Total Solar Eclipse of 14 December 2020

Climatology of the 2020 December 14 Total Solar Eclipse

Figure 1: Overview of the path of the 2020 total solar eclipse. Eclipse track from Xavier Jubier.

The path of the lunar shadow in 2020 brings eclipse seekers back to Chile and Argentina for another shadow crossing less than 18 months after that of 2019, but this time it comes in the Southern Hemisphere summer and at a location 1000 km to the south (Figure 1). The climate along the 2020 eclipse track is largely controlled by three factors: the large sub-tropical high-pressure systems (anticyclones) over the South Pacific and South Atlantic, the cold Humboldt Current along the coast of Chile, and the 3 to 4-km-high Andes Mountain barrier that creates a vast rain shadow over much of south central Argentina. In both Chile and Argentina, summer is the dry season, with the amount and frequency of rainfall well below the winter maximums.

The early morning portion of the eclipse track moves across the South Pacific anticyclone (Figure 2), but it’s a region with no convenient islands, reachable only by ship and over some distance at that. The anticyclone forms beneath descending equatorial air, a process that warms and dries the high- and mid-levels of the atmosphere. The sinking air column cannot descend right to the surface, and so a small marine layer, about 1.5 km deep, remains, where it accumulates moisture from the water surface below. The result of these dynamic processes is a very stable atmosphere with a strong temperature inversion above a moist marine layer. In mid-ocean, skies in the marine layer is usually dotted by small non-precipitating cumulus clouds (Figure 3), usually with generous openings in the cloud cover.

Figure 2: Map of mean sea level pressure (MSLP) for December along the track of the eclipse. Data source: NCDC.

On the east side of the anticyclone, where the track curves toward the South American coast, the shadow moves away from warm sub-tropical water and encounters cold Antarctic water carried northward by the Humboldt Current. At the same time, the track moves away from the core of the Pacific anticyclone and into a region of gradually increasing cloud. The cold surface water reinforces the low-level inversion, suppressing the development of cumulus cloud, and instead traps the marine moisture near the surface where it forms extensive sheets of broken to overcast stratus and stratocumulus clouds (Figure 3). Fortunately, the highest levels of this marine cloud seem to prefer to remain offshore in the summer months.

Track of the lunar shadow across the Southern Hemisphere.
Figure 3: The eclipse track over the Pacific and South America as seen from the GOES 16 satellite. The cloud patterns offshore are typical of the early summer season in the Southern Hemisphere. Image: NASA.

The Pacific and Atlantic sub-tropical highs are at their strongest from December to February, weakly joined by a ridge of high pressure that crosses the South American continent along and just north of the eclipse track (Figure 2). A summertime low over Paraguay extends a trough southward to the eclipse track. This puts the land-based portion of the eclipse in a zone of weak winds and occasional convective instability, sandwiched between highs to the east and west and lower pressures to the north and south. Moreover, in Chile and western Argentina, the considerable influence of the Andes dominates the wind and weather, as the upper flow has a fairly steady westerly direction and so must rise over the mountain barrier on the Chilean side and descend over Argentina. December is well into the dry season for this latitude, but the flow across the Andes barrier gives Chile a modestly wetter and cloudier climate than Argentina.

Cloud cover map along the eclipse track.
Figure 4: Cloud amount along the eclipse track extracted from 17 years of MODIS satellite imagery. The data are compiled from the 1:30 pm (local time) overhead pass of the Aqua satellite. Data: NASA.

Over the South Atlantic, the shadow track turns into the South Atlantic anticyclone (Figure 2), eventually ending its global sojourn 370 km west of the coast of Namibia.

The Climatology of Chile

Map of Chile along the eclipse track.
Figure 5: Topographic map of Chile along the eclipse track.


The latitude of this eclipse—about 40 degrees south—puts it on the border of the Southern Hemisphere’s temperate zone, so eclipse seekers should be ready for a large variety of weather systems that might affect the eclipse track. This is particularly true in Chile, where the Andes Mountain barrier tends to hold up passing disturbances, causing cloud cover to linger when it ought to be passing on. The Pacific anticyclone still has some influence on the weather, however, keeping deep convective buildups (thunderstorms) to a minimum.

The eclipse first comes ashore at Isla Mocha, a small, rocky, and sparsely populated island lying 30 km off of the Chilean mainland. For those with a sense of adventure, the island offers a 1m12s eclipse from its southern tip, which lies just 9 km inside the north limit. The island is noted for its natural habitat, hiking trails, fishing, birding, and the large number of pirate wrecks lying on the sea bottom along its coast. An altercation between the local inhabitants and Sir Francis Drake’s crew in 1578 left Drake with a large scar across his face. The locals are friendlier now and access, by air or boat, is relatively easy and inexpensive from the nearby mainland.

A few seconds beyond Isla Mocha, the lunar shadow moves onto the South American mainland and is welcomed by a low-lying landscape and a small water body, Lago Budi. This low-lying terrain provides a convenient route for ocean stratus and fog to spread inland, which it does occasionally when marine cloud piles up offshore. On the south side of the central line, the rugged terrain reaches heights of 500 m or more, while to the north, hilltops crest at about 200 m. This leaves an opening on the northern side for more aggressive marine stratus to push inland, swallowing the landscape between Teodoro Schmidt and Temuco; the cloud seems to be reluctant to push as far as Villarrica.

Map of cloud cover over Chile and Argentina
Figure 6: Average December cloud amount derived from MODIS satellite observations. This image is an enlargement of Figure 4.

In spite of the occasional influx of ocean stratus, the cloud map in Figure 6 and the cloud cover graph in Figure 7 gives the coastal region west of Teodoro Schmidt the most promising weather prospects in Chile. Closer inspection of Figure 5 reveals that the best cloud prospects along the coast are even better on the south side of the track where the terrain is roughest. There are several small communities in this area that might serve as a base: Nueva Toltén (1m55s) and Hualpin (2m04s) in particular. Further inland, with cloud statistics that less encouraging, are Loncoche (1m53s) and Lanco (1m15s). All of these communities lie along decent north-south highways that will allow last-minute across-track travel.

Figure xx: Average cloud amount along the central axis of the eclipse track derived from MODIS satellite observations from 2002-2016.
The position of communities along the track is based on their longitude; in most cases, they do not lie exactly on the central line. Data: NASA.

Figure 7’s graph shows that beyond Teodoro Schmidt, average cloud cover increases modestly, reaching a maximum east of Gorbea before declining toward Villarrica. The downward trend in December cloudiness toward Villarrica is due partly to the distance from the ocean and partly to the cooling effects of Lago (Lake) Villarrica, which suppresses convection. A short distance east of the lake, terrain along the eclipse track rises sharply as the track moves into the rough-and-tumble topography of the Andes Mountains. Notable are three major volcanoes: Villarrica (2800 m), Quetrupillan (2300 m), and Lanin (3600 m). Though the graph in Figure 7 shows cloud cover increasing as the Argentine border is approached, the increase is modest.

An interesting aspect of the weather in this part of Chile is that convective clouds are often found in the morning hours when the Terra satellite passes (10:30 am), but dissipate by the time its companion orbiter, Aqua, passes overhead at 1:30 in the afternoon. That’s an unusual behaviour, as convective cloud typically builds through the afternoon as the ground warms. Because of the presence of the Pacific anticyclone offshore, dry air can be found a short distance aloft above a weak inversion. As the ground warms in the early part of the day, dry air mixes downward to dissipate the small buildups. It works best when the convection is scattered (less than 50 percent cloud cover) and fails completely when an invasion of marine moisture gives rise to heavy stratocumulus and cumulus cloudiness.

Table 1: Average December cloud and weather statistics from ground-based observations. These data are for the time of the eclipse.

Surface-based observations are in short supply along the eclipse track (Table 1), with only Temuco (at the north limit) and Villarrica lying within the umbral path. Measurements at Temuco show an average of 53 percent of the maximum possible sunshine in December. Cloud-cover observations by ground observers at the airports show that Villarrica is a few percent less cloudy than Temuco, and so the percent of possible sunshine there is probably around 60 percent. Percent of possible sunshine is the best measure we have of the chances of seeing the eclipse, though measurements are probably a tad pessimistic since the Sun has to be a bit over the horizon before the sunshine recorder is triggered.

Observations from Villarrica airport show that nearly 2/3rds of skies at eclipse time had scattered cloud or less—conditions that would likely allow for a successful view of totality, though a little movement might be needed to get out from individual clouds. The one-third of skies that have broken or overcast conditions comes from synoptic-scale weather systems that arrive with passing low-pressure disturbances and active fronts, along with the small amount of rainfall. There’s not much that can be done to escape them, as the cloud tends to pile up against the mountains.

Among all of the destinations in Chile, Villarrica and Pucon are likely to be the most popular because of the scenic lakes and volcanoes in their environs. Topographically, the area lies on the east side of a rolling plain in which elevations vary between 100 and 300 m, so terrain plays little part in the production of cloud. What is important is the several lakes, the largest of which is Lago Villarrica. The lakes suppress the formation of convective cloud on the lee side on days when the low-level atmosphere is unstable. Though usually light, prevailing winds are from the north and northeast, which favours the lake’s south shore where Villarrica and Pucon are situated.

Because of the influence of the Pacific anticyclone, convective clouds are restricted by a low-level inversion and typically do not build very deeply during the afternoon. This makes it more likely that the cooling associated with the approaching lunar shadow will dissipate much of the cloud cover though there is a substantial possibility, if the cloud cover is heavy, that the cumulus buildups will change to a heavy stratocumulus layer.

From Pucon, Highway 199 roams along the valley of the Rio Pucon, past small farms, lakes, and forested slopes. It’s a pleasant drive, all the way to the Argentine border, mostly on a narrow paved road with numerous small communities. The sun is high at mid-eclipse, 72 degrees above the horizon, and so there are no significant problems with the terrain. Highway 199 runs parallel to the eclipse central line until Curarrehue, and then turns southward and then eastward, across the lower slope of Volcan Lanin, to cross into Argentina at an elevation of about 1200 m.

The vegetation along the highway reflects an increasingly drier climate, a testimonial to the gradual decrease in precipitation as the mountains behind block moisture from the Pacific. Average December cloud cover increases as we move into the Andes cordillera according to the satellite observations in Figure 6’s graph, but it’s not a dramatic increase, remaining below 50%, and, in fact, declining slightly before reaching the border with Argentina.

The three major volcanoes under the eclipse track offer some intriguing observational possibilities. An 8- or 9-hour hike will get you to the top of Villarrica and Quetrupillan (Lanin is usually climbed from Argentina). A ski lift will take you part way up the slope on Villarrica, but the view probably cannot compare with that taken from the top. The peaks of the volcanoes will be well above any low-level cloud; mid- and high-level cloud probably comes with wind and weather that may prevent an expedition to the top. Guides who take hikers to the top (it’s a hike, not a climb) tell me that if permission is given by authorities and the weather cooperates, it should be possible for eager eclipse watchers to do so from the top. One caution though—Villarrica is still active, last erupting in March, 2015.

Climatology of Argentina

Topographic map of Argentina along the eclipse
Figure 8: Topography along the track of the eclipse over Argentina.

In Argentina, the sunniest and driest regions lie up against the east side of the Andes, with the very best cloud prospects near the small community of Peidra del Aguila. According to the graph in Figure 7, cloudiness is highest along the Chilean border, where the peaks of the Andes often generate high-level cloud as the air is lifted to flow over the 4000-m peaks. Once away from the heights, east of Junin de los Andes, the mean cloud amount drops below 40 percent and remains there until the track reaches the Atlantic coast at San Antonio Oeste.

While cloud-cover statistics are very encouraging for the Argentine portion of the track, eclipse observers should not be too complacent. The airport observations in Figure 8 show that one-third to one-quarter of December days have broken to overcast cloud at eclipse time. A part of this cloud—perhaps as much as 70 percent—comes from high-level flows across the Andes, carried into Argentina by strong winds aloft. In contrast to Chile, afternoon convection, particularly thunderstorms, is a part of the Patagonian weather. Neuquen experiences an average of 1.4 thunderstorms in December, with a maximum of 3 in the past 20 years. Junin, on the other hand, has averaged more than six December storms over the past 20 years.

Patagonia is notable for its winds, but wind statistics lose the extreme values when averages are compiled. Summer is wind season, with values sometimes approaching or exceeding 120 km/h. . Fortunately, the eclipse track travels in the weak pressure gradient between the Pacific and Atlantic anticyclones, and so daily winds are more subdued than farther south—say 15 km/h with gusts to 30 on an average day. In the mountains, wind will depend on the lay of the terrain and exposure. Bariloche averages 25 km/h with gusts to over 40 in a typical December day; Junin is more sedate at 14 and 28 km/h. The main feature of Patagonian winds is they are a regular feature of each day.

At the end of the land-based portion of the Moon’s shadow path, the Atlantic Ocean becomes an important part of the climatology. Figure 7’s graph shows that cloud piles up at San Antonio Oeste, but then declines as the centre line skips along the shoreline of Golfo San Matias. The south side is cloudier than the north, mostly because it’s over water.

The eclipse path crosses a rough and tumble landscape from the mountains to the coast, with limited accommodation and facilities. Close to the central axis of the path, Peidra del Aguila, a small but pleasant-looking community, offers the best eclipse weather prospects. There are limited amenities, but the town is probably destined to become a popular viewing point, since it is relatively easily reached from Bariloche (200 km) or San Martin (200 km) along Highway 237, or from Neuquen (235 km) in the north. Piedra del Aguila is 22 km south of the central axis, but a short distance north, a dam site across the Rio Limay boasts a small park that is 7 km closer. The landscape at the centre line itself is open and rather barren, offering little protection from the wind but lots of space to gather.

Fourteen kilometres north of Piedra del Aguila, a washboard gravel road branches northwestward to Santo Tomas, 20 km farther on. A village of some 240 inhabitants, Santo Tomas has the distinction of lying atop the centre line.

From Junin, eclipse travellers can head northward to tiny Pilolil or northeast to Catan Lil. Google Earth shows that the road to Pilolil (it’s in the wrong place on GE – look a few km farther north) is a rough and unforgiving gravel adventure. Catan Lil is a bit of an anonymous point on Google Earth, with only a few estancias nearby to suggest human settlement. It’s a landscape of rolling hills with some trees to break the wind if need be. Though the location is approached by a rather roundabout route from Junin, the highways are paved all the way.

Still farther east, the centre line can be approached from General Roca (150 km) along Highway 6. The central line here is also a barren treeless landscape of low hills, but it’s the closest convenient spot to the point of maximum eclipse. Wind could be a problem.

Beyond the point of maximum eclipse to the coast, the only easy access points to the shadow axis is along Highway 23 from San Antonio Oeste through Valcheta to Ministro Ramos Mexia. At its maximum, the highway is only 13 km from the central line, so more than anywhere else on the track, this route allows for “running room” if quick movement is needed on eclipse day. Don’t look for a lot of scenery and the route is rough in some spots.

Summing Up

All-in-all, the weather prospects are promising in Chile and encouraging in Argentina. Terrain and landscape pose some difficulties, mostly in limiting the ability to move, except in eastern Argentina. Mobility on eclipse day and attention to forecasts and other weather resources will be valuable if the weather is challenging.



Climatology and weather for celestial events