Introduction
On Sunday, 2010 July 11, a total eclipse of the Sun is visible from within a narrow corridor that traverses Earth’s Southern Hemisphere (Espenak and Anderson 2006). The path of the Moon’s umbral shadow crosses the South Pacific Ocean where it makes no landfall except for Mangaia (Cook Islands) and Easter Island (Isla de Pascua or Rapa Nui). The path of totality ends just after reaching southern Chile and Argentina. The Moon’s penumbral shadow produces a partial eclipse that is visible from a much larger region covering the South Pacific and southern South America.
Umbral Path and Visibility
At 18:15 UT, the Moon’s umbral shadow first makes contact with Earth as the path of totality begins in the South Pacific. The eclipse track is 179 km wide and the duration of totality on the central line is 2 min 42 s. Regrettably, the nearest land is either 700 km to the northwest (Tonga) or 1800 km to the southwest (New Zealand). It is most unfortunate that the dearth of solid land with the track is one of the most noteworthy characteristics of this eclipse.
As the shadow travels northeast, the path grows wider and the duration increases. At 18:19 UT, the track just misses Rarotonga—the largest and most populous of the Cook Islands—by just 25 km. Rarotonga’s 14,000 inhabitants witness a very deep 0.993 magnitude partial eclipse. Two minutes later (18:21 UT), the umbra makes the first of its very few landfalls after covering 1450 km of open ocean. Mangaia (Auau Enua)—the second largest and most southerly of the Cook Islands—is only 15 km south of the central line. Its normal population of 1900 will undoubtedly grow with many new visitors there to enjoy 3 min 18 s of total eclipse. The Sun’s altitude is 14°, the umbra’s velocity is 2.7 km/s, and the path width is 200 km.
Leaving the Cook Islands behind, the umbra passes tantalizingly close to the Society Islands of French Polynesia. Alas, such exotic destinations as Moorea, Bora Bora, and Tahiti all lie outside the eclipse track. The southeastern coastline of Tahiti lies just 20 km beyond the northern limit and gets a 0.996 magnitude partial eclipse at 18:28 UT. In comparison, Papeete—Tahiti's capital (pop. 131,000) on the northwest coast—experiences a 0.984 magnitude partial. Because of its close proximity to the path, one or more eclipse cruises will be based out of Tahiti. While they cannot offer the rigid stability of a land-based observing site, the mobility of cruise ships can increase the possibility of avoiding clouds.
After the Society Islands, the Moon’s shadow passes over a number of small atolls of the Tuamotu Archipelago, only a few of which are inhabited. Hikueru—a 10 × 15 km oval-shaped atoll—has a population of ~125 and a small territorial airport. It may be suitable for small expeditions who can expect the total phase to last 4 min 20 s. One of the more isolated atolls of the Tuamotus is Tatakoto. The 4 × 14 km wide islet has a population of ~250 and is within 20 km of the central line. The duration of totality is 4 min 35 s with the Sun 36° above the horizon (18:48 UT).
The umbra now embarks on a lonely trek with no landfall for the next 1.4 h as it races 3300 km across the South Pacific. During this period, the axis of the Moon’s shadow passes closest to the center of Earth (gamma = –0.6788) as the instant of greatest eclipse is reached at 19:33:31 UT (latitude 09° 45′S, longitude 121° 53′W). The maximum duration of totality is 5 min 20 s, the Sun’s altitude is 47°, the path width is 258 km, and the umbra’s velocity is 0.60 km/s.
The seclusion of the lunar shadow’s solitary journey is finally interrupted at 20:11 UT when it encounters Easter Island, a.k.a. Isla de Pascua or Rapa Nui, as the Polynesians call it. Renowned as one of the world’s most isolated inhabited islands, Easter Island is 3,600 km west of Chile. It possesses 887 enormous monolithic statues created by the native Rapanui people hundreds of years ago. Dominated by three extinct volcanoes, the triangular-shaped island measures approximately 11 × 23 km. From the capital, Hanga Roa, totality lasts 4 min 41 s, with the Sun at an altitude of 40°. The 3,800 inhabitants of the isle are accustomed to tourism, but the eclipse is expected to bring record numbers to this remote destination.
After Easter Island, the Moon’s umbra covers another 3700 km of ocean during 38 min before beginning its final landfall along the rugged shores of southern Chile at 20:49 UT. The shadow is now an extremely elongated ellipse traveling at a velocity of 8.7 km/s and accelerating. The central line duration is 02 min 57 s with the Sun 5° above the horizon. However, the desolate, fjord-lined coast of the Chilean Archipelago affords no suitable locations for eclipse observing. Quickly crossing the Andes, the shadow enters Argentina where it encounters El Calafate, a tourist village of 8000 located on the southern shore of Lake Argentino. The Sun’s altitude is only 1° during the 2 min 47 s total phase, but the lake may offer an adequate line-of-sight to the eclipse hanging just above the Andes’s silhouetted skyline.The path ends 130 km southeast of El Calafate as the umbra slips off Earth’s surface and returns to space at 20:52 UT. Over the course of 2 h 39 min, the umbra travels along a track approximately 11,100 km long that covers 0.48% of Earth’s surface area. It will be 29 months before the next total solar eclipse occurs on 2012 Nov 13.
Weather Prospects for the Total Eclipse
Weather Overview
The 2010 total eclipse comes at the depths of the Southern Hemisphere winter, ordinarily a time of frequent storms and alternating high- and low-pressure systems that bring a lot of changeable weather, winds, and cloudiness. Fortunately, latitude comes to the rescue for the first half of the shadow track, as it travels in and north of the belt of high-pressure anticyclones that girdle Earth at about 30°S. This high-pressure belt is a region where the air descends from higher levels in the atmosphere, warming and drying by adiabatic compression. It is a zone of mostly sunny skies and pleasant temperatures, akin to the Caribbean in the Northern Hemisphere winter, but it is not without its temperamental weather. Cold fronts from storms in the “Roaring Forties”—latitudes between 40°S and 60°S—are able to move into anticyclonic barrier, bringing showery weather and cloudy skies to the eclipse path when they do. Beyond Easter Island, the eclipse track dips into the Roaring Forties, and cloudiness—at least over the eastern Pacific and the coast of Chile— becomes much heavier.
In addition to passing cold fronts and the impact of the Roaring Forties, there is a semi-permanent feature of southern meteorology known as the South Pacific Convergence Zone (SPCZ). The SPCZ is a band of low-level wind convergence lying over the warmest waters of the southwest Pacific and so, like the ITCZ along the equator, is a region of frequent showers and thundershowers along with the associated cloudiness. In July, alas, the SPCZ lies at the northern limit of its annual range, stretching from the Solomon Islands near New Guinea, across Samoa and the Cook Islands. In recent years, perhaps in response to global climate changes, the SPCZ has tended to move north and east, to a position that more directly affects the eclipse track. To avoid the influences of the low-latitude storms and the thunderstorms of the SPCZ, eclipse watchers must head to the north and east—to the extremities of French Polynesia or beyond.
In spite of the weather factors that promote cloudiness along the eastern and western extremities of the eclipse path, the western Pacific is actually in the midst of its seasonal dry spell during July. Closer to the South American coast, just the opposite is true, though at Easter Island, the difference in precipitation between the wet and dry seasons is less pronounced than at Tahiti and the Cook Islands. At the end of the path in Argentina, where winter influences would be expected to bring the most difficult conditions, the Andes Mountains act as a very effective barrier to the Pacific storms and the weather is quite promising instead.
Islands in the Cook Islands and French Polynesia are either mountainous volcanic peaks (Tahiti, Mangaia, and also Easter Island) or low, flat atolls (Tuamotus). The latter are too small and low to affect the flow from the sea, so the weather observations from those sites reflect the conditions on the water. On the other hand, the mountainous islands impose a considerable orographic modification on wind, cloud, and precipitation—generally to increase cloud and rainfall and divert the winds. The humid tropical air is always ready to form clouds if lifted by any of several processes. Large islands are darker than the sea, and warm more readily in sunlight. Warm air, being buoyant, rises upward, forming clouds at some small distance above the surface. Winds blowing onto the land are compelled to rise as they encounter the mountainous topography, adding to the impact of the solar heating, and usually cloaking the mountain ridges and peaks with a cap of cloud, especially in the afternoon hours.
The reverse process occurs at night in the case of solar heating, and on the lee side of the terrain in the case of orographic lifting. Winds blow downslope on crossing the highest point of the terrain, so clouds dissipate and rain ends. The whole process is complicated by the complexities of the topography, but in general, the lee side of the mountains on Mangaia and Easter Island will have a slightly greater tendency to sunny weather. The degree of impact will depend on the height and lie of the terrain, and on both islands the cloud-producing processes will dominate those that dry out the air.
Cook Islands
Mangaia, the only Cook Island within the eclipse path, has a latitude that puts it securely within the influence of the SPCZ though the Convergence Zone has a mixed personality, sometimes quiet and barely evident, other times especially active and full of convection and rain. Cold fronts, migrating northward from lower latitudes, reinforce the SPCZ or attend the islands with their own independent weather. It is easy to be pessimistic about the weather prospects, but the climate statistics for Mangaia give reason for some optimism: July is the driest month at nearby Rarotonga, with an average of about 100 mm of rainfall and Mangaia follows the same pattern. Mangaia’s cloudiness is similar to that of Rarotonga with an average cloud cover of 64% calculated from the observed frequency of the various cloud categories. Rarotonga reports an average sunshine amount of 52% and it is probably only slightly less than this at Mangaia.
Mangaia Island is 9 km in diameter, rugged, with a modest 170 m peak in its interior. It is mostly tree-covered in its interior and so combines the cloud-producing features of low albedo and a rising topography, although this forcing is modest compared to Tahiti. On most days when the SPCZ is weak or distant and the skies are sunny, the afternoon convective clouds are small and confined to the interior. Clouds such as these will dissipate quickly in the cooling that accompanies an eclipse. When more organized weather visits, the small dimensions of the island and limited terrain are unlikely to have much influence, either to reinforce the rainfall on the windward side or to dissipate the clouds to leeward. Whatever influence the island can muster will be confined to the lowest cloud levels. There is no prevailing wind at Mangaia, but the stronger weather systems tend to come with easterlies and southeasterlies.
On quiet days, rains on Mangaia tend to come in the afternoon after a sunny morning, and may be quite heavy for a brief time. Daylong rains are more unusual, but do occur from time to time. This diurnal pattern favors the eclipse, which occurs in the morning hours, before maximum heating and maximum cloudiness. Winds blowing against Mangaia (on the windy days—one-third of the wind observations are calm) may cause the formation of an arc of cloudiness offshore where the winds converge and are diverted to flow around the island. These arc clouds (much like the bow wave of a boat) will likely remain offshore during an eclipse.
Tahiti and French Polynesia
At Tahiti, July is the second-driest month and at Hereheretue, in the Tuamotu Islands, July is the driest. While this pattern is similar to that at Rarotonga, the amount of rain in July is about half that in the more southeasterly Cook Islands. The drier weather is reflected in the cloud cover statistics, with average cloudiness dropping to between 44% and 53% across much of Polynesia, a figure 10% to 20% less than in the Cook Islands, in large part due to the reduced influence of the SPCZ. Sunshine statistics are also generous, though somewhat erratic, with very encouraging measurements of 65% to 70% of the maximum possible in most of the islands.
Periods of bad weather are often associated with the passage of cold fronts that arrive from the southwest, sometimes lingering for several days. The stronger fronts have a tendency to stall near the islands and the eclipse track. Even though Tahiti is in the midst of its dry season, a persistent frontal band can drop large amounts of rain for several days in a row. Should a cold front occur on eclipse day, the only escape would be to sail out from under it, most likely by heading eastward down the path.
With only a limited number of places to stay on Mangaia and the atolls of French Polynesia, most southwest Pacific observers will choose to watch this eclipse from shipboard. From a climatological perspective, ships should place themselves as far eastward along the track as schedule permits. This puts the SPCZ well behind and increases the probability that temperate-zone cold fronts will be left behind. Cloud systems tend to become smaller and more disorganized in the more northerly latitudes, and thus easier to avoid when eclipse day arrives. Closer to Tahiti, the SPCZ cloudiness will have to be watched cautiously, though the island typically marks the easternmost extent of its influence.
In satellite imagery, high- and mid-level clouds associated with the SPCZ tend to move from west to east in the upper level flow. Low-level clouds usually move in the opposite direction, but are much more variable and cannot be counted on from one day to the next; frequently they just hang around without seeming to go anywhere. When strong highs pass to the south of the island, the pressure gradient is compressed and stronger than normal easterly trade winds—known as a Mara’umu—can bring winds of 50 km/h and 3 m wave heights in the seas, sometimes lingering for days or even over several weeks. These enhanced trade winds bring heavy rains to the windward side of Tahiti, but such orographic effects will not be a factor along the eclipse track as there are no islands with a significant topography under the lunar shadow’s path in French Polynesia.
Frontal clouds will come up from the south or southwest, usually in the form of bands that are 100 or 200 km wide. On top of this complicated pattern is the tendency for clouds—especially low-level clouds—to form and dissipate over one- to three-hour periods, making prediction from satellite imagery very difficult. “Chasing” an opening in the clouds may be a frustrating experience so positioning the ship where climatology is most favorable at the start will make eclipse-day planning a less hectic event. Positions to the northeast along the path will also increase the eclipse duration. Cyclones should not be a problem, as the hurricane season runs from November to March.
When the SPCZ keeps to the south (its more usual position) and cold fronts are not in the area, westward-moving cloud clusters known as “easterly waves” may be the only weather feature to watch for. Easterly waves are more-or-less circular areas of convection with varying dimensions, up to 200 km or thereabouts. They may bring heavy overcast or scattered thundershowers, but are readily seen and predicted in satellite images.
A careful watch on the satellite images will show “zones” of descending air where both high and low-level clouds tend to disappear. These zones will not be very distinct, and they will not be completely free of cloud, but once identified, can be counted on for favorable circumstances for a half-day or longer. From the ship’s deck, such areas will have smaller convective clouds (primarily shallow cumulus), and thinner high-level cirrus.
A ship’s mobility will increase the chances of seeing the eclipse by an estimated 5%, limited, in large part, because cloud patterns are not easily predicted. The biggest advantage to be given to a shipboard site is the ability to move east of the Tuamotu Islands to tap the best climatology along the track. For those eclipse watchers who are determined to settle on land, the Tuamotus offer a few places with airports, including Hao, Hikueru, Tatakoto, and Anaa. Other islands will have to be reached by boat, a prospect that greatly extends the travel time.
Tropical Cyclones
In the southern Pacific, the tropical cyclone season runs from November to April. For regions along the eclipse path, the frequency is relatively low, with about nine storms per year on average across the whole basin east of Australia. While statistics are somewhat poor, the Cook Islands near Rarotonga (including Mangaia) experience one tropical storm every seven years, while in Tahiti they are about half that rate. In El Niño years, cyclones tend to be widespread between 10° and 30° S latitude, from Australia to 130°W, which pretty much covers the whole track through the Cook Islands and Polynesia. In La Niña years, cyclones tend to be fewer in number, forming and traveling much closer to the Australian coast. In any event, the possibility of a tropical cyclone is virtually nil during July.
Easter Island
Easter Island lies on the south side of the anticyclonic belt that circles Earth at 30° S latitude, and as a consequence, is much more exposed to the influence of the westerlies and storms in the Roaring Forties. In July, Easter Island is in its wet winter season and sunshine is at a premium. Still, it is an exotic destination, and the sunshine statistics show a percentage of the maximum possible (50%) that is similar to that at Rarotonga (and Mangaia) in the Cook Islands. The prospect of stunning photographs of the eclipse over the Moai has tremendous appeal.
Easter Island has three large volcanoes and a number of smaller ones, and the cloud on the mountaintops is a persistent feature of the winter weather. The weather is extremely changeable when it is inclined to be cloudy, and there is no advantage in chasing from one site to the other at the last minute to find a sunny haven. There is a strong convective element to the cloud types, even when large weather systems reach the island, and because of this, clouds can form and dissipate within minutes. On sunny days, clouds will tend to form in the afternoon, but will dissipate as the eclipse approaches.
Given the nature of the cloudiness described above, there are still a few tricks to help pick a successful eclipse site. Do not go uphill unless the day is spectacularly sunny. Especially, do not locate on the upwind side of a volcanic hill. Coastal sites exposed to the wind may have a little less cloudiness if the wind is not too strong, as the cooler air from the sea will suppress the immediate formation of cloud as it reaches land. Sites in the lee of the larger volcanoes may be a little sunnier if the weather is not too thick, but usually, the clouds will form on the slopes and blow downwind; the flanks of the larger peaks may offer safer sites. There is no strong prevailing wind—they can come from any quarter according to the weather of the day.
Given all of the complexities of the wind and weather, the south coast seems like the safest bet, perhaps at Tongariki where the Moai offer great visual appeal. With easterly or westerly winds, the village at Hanga Roa is promising, but southerlies or northerlies will carry clouds from the peaks of Terrevaka or Rana Kao onto the town. Northerlies at Tongariki will have a slight downslope flow, which tends to dry the airout a bit, but the volcano Pakaiki lies just to the east and flow from its peak will have to be watched carefully. The beach at Anakena is promising under a northerly onshore flow, and may be one of the best sites for a large group because of the facilities available there.
Unless there is a large and active weather system over the island on eclipse day, there will certainly be mixtures of sun and cloud that will make site selection difficult. If mobility is an option, eclipse watchers should wait until the last possible moment to assess the character of the cloud and wind before picking a viewing site.
South America
The Chilean Archipelago, while imbibed with towering forested slopes that fall into dark mysterious water, is also exposed to the full force of the westerlies and nearly devoid of community, thus making for a poor or impossible eclipse site. Once across the Andes however, and into Argentina, the weather improves significantly and the eclipse comes to its sunset ending near the resort town of El Calafate. The Andes block the flow of the westerlies, stripping them of their moisture and clouds, and leaving a drier and sunnier airflow to descend onto the plains of southern Argentina. No sunshine data are available for Argentina, but cloud-cover statistics show an encouraging average cloudiness for July at El Calafate of 55%. While this is about 10% higher than Tahiti, the data are similar to the values in the Cook Islands and parts of Polynesia.
The winter season brings cool temperatures, although nothing like the winters in the Northern Hemisphere. Average highs reach 6°C and average lows descend to a chilly (for the Southern Hemisphere) –5°C.
Because the Sun is close to setting during the eclipse, sight lines will have to be carefully arranged to avoid the distant mountains. That will be a tough challenge, as the eclipsed Sun is only 1° above the horizon, although the presence of several lakes aligned toward the west and northwest will help. The long view through the atmosphere will increase the probability that even a small amount of cloudiness will block totality.
Summary
French Polynesia is the clear-cut choice for the best weather prospects, but land-based sites are scarce and most observers will opt for a shipboard eclipse experience. Mangaia and Easter Island are the largest islands in the track, with more-or-less the same chances of sunshine—about 50%. If observing requirements dictate solid ground, then the choice is among one of these or the few reachable islands in the Tuamotus. Easter Island has, by far, the most developed infrastructure and the most convenient travel, but the small French Polynesian atolls offer the best weather. Easter Island, of course, has that aura of mystery that will more than compensate for the limited weather prospects.
Argentina is not a good choice if the eclipse alone is your goal. The very low altitude of the Sun, mountain-toothed horizon, and modest chances of sunshine suggest that more tropical destinations would be better.
Credits
Reprinted from NASA’s Annular and Total Solar Eclipses of 2010, Espenak and Anderson 2008.
Additional maps and charts from Jay Anderson, http://www.eclipser.ca/
