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Climate of Australia

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Latitude, terrain, and proximity to the sea dictate the climate of Australia. The vast interior of the continent is a desert, while semi-arid grasslands frame its perimeter. To the southeast, the cities of Canberra, Sydney, Melbourne, Hobart, and Adelaide enjoy a temperate climate with plentiful, though variable, rainfall. Perth, at the southwest corner of the continent, experiences a warm sub-tropical climate. But in the north, along the track of the eclipse, tropical and equatorial conditions hold sway, and so the eclipse traveller must cope with high humidity and a greater likelihood of cloud cover than those making a trip “down under” might ordinarily expect.

The northern half of the Northern Territory – the “Top End” – has two very distinct seasons: a wet from November to April and a dry from May to October. These opposing seasons come from the back-and-forth sloshing of tropical maritime air that forms over the waters north of Australia. More extreme humidities arrive with the equatorial airmass, which is able to penetrate only as far south as the northern parts of Queensland, and then only later, in mid-summer months. Because this November eclipse comes at the early part of the rainy monsoon season (Figure 1), cloud cover is not so heavy as in the months following, and there are good prospects for a sunny day on the 13th. Temperatures are warm – mornings range around the upper 20s – and the humidity is high, typically, around 80 percent at eclipse time.

 

Figure 1: Monthly Precipitation for Darwin and Port Douglas.

 

Queensland and the Northern Territory are sandwiched between a semi-permanent band of high-pressure systems in the south and the Intertropical Convergence Zone (ITCZ, but known locally as the monsoon trough) in the north (Figure 2). Between the two systems is a band of easterly and southeasterly trade winds. During the summer months, the monsoon trough sags southward to lie across northern Australia, but in November, the transition is in its early stages (the “buildup”) and the climate is not so humid and wet as later in the season. The ITCZ is a broad low-pressure trough that marks the axis of convergence between humid southeast trade winds and the wetter tropical air from north.  In Figure 2, the average position of the November ITCZ lies well to the north of Australia, but the low-pressure band is broad and transitory. There is also a weak low-pressure centre over the Northern Territory that contributes to the convergence in the region. The consequences of the convergence in the trought and the low-pressure system is the creation of a zone of high cloudiness and precipitation that straddles northern Australia.

In November, the buildup is in its early stages and the influence of the monsoon trough is muted. The wettest weather is around Darwin, with progressively less rainfall and fewer days with rain eastward along the track (Table 1). The cycle of winds and weather are evident in the wind rose diagrams for stations along the shadow path. Over the Northern Territory, winds tend to blow from all points of the compass as trades and monsoon compete for dominance. Over Queensland’s “Far North”, particularly along the coast, the trades are very dominant in November, and the more humid air from the north hardly ever encroaches. Because of this difference, the cloudiness over the Top End is noticeably higher than over Queensland (Figure 3), but neither has a completely sunny disposition.

Figure 2: Average sea-level pressure pattern for November. The position of the ITCZ is shown by the dashed line but the width of the low-pressure trough is an indication of its mobility. The weak low over the Northern Territory is likely promoted by the heating of the land surface.  Data: Hadley Centre.
Figure 3: Number of sunny and cloudy days at selected sites along the eclipse track. Note that Northern Territory sites are generally more cloudy than those over Queensland. Australian sources do not provide a precise definition of “sunny” and “cloudy” but together they typically account for half of the days of November. Presumably the remaining days have mixed cloudiness. Cairns Aero has the greatest number of sunny days, but inland sites such as Palmerville and Mareeba have fewer cloudy days.

 

The easterly trade wind flow over Queensland is complicated by the presence of the Great Dividing Range that lies tight against the coast in the vicinity of Cairns and Port Douglas. The easterly trades push up against the Range, and, being forced to rise, bring a heavier cloudiness and precipitation to the seaward face than to the inland lee side.

Figure 4: Satellite-derived mean cloud cover for November based on observations from 1980-2000. Data: NASA Pathfinder.


Mountains are chancy places to watch an eclipse – especially mountains that are bathed in tropical moisture – and so the high ground between Port Douglas and Maitland Downs, reached along the Peninsula Developmental Road, is probably best avoided. This is unfortunate, as the peaks are host to Daintree National Park, the Earth's oldest tropical rain forest. Port Douglas itself lies close against the highest peaks, but the central line passes south of the city, crossing a more gentle coastal plain and so provides a less cloud-prone location in which the view the eclipse. The cloud and sunshine statistics for Cairns (Table 1) are representative of the region, and offer a promising 66 percent frequency of sunshine in November. Because the sunshine data refer to the entire day, and because the eclipse comes in the sunnier morning hours, the probability of seeing the eclipse is likely greater than the statistics imply. Cairns actually has the highest frequency of “sunny” days in Figure 3.

Once over the Great Dividing Range, the trade wind flow descends onto the outback plains, drying along the way. Palmerville, well inland, has the lowest cloud amount (but not the highest number of sunny days) according to Figure 3 and Table 1. The calculated cloud amount for the station, 35%, is much lower than any other site along the track and somewhat at odds with other nearby stations. Nevertheless, both human observations and satellite images give the nod to this part of Queensland. A more convenient site may be found near Maitland Downs, on the west side of the Dividing Range and also on the central line. The town is perilously close to the highest peaks of the Range, but at least lies on the leeward side where theory predicts an improvement in cloudiness. Most likely the area has cloud cover statistics similar to Cairns and the Port Douglas coast (Figure 4). The road to Maitland Downs lies within the track of the shadow, so that if an expedition is held up by bad weather, the possibility of seeing the eclipse is not lost. The road to Palmerville requires a significant deviation from the shadow path before turning back into it.

Satellite images show that much of the scattered to broken sky cover over both Queensland and the Northern Territory during the daytime frequently consists of small cumulus clouds (Figure 5a, 5b). Because this is an early morning eclipse, the ground likely will not have warmed enough to cause the development of cumuli; even if they have, the onrushing eclipse shadow will drop temperatures back again and dissipate the convection. For the most part, eclipse expeditions will have to assess the effects of the terrain. Where winds blow against a rising topography, cloud is more likely to form. Where it blows downslope, clouds tend to dissipate. If there is cloud or fog in your vicinity which evaporates as the Sun rises, there is a possibility that it will reform during the cooling that comes with the approach of totality. Fog should be taken especially seriously.

Figure 5a: 2006 November 14 0000 UTC
Figure 5b: 2007 November 14 0000 UTC
Figure 5c: 2008 November 14 0000 UTC
Figure 5: Satellite images at 00 UTC (3 1/2 hours after eclipse) showing a selection of cloud patterns along the eclipse track. The tendency toward heavier cloudiness along the coast is evident in all three images, but in 2006 and 2008, the cumulus cloudiness would not have had time to form before the early morning eclipse. In 2007, the cloud band barely respects the Great Dividing Range, spilling over it and into the outback interior. Clouds in the Northern Territory are less organized by the topography.

During the night, cool air from higher slopes on the mountains tends to flow downhill. This descending flow acts against the formation of cloud, giving overnight and morning hours clearer skies than might otherwise be the case. As the Sun rises and the land heats, this downslope flow gradually reverses. Around Cairns and Port Douglas, the trade winds oppose this downhill flow and, being stronger, overcome it entirely as seen in the wind rose for Cairns (Figure 6). On the inland side of the Dividing Range, the trade winds and topographic flows reinforce each other (or at least, don't oppose), giving an extra impetus to the formation of clear skies. All of these effects are relatively minor, and the cloud cover statistics in Table 1, being for the time of the eclipse, take such nuances into account. However, if the trade wind flow is a light one on eclipse morning, sites between Cairns and Port Douglas may benefit from the proximity of the nearby mountains and the local winds.

Figure 6: November wind roses for Gove, Cairns, and Palmerville. The southeast and south winds at Cairns reflects the steady trade winds along the coast. At Palmerville, trade winds continue to dominate, with occasional excursions of northerly airmasses. Gove's wind rose depicts a mixture of easterly trade winds and winds from the equatorial regions to the north.

 

Cyclones

Cyclones (hurricanes in North America) are a normal part of the climatology of the Far North and Top End. Fortunately, they are rare in November and extremely unlikely to play a role on eclipse day. On average, 4.7 tropical cyclones a year affect the Queensland Warning Area during the summer months (November to April). In the 20 years from 1986 to 2005, only six cyclones passed within 100 km of Cairns and none were in November. The number is even smaller for the Northern Territory – three cyclones in total and none during the time of the eclipse.

Heavy rain from other causes is more likely, even at the start of the monsoon season, and significant amounts will bring a halt to travel inland. The average monthly rainfall in Palmerville is only 62 mm for November, but the month has recorded as much as 317 mm of rain (and as little as 1.3). An early start to the wet season or a heavy beginning could make expeditions into the interior very difficult or impossible.

Table 1: Climate statistics from surface-based observations for sites along the eclipse track. POPS = percent of possible sunshine. Clear refers to no cloud at all; "few" indicates 1-2 oktas (8ths) of cloud; "scattered" to 3-4 oktas; "broken" to 5-7 oktas. "Obscured" refers to ground-based layers such as fog that hide the sky. Stations with an asterisk lie within the total eclipse zone.

 

Top End and Far North Travel

After the weather, one problem remains – inland areas are sparsely populated, poorly supplied by roads, subject to flooding and closure, and not capable of handling large groups of tourists. Travel near Cairns, out onto the Atherton Tablelands, is comfortable, and the central line can be approached along the Peninusula Developmental Road, past Mareeba and Mount Molloy. Doing so will place you behind the higher parts of the Great Dividing Range, on the west side of highly recommended Daintree National Park.

For the more adventuresome, who wish to go well inland, the route from Cairns toward Kowanyama is by way of the Burke Developmental Road (BDR). The road is paved almost to Chillagoe and is comfortably passable for buses and 2WD vehicles to that point. Beyond, the route roughens, and 4WD vehicles are recommended at all times, especially as November heralds the start of the wet season. The road is sprinkled with minesites, old and new, and described as “good” in the dry. From Chillagoe, the BDR heads northwestward, gradually approaching the south limit of the shadow zone. About 100 km along and 35 km before Gamboola, the road reaches the Mount Mulgrave/Palmerville Road, which turns northward toward the shadow path, and eventually, at Palmerville, reaches the centre line. This is not a route for the uninitiated. Roads can be closed at any time due to flooding and service stations are non-existant. It would be sensible to go with local guides. All-in-all, a grand adventure, but not one to be taken by bus-loads of casual tourists.

In the Northern Territory, nearly all of the eclipse track is confined to Arnhem Land Aboriginal Land.  in which. A small number of tour outfits operate in the area, but individual tourists are not permitted. Advance planning will be necessary, but it is difficult to believe that there will be no expeditions into the area.

In Short

For most of us, this eclipse belongs to Queensland’s Far North. Cloud prospects are good along the coast, especially in the region between Cairns and Port Douglas, and even better in the outback, on the back side of the Great Dividing Range. Arnhem Land is more remote and more cloudy. Vessels offshore would seem to have about the same prospects as on the beaches, though their mobility will bring considerable advantage if they have time and room to move.

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