through New South Wales and then into Queensland. The point where the three states meet is known as Cameron's Corner and the region is the Corner Country. By now, the Sun is very close to the horizon at mid-eclipse, reaching that point just beyond Old Tickalara. The path continues northeastward, right across Australia and into the Pacific Ocean, but gives watchers only a brief view of the eclipsed sun before it sets.
The Corner Country is a remote part of Australia, well into the Outback. Roads are not paved and are largely deserted. Travel requires food, extra fuel, water and spare parts. The area around Old Tickalara is very dusty; one traveler reports only three homesteads between Cameron's Corner and Thargomindah, a 340- km route that more-or-less follows the eclipse track and its extension northeastward.
A low-pressure trough then follows, separating the departing high from the next in the sequence. This trough often contains a cold front, substantially modified after passage over the Bight, but containing enough moisture to create the relatively cloudy climate of coastal regions. Most of the cold front cloudiness does not penetrate far inland before it encounters the dry desert air of the interior and gradually dissipates. This is not an incontrovertible rule however, for the stronger systems are quite capable of pushing cloudy weather right across the length of the eclipse track as far as the Great Dividing Range. Southern Australia has a flat terrain that offers little physical impediment to weather moving in from the south.
The highs and troughs moving alternately past the continent bring a repeating pattern of cloud and sun, cool and hot weather. While the cycle from high to high or trough to trough has a modest degree of regularity (repeating at 5-6 day intervals), the weather in Australia is not a monotonous series of predictable events. Troughs vary in strength, some with fronts, some not. Highs may stall, bringing persistent weather for several weeks before moving on to allow the normal cycle to return. Departures from the average are common and bring weather that is far from average.
In northern Australia, cloud and precipitation are influenced by the Intertropical Convergence Zone that has moved southward onto the continent by December. The presence of the ITCZ signals the beginning of "the Wet", a season of frequent and widespread showers and thundershowers. Moisture from the ITCZ can be induced to flow southward under the right conditions, but typically remains in northern regions away from the eclipse path.
One of the elements that may induce a southward excursion of the moisture is a strengthening in the Pilbara heat low. As the low intensifies, northerly winds increase and push tropical moisture southward across Western Australia. This moisture is typically caught by an inter-anticyclonic trough (with its cold front) and is turned eastward toward the center of the continent and the eclipse track. The combination of tropical moisture and the atmospheric dynamics of the trough can create a large area of broken cloud that may linger over the track for several days. The combined system often contains enough instability for showers and thunderstorms. Satellite photos show characteristic arched bands of cloud with this type of pattern that persist from their first appearance in the Bight until they pass Adelaide several days later. The low may also be reinforced by a tropical cyclone moving onto the continent from the Indian Ocean, with dramatic consequences for cloud cover across much of the country. This is a relatively uncommon event, occurring only about once a year. On December 6, 2000, Tropical Cyclone Sam moved into northwestern Australia, joined the Pilbara low, and pushed a large area of cloud southward for the next five days. In what might be a good omen for this type of meteorological event, the cloud did not reach the south coast and only the northern part of the eclipse track was affected.
Figure 22 shows contours of the number of clear days in December based on surface observations of cloud cover from Australian weather stations. The frequency of clear days increases from 8 to 11 days per month at Ceduna where the eclipse track first reaches land, to 19 days at the end of the track where the sun sets at mid eclipse. Values for the percent of possible sunshine (Table 20) reach almost 80% at Oodnadatta, and nearly 75% at Woomera statistics 10 to 15 percent better than the best in Africa. While Australia quite clearly has sunnier weather, the low altitude of the sun will require a nearly clear horizon in order for the eclipse to be seen. This is a much more stringent requirement than a low frequency of cloud cover at midday and the choice of Australia versus Africa will be a tough one to make.
The seasonal frequency of completely clear skies at sunset ranges from 15 to 35 percent (Table 20) with the larger amount inland. Scattered cloudiness (up to half of the sky covered) will bring a modest probability that the horizon will be obscured. Some of this cloud will be convective and will dissipate as the Sun approaches the horizon and temperatures fall, but this cannot be counted on if a cold front has made a recent passage through the area. Larger cloud amounts, when more than half the sky is covered, will bring progressively slimmer chances of seeing the eclipse. If we make the reasonable assumption that there is a one-third chance that scattered cloud will block the Sun and a two-thirds chance for broken cloud, then the chances of seeing the eclipse will range from about 55% on the coast to about 65% inland. All in all, the prospects for a successful horizon eclipse in Australia likely exceed the best chances in Africa by about five to eight percent. To achieve this advantage, a site inland from the coast must be selected.
Departing anticyclones bring hot northerly winds and coastal regions can see temperatures nearly as great as the interior. In due course, another high-pressure system will approach from the west and winds will turn to the south. This brings temperate ocean air inland, resulting in an abrupt drop in temperature known as a "cool change." A 10° Celsius change in 30 minutes is typical and 20° Celsius is possible. Though the eclipse occurs while temperatures are falling after the daytime high, equipment set-up will take place at the hottest time of day. Temperatures in the forties occur two or three times on average each month at inland sites. Readings this high require great caution, for heat stroke is a very real possibility. Plenty of water, misters, hats, a slow pace and occasional refuge in shade would be well advised, along with emergency plans if the heat should prove overpowering. Equipment also suffers in such heat and planning should include provision for cameras and telescopes. It is worth noting that official temperature measurements are made in the shade and readings in full sunlight are much higher.
The daily sea breezes have long been recognized as a distinctive feature of the local climatology and are given popular local names such as Eucla Doctor, Albany Doctor or Fremantle Doctor. Sea breezes arise because of the temperature difference between the land and the water; the hot air inland rises, drawing air from the cooler South Australian Bight onshore. Sea breezes typically begin in mid-morning along the coast, but the onset can be delayed or even prevented if opposing winds driven by a larger weather system hold them at bay. Depending on the time of onset and the help or hindrance of prevailing winds, sea breezes may extend as far as two or three hundred kilometers inland where they bring a pleasing drop in temperature and an increase in humidity. The effect of the eclipse on these winds will be an interesting observation, as will their effect on shadow bands.
Dust is an unpleasant component of any strong wind in Australia as the dry interior provides a ready supply of material. The initial gust that signals the arrival of the sea breeze is often very strong and then dies quickly to a steadier and lighter flow. Under such circumstances, it may be sufficient to protect equipment for only a short period, but a lingering dust haze may impede the view to the setting sun. Dust devils, a whirling column of dust, are also common in Australia in the dry season and contribute to the general haziness of the atmosphere. Some can become quite strong and lift a considerable quantity of dirt into the air. Dust storms are also legendary in the country, being driven by particularly strong cold fronts or by the outflow from strong thunderstorms.
Save for the fact that the eclipse occurs at sunset, Australia would be an easy choice over Africa if weather were the only factor to be considered when selecting a viewing site.
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