| | Figure 1: 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. |
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 1). The ITCZ is a broad low-pressure trough that marks the axis of convergence between humid southeast trade winds and wetter equatorial air from north; it is a region of heavy cloudiness and frequent thunderstorms. Between the ITCZ and the highs lies the 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 (Figure 2). | Figure 2: Monthly Precipitation for Darwin and Port Douglas. |
If the approach of the ITCZ were not enough, the summer warming of the Australian continent also creates a weak low-pressure centre over the Northern Territory ("Top End") that contributes to the convergence in the region. The consequence of these two mechanisms - the low and the ITCZ - is the creation of a zone of high cloudiness and precipitation that straddles northern Australia. Add to these a moist onshore flow along the east coast of Queensland that impacts a steeply rising coastal mountain range and we have the makings of a challenging eclipse-viewing environment. | | Figure 3: 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. |
Fortunately, the buildup is in its early stages in November, and the onset of cloudy weather is just beginning. The wettest weather is around Darwin, with progressively less rainfall and fewer days with rain eastward along the track (Table 1). The correlation of winds and weather is evident in the wind-rose diagrams for stations along the shadow path (Figure 3). Over the Northern Territory, winds tend to blow from the north and east as trades and monsoon compete for dominance. Over Queensland's "Far North", particularly along the coast near Cairns, the southeast 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 (Figures 4, 5), but neither has a completely sunny disposition. | | Figure 4: 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. The mountain barrier is not continuous (Map 2), but instead is interrupted by lower sections that do not provide much of a barrier to the ocean moisture. The coastal cloudiness is evident in the three satellite images in Figure 6, but in only one case (2007) was it heavy enough to persist through the eclipse cooling. | | Figure 5: Satellite-derived mean cloud cover for November based on observations from 1982 - 2007. Data: CIMSS/SSEC/UW-Madison |
The interaction between terrain and trade winds brings a complex cloudiness to the Cairns region that will challenge eclipse seekers. Eclipse day may see deep heavy cloudiness from complex low-pressure troughs that cross the region, much like the frontal cloudiness in more temperate climates. Or, the critical day may see the arrival of a disturbance in the trade winds (so-called easterly waves) that cloak the ocean-facing mountain slopes with overcast skies and drenching showers; the same mountains will break the disturbance into smaller bits so that the inland lee side experiences a broken cloudiness that might permit a view of the eclipse to lucky travellers who happen to be in just the right place. Figure 5, above, shows the average effect of all of the various forces that create clouds along the Australian part of the eclipse track. Offshore waters and the immediate beach front have a lower average cloudiness than any of the inland areas over Queensland. Cloud is heaviest in a band that parallels the coastline and lies just inland, reflecting the tendency for the rising terrain of the Great Dividing Range to form clouds in the prevailing southeast winds. In the morning hours, this terrain-induced cloudiness is reinforced by the heating of the ground by the rising sun, building convective clouds very early - surprisingly early - in the day. In most cases, this inland cloud is convective in personality, and brings frequent rains when upper-level disturbances give them a little extra energy. The blocking effect of the mountains actually extends a little offshore, because the air "piles up" along the coast extending the influence of the mountains a few kilometres out into the Coral Sea. The most fortunate aspect of the Queensland coastal weather pattern is that the cooling associated with the eclipse will probably delay the start of strong convection for an hour or two in the morning. Once the Dividing Range has been crossed, cloud cover decreases once again, though not to the levels along the shoreline. Air that has crossed the mountains descends to lower terrain, warming and drying in the process. However, the terrain on the west side of the Dividing Range takes some distance to smooth out – almost to Palmerville (Figure 8 below) – and so the cloud-making effects of the topography continue for 100 or 150 km beyond the highest coastal peaks, though not so efficiently as on the seaward-facing slopes of the Dividing Range. Depending on the weather of the day, the Great Dividing Range may prove to be a benefit or a liability. | Figure 6a: 2006 November 14 0000 UTC | | Figure 6b: 2007 November 14 0000 UTC | | Figure 6c: 2008 November 14 0000 UTC | | Figure 6: 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. |
If Urania (the muse of astronomy and meteorology) smiles upon the entire coast, it may dawn completely clear, though Table 1 below suggests that a morning without any cloud at all is a very unlikely happenstance. Even on a very sunny morning, there are always a few patches of cloud on the slopes or offshore, and so clear skies are rare (a typical condition for tropical environments). Under "average" conditions – one without an incoming disturbance of some sort – the best weather is likely to be on the coast and a kilometre or two inland, though it will also be pretty decent in the outback west of the coastal mountains. If the weather is heavy however, and cloud is piling up on the coast, a better location will probably be found on the sheltered side of the Dividing Range. If the disturbance is a big one, with lots of cloud at all levels, then everyone will be cloudy, though complete overcast is rare. The Queensland coast could be a frustrating place to find a view of the Sun, as heavy weather days tend to have numerous holes in the cloud, but they are ephemeral openings that come and go at quarter-hour intervals. Chasing such a hole may be completely impossible, at least at Port Douglas, given the crowds expected for the Solar Eclipse Marathon, due to start at third contact. Theoretically, November is cyclone season, but it would be history making if one were to occur along the eclipse coast. Most likely is that the day will dawn partly sunny with patches of low-level cloud here-and-there over the water and along the shoreline – a typical Cairns morning in November, and one guaranteed to stress to eclipse watchers as they attempt to determine the motion of clouds and their chances at a different spot. | | 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. |
Beach-front sites have the advantage of a clear view across the water without concerns for telephone wires or hills that might block the view of the rising Sun and early views of this low-altitude eclipse. From central Cairns, a line of hills toward the southeast (Cape Grafton) won't block the view toward the rising Sun, but the hills are prone to hosting clouds that might prove to be a problem at the critical moment. Instead, try the northern suburbs of the city and the plains to the north, as far as Clifton Beach, which offer a gently sloping unobstructed view to the eclipse with the added advantage of a longer duration of totality. The beaches north of Cairns (Figure 7) will likely be crowded with eclipse watchers on November 13. Beyond Clifton Beach, the coast rises sharply from the water's edge, opening up again as the highway reaches Port Douglas. There are limited opportunities to stop and set up in the area. | | | Figure 7: Left: early morning beach scene near Port Douglas. Right: the junction near Mount Molloy. The terrain and vegetation are typical of the area. |
Port Douglas lies near the southern edge of a flat coastal plain that extends toward Daintree, but is bounded to the west by the highest part of the Great Dividing Range. The slopes of the mountains are quick to form cloud, but the plain resists that tendency until slightly later in the day. Hotels along the coast may prove superb sites to see the eclipse, but not all of them are well exposed to the ocean and may require a trip down to the beach to realize the best views. Nevertheless, there are plenty of places to stop and watch, though you are likely to be joined by thousands of others. A huge concern for movement in and out of Port Douglas is the Solar Eclipse Marathon that is scheduled to begin at third contact. Simply put, last-minute manoeuvring is probably impossible, but an early start, perhaps to go inland to the outback or north toward Daintree, should find few impediments. More details about the marathon can be found online. If the weather is favourable, an oceanside view from a convenient hotel is probably the most comfortable situation that this eclipse can offer.
The cloud and sunshine statistics for Cairns (Table 1) are representative of the region, including Port Douglas, 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 a little greater than the statistics imply. Cloud-cover statistics in Table 1 are those for the time of eclipse, or within a few hours of it, but mirror the information contained in the sunshine data. In Figure 4, Cairns has the highest frequency of "sunny" days in the region. On "typical" quiet-weather days, low-level cloud can usually be seen lying over the nearby ocean, drifting onshore in patches, with many open areas in the scattered-to-broken sky. These offer tantalizing prospects for a chase, especially if they are only a few kilometers distant, but the motion of the patches is quick enough that catching them in time for an eclipse might be difficult, or certainly challenging. Scattered patches of low cloud can also be found plastered against the steep slopes of the mountains a few kilometres inland; they will not impede a view out toward the sea for a coastal viewing site. Perhaps the best chance of success along the coast is to "hunt" from a vehicle moving on the highway north of Port Douglas, where fields are open and places to stop are readily available, provided, of course, that traffic is flowing and there are not hundreds of others with the same idea. This is a viable option only with an early start in the dark before you can actually see what the cloud cover looks like - you are gambling on the advantages of a little bit of mobility versus playing the probabilities on eclipse day by remaining fixed. If, on the sunrise, it turns out that the other side of Port Douglas looks better, you may encounter some difficulties in crossing through the town in the marathon crowds. All-in-all, coast observers might be better advised to sit and take whatever nature offers, but it will be a heartbreaker if the eclipse is visible a few hundred meters away while being obscured at your chosen location. From Port Douglas to Mount Molloy, the highway is a twisting, climbing road surrounded by dense vegetation with few outlooks toward the sea. There are almost no opportunities to pull over and watch the sky, and since the terrain faces into the trade winds, there is a high probability of cloudiness as the air flows through the gap in the mountains. Once at the Mount Molloy crossroad (Figure 7), the ground smoothes out a bit, but trees line the highway and it is difficult to find a suitable view. The surrounding terrain is still very rough, but even on cloudy days it is surprising how even small hollows in the topography can chew holes in the cloud cover, as long as it is not too thick. Viewing sites at all inland locations will have to give up a view of first contact and there is a danger that the eclipse itself could be hidden behind a hill if the site is not chosen carefully. During a scouting trip with Aram Kaprielian of TravelQuest in November 2010, the weather over Queensland was very active, with a small easterly wave approaching the coast. Dawn came with about 50 percent cloud cover, but it was a turbulent morning, with blue-sky openings moving rapidly across the sky. Ragged cumuliform clouds (overlain by thin cirrus and mid cloud) tended to form on the higher ridge lines to the east, and move off in the flow, leaving a relatively open cloud cover overlying the low spots on the terrain - which, fortunately, is where the Sun was located from our observing site at Wetherby Station. Eclipse time came with a visible though partly obscured Sun, low enough that power lines in the distance could have been a problem, though a move of a hundred meters fixed that problem. Clouds move very rapidly, so that chasing an opening would be all but impossible in active-weather circumstances. | | Figure 8: High-resolution topographic map of the Cairns-Port Douglas area. Careful attention to the fine details of the terran may provide clues to find more favourable eclipse sites. |
Perhaps the most promising option is to move well behind the high peaks of the Great Dividing Range, to a location near Mount Carbine or some other place where the local terrain provides some shelter from the clouds (Figure 8), though there are no sites along this part of the eclipse track that are completely free of mountain influences. There is a danger here that the peaks, which include the highest along the chain, will form cloud on their summits, but one of the peculiarities of the trade-wind zone of the Earth is that high-level winds blow in the opposite direction to those at low level. Cirrus and higher mid-level cloud will move from west to east, away from the observer, while low-level clouds will do just the opposite. In other words, keep an eye on the sky in both directions! The highway from Mount Molloy to Mount Carbine (the Peninsula Development Road) lies in a shallow valley, with highest peaks to the west and lower ones to the east. At Mount Carbine, Highway 81 turns westward, through a gap in the hills. From a strictly topographic point of view, this might be one of the best conveniently accessible spots along the track. There are stories that some landowners in the area are setting up eclipse-viewing sites for travellers. After Mount Carbine, the Mulligan Highway (Peninsula Development Road in Google maps) turns west and runs parallel to the centre line. After a time, it turns northward to cross the line a little south of Maitland Downs. The town is perilously close to the highest peaks of the Range, but of all of the convenient Queensland choices, is the one most protected from the marine winds and moisture. 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.
Students from the Atherton State High School scouted the area in 2010 and their report has important cautions for eclipse seekers who plan on siting along this highway. Their conclusions can be summarized in the following points:
The terrain to the east of the highway frequently blocks the view toward the eclipsed Sun Tall trees also line the highway, though open spots can be found Power lines along the east side of the highway may lie across the Sun at the critical moments Regulations from the Department of Main Roads require that persons within 10m of the highway wear suitable safety gear. Parking is prohibited in the zone, which imposes a requirement to ask landowners along the route for permission to set up.
The Atherton students' highest recommend locations were 6 and 9 km north of the centre line where open fields are present and one location 6 km south of the centre line, at Whumbal Creek. These locations are easily found on Google Earth, though the view there does not suggest anything unusual about the sites.
For the adventuresome (in small groups), Palmerville, well into the outback, has the lowest cloud amount (but not the highest number of clear days) according to Figure 4 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. The road to Palmerville requires a significant deviation from the shadow path before turning back into it. Travel in the Queensland outback is not for the novice.
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 6a, 6b). Because this is an early morning eclipse, the ground likely will not have warmed enough to cause the development of cumuli; if it has, the onrushing eclipse shadow will drop temperatures back again and dissipate the convection.
| | Figure 9a: Overnight infrared satellite image of the Queensland coast acquired 13 November 2009 1800 UTC | Figure 9b: Overnight infrared satellite image of the Queensland coast acquired 13 November 2010 1800 UTC |
Both infrared and visible-light satellite images are available over Australia, but this eclipse poses a bit of a dilemma for the mobile watcher. Because of the early morning passage of the shadow, visible images are not available and decisions will have to be made on the basis of low-resolution infrared pictures (Figure 9). Since the infrared images are constructed from thermal radiation, low cloud will have poor definition against the surface, appearing as a slightly lighter shade of grey against a darker background. Animation of the images will be helpful in identifying cloud from ground. Even with high-resolution IR images, details will not be as sharp as with visible images (which have four times the resolution) and the fine detail of the cloud structure and pattern will only be hinted. Fog patches on the ground may not be distinguishable at all.
| | Figure 10: the influence of el Niño conditions on cloud cover along the eclipse track. Data: NASA Pathfinder. |
The Impact of El Niño
It would be exceptionally unusual – probably unprecedented – if the strong La Niña conditions that plagued Australia during 2010 and 2011 lasted until the date of the eclipse. It is much more likely, though not probable, that the ENSO (El Niño Southern Oscillation) cycle will turn into neutral conditions or an El Niño. The question becomes then, will an El Niño have an impact on cloud cover (since neutral conditions are accounted in the average statistics quoted above)? Figure 10 above attempts to answer that question, and it seems that the El Niño side of the cycle will bring in slightly more favourable cloud conditions. This figure shows the change in cloud cover during El Niño years over the Australian part of the eclipse track. It was constructed by subtracting the average November cloudiness at the start of El Niño years from the average of all years.
Over the Australian continent along the eclipse track, average cloudiness is typically reduced between 5 and 10 percent, with the greatest change over the Gulf of Carpentaria. In the Cairns-Port Douglas area, the reduction amounts to 10-15 percent. By the date of the eclipse, the onset of El Niño conditions should be apparent if they are going to happen. El Niño events are associated with drier than normal conditions over northern Australia, and so there is no surprise that cloud cover is reduced should one begin in late 2012. If El Niño begins in late 2011, chances are that it will have run its course by eclipse time, though a few of the episodes have been known to extend over two years. 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.
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 sllightly better prospects than sites on the beaches, especially as their mobility will bring considerable advantage if they have time and room to move.
2011 Update
Below are satellite images from November 2011 showing the pattern of daily cloud cover around Cairns and Port Douglas. The visible light images from 2033 UTC show the cloud cover at the time of the eclipse, but these will not be available to the watcher before the event is over, and planning will instead have to rely on low-resolution 1833 UTC or earlier infrared images.
The infrared satellite images above are those available from the Brisbane Storm Chasers web site (below) and are enhanced to show the dark grey low clouds more clearly. The difficulty in identifying small patches of low-level cloudiness is demonstrated by comparing the 18:33 UTC infrared images to the early morning high-resolution visible-light images directly above each of them. Higher resolution infrared images are available, but these will not be as revealing as the visible images.
Links
Satellite images
Real-time satellite images and animations of Australia from WeatherZone: www.weatherzone.com.au/satellite.jsp
High-resolution satellite images from the Brisbane Storm Chasers web site: realtime.bsch.au.com/currentwx.html
Archive satellite images from Dundee University (registration required): www.sat.dundee.ac.uk/
Forecasts
Queensland forecasts from the Bureau of Meteorology in Australia: www.bom.gov.au/weather/qld/
Northern Territory forecasts from the Bureau of Meteorology: www.bom.gov.au/weather/nt/
Numerical Model Output
Cloud forecasts from the U.S. GFS model from SkippySky Astro-Weather Forecast: http://www.skippysky.com.au/
NCEP Central Operations: http://mag.ncep.noaa.gov/NCOMAGWEB/appcontroller
Eclipse tracks courtesy of Fred Espenak
Updated: November 2011
|
,当Kp指数达到6或以上时，我国境内有可能观察到极光；当前极光活动水平
，极光为9时漠河可能见极光，
级，当地磁暴级别达到G2或以上时，我国境内有可能见极光
置顶卡
变色卡
千斤顶
显身卡