This eclipse comes in the depths of the Southern Hemisphere winter, but the track’s sub-tropical latitudes takes the sting out of what might otherwise be a very cloudy environment, providing several good opportunities for a successful view of the eclipse. From its beginnings in the mid Pacific near Pitcairn Island (Figure 1) to its end over the suburbs of Buenos Aires, the track straddles the semi-permanent anticyclone (high-pressure cell) that stretches along the 30-degree south latitude (Figure 2). This Pacific Anticyclone and its cousin, the Atlantic Anticyclone, dictate much of the character of the weather along the shadow track. Over land, the cloud is further moderated by the rugged terrain along and across the Andes Mountains that both enhances and subtracts from the cloudiness according to exposure to the prevailing winds.
The Pacific anticyclone brings large-scale atmospheric subsidence that warms and dries the air column aloft, creating a strong temperature inversion at about the 500-m level along the Chilean coast. Below this marine inversion, the air is cooled and moistened by contact with the cold Humboldt Current that flows northward from the tip of South America to northern Peru. Atmospheric mixing in the surface layer lifts the moist air upward until it becomes saturated, forming an extensive blanket of low stratocumulus clouds. This marine cloud has an on-again, off-again presence along the coast of central Chile (Figure 6), but when present, usually spreads inland a short distance, especially at night. The amount of this ocean cloudiness depends on the height of the inversion and the temperature of the underlying ocean or land surface, but in general, the cloud is thinner within about 200 km of the coast than in mid-ocean (Figure 3).
Ocean temperatures are relatively constant through the day, but temperatures over land rise and fall in the normal diurnal cycle. The marine cloudiness over land is usually overcome about mid-day by warming surface temperatures in nearby clear skies and the clouds are gradually eroded from the edges inward. At night, land cools, the inversion reforms, and marine cloud may return inland if winds are right. The nighttime advance of marine cloudiness is limited by the terrain, which prevents the moisture from spreading more than a short distance onto land except into river valleys that penetrate the mountain barrier.
Winter is a time of low-pressure storms that carry frontal systems with extensive cloud shields that intrude on the anticyclones, spreading high- and mid-level clouds across the eclipse track. These frontal systems and their associated lows are the main source of the cloud in inland parts of central Chile and over much of Argentina. For the most part, the clouds are not heavy enough to generate precipitation over lower coastal areas of Chile, but on occasion – once every two to four years – a passing front will drop several tens of millimetres of rain with dramatic effect. More significantly for eclipse travellers, even modest amounts of high-level cloud will reduce the amount of sunshine that reaches the ground, limiting the rise of surface temperatures and permitting marine cloud to persist through the day. Moderate to strong onshore winds that push cloud onto land may also cause low cloud to persist through the day.
Inland, beyond the reach of the low-level marine stratus cloud, the rugged terrain is both a good cloud manufacturer and a cloud eater. As the prevailing westerly winds rise over the peaks, the air cools by expansion and condenses into clouds. Conversely, on the downwind side of the terrain heights, air descends into valleys, warming and drying by compression as it sinks. The ups and downs of cloudiness caused by these alternating ascents and descents are prominent features of the cloudiness depicted in Graph 1 and in Figure 4.
East of the Andes, the Moon’s shadow passes across Argentina’s Cuyo and Pampas regions on its way toward a sunset ending near Buenos Aires. The Cuyo, tucked up against the Andes and still beneath the sub-tropical anticyclones, has an arid climate that offers the driest and sunniest locations from which to watch the eclipse (Graph 1) though by no means cloud free. In contrast to eclipse sites in Chile (where precipitation reaches an annual maximum), July is one of the driest months of the year in the Cuyo. Even when the rest of Argentina is covered in a solid overcast of low cloud, there is often a narrow band of sunny weather tucked up against the eastern slopes of the Andes.
East of the Cuyo region, Argentine topography settles onto a broad and fertile Pampas — a flat terrain that angles slowly downward, from about 1000 metres at Rio Quarto to sea level near Buenos Aires some 700 km distant. The Pampas becomes increasingly humid from west to east, with July rainfall rising from 16 mm at Cordoba to 66 mm at Buenos Aires. This long, sloping terrain with few topographical barriers is open to intrusions of moisture from the Atlantic and to passing low-pressure disturbances, which come with considerable frequency.
Cloudiness over the Cuyo and the Pampas arrives from several sources: the upper remains of Pacific disturbances that have been enhanced on the Andean peaks and blown downwind; from extensive areas of fog and low cloud that form in the long, cool nights of winter; and from frontal systems that approach from the Atlantic side. The first of these brings relatively thin cloudiness at high levels over the Cuyo, but the other two can be disastrous for eclipse viewing and are the main reason why the cloud levels are so high toward the end of the eclipse path. Satellite photos often reveal extensive sheets of solidly overcast low-level clouds across the Pampas that take days – sometimes more than a week to dry out.
Argentina’s terrestrial setting and flat topography means that Argentina can experience severe winter storms that occasionally reach the eclipse track, disrupting travel and communication and bringing extremely cold (for the latitude) conditions. Though these are uncommon intrusions, eclipse travellers should be prepared for cold weather before setting out.
The presence of more than a half-dozen world-class observatories in the mountains near La Serena (Figure 5) testifies to its excellent and largely cloud-free skies, but July is not a kind month for eclipse observers or astronomers. Observations from La Florida airport, about 7 km inland from the coast at La Serena, show that the city receives only about 53 percent of the maximum possible sunshine during the month (Table 1). Cloud cover mirrors the sunshine measurement with an average amount of 45 percent at 21:00 UTC (totality is at 20:39). In spite of these somewhat pessimistic statistics, a frequency graph of daily sunshine hours for the city (Graph 2) shows a large number of clear or mostly clear days.
A significant part of the cloud at La Serena comes from the marine stratus that pushes onshore, but satellite observations (Figure 6) show that this low-level cloud typically evaporates around noon leaving the rest of the day under sunnier skies. Graph 2 shows a large number of days with 4 or more hours of sunshine, and most of these hours are probably accumulated in the afternoons when the low cloud has dissipated.
Satellite images also show that the marine cloudiness does not usually penetrate a large distance inland and so it is possible to move into a much sunnier climate by a small migration to the east. From La Serena, that would mean a trip up the Elqui Valley toward Vicuña and beyond. While no cloud statistics are available from Vicuña, the satellite measurement of cloud cover there shows a 11 percent drop in average cloudiness compared to La Serena (Graph 3). And, if the day should turn cloudy, the region is famous for its pisco brandy to mollify the disappointment.
Chile’s climate is very dry to the north of the eclipse track and the transition is very abrupt, so that the question of whether to go north along the coast (even beyond the centre line) or inland along the Elqui Valley for the sunniest skies is critical for picking the best site. While Graph 1 shows us the trend in cloud cover along the central axis of the shadow track, it says nothing about the cross-track variation in cloudiness, which is particularly variable in this part of Chile. In Graph 3, I show the satellite-measured cloud fraction (in percent) for selected locations within the zone of totality, but scattered across the track. As can be seen from the value for La Silla Observatory, there is a strong trend to sunnier skies on the north side of the eclipse track beyond La Higuera. This trend does not extend into Argentina.
There are a number of agricultural meteorological sites that measure, among other parameters, the amount of solar radiation. This is not a particularly reliable comparative measure of the difference between stations, as sites in the Elqui Valley will see much less sunshine than stations on the coast because of shadowing by the terrain. Nevertheless, there is a small but telling increase in solar radiation from the coast to deep in the Elqui Valley. Beyond Varillar, the insolation decreases toward the Argentine border, likely because to the much deeper valley along the route and the gradual increase in cloudiness with elevation.
By comparison, at Punta de Choros, on the coast and just north of the central line, solar radiation is comparable to or slightly higher than stations in and around La Serena, but the difference is small and less than the amount of radiation measured inland up the Elqui Valley. This is confirmed by the similar cloud frequencies between La Serena and La Higuera in Graph 3. However, the dramatic decline in cloud amount at La Silla shows that the impact of the dry Atacama Desert to the north begins to be felt north of the central line and inland from the coast.
These results suggest that eclipse plans should favour the Elqui Valley, particularly at Vicuña or where the valley turns southward toward Pisco Elqui, or locations on the north side of the central line. Inland locations must be chosen carefully to ensure a view of the late afternoon Sun (Figure 7). Past Vicuña, the valley narrows significantly and eclipse-viewing locations must be chosen very carefully to avoid the Sun being shadowed by the terrain. At Varillar itself, the terrain is probably too high to allow the eclipse to be viewed.
If eclipse day promises to be sunny along the coast, the small community of La Higuera, which lies 60 km (by road) north of La Serena and only 4 km north of the central axis of the shadow track, may prove to be particularly attractive. La Higuera is protected from the marine cloudiness by a range of 1000-metre hills that lie only a few kilometres from the coast. In Figure 8, taken in June 2017 during a TravelQuest scouting trip, we see the cloud spilling over the hills and then dissipating in the descent into the community. When the coast at La Serena is overcast, La Higuera may be free of cloud, though it would be unusual for marine cloud to persist to the late afternoon eclipse time in either location.
For those looking for a more grandiose view of the eclipse, along with a 2300-m altitude with more transparent skies, La Silla Observatory may prove to be irresistible, in spite of a 40-second loss of eclipse duration. Check it out in Google Earth streetview. For the most adventurous, the Paso de Agua Negra on the border between Chile and Argentina offers a 4800-m observing site though at the cost of a very high probability of cloud (Graph 3). The highway between the two countries crosses the pass only 400 m north of the central line.
The Andes Mountains form an impenetrable barrier to Pacific moisture except at the upper levels of the atmosphere. In contrast, the flat plains of central Argentina do not pose much of an obstacle to Atlantic moisture and so weather systems are able approach and envelope the eclipse track from the south and east. Fortunately, the Cuyo region is a long way from the Atlantic and moisture reaches the foothills from that direction only occasionally. This gives the Argentinian slopes of the Andes a small but especially favourable climate for eclipse observation.
All the available evidence – satellite and ground-level measurements – points to a location up against the eastern slopes of the Andes as having the best chances of seeing the eclipse. In particular, the small hamlets of Bella Vista and Iglesia (north of Bella Vista) lie on an open plain where satellite imagery promises the lowest average cloud amount anywhere along the track. Bella Vista lies only 33 km from the sizeable town of Rodeo and 80 km from the much larger San José de Jáchal. The two communities have good visibility west toward the lowering Sun (11° high at mid-eclipse).
July finds Bella Vista, San José de Jáchal, and San Juan in the midst of their winter dry season with little precipitation in the month. Nights are cool because of the season and the altitude (1200 m at Jáchal; 1900 m at Bella Vista), with temperatures frequently dropping below freezing and occasionally as low as -10 °C at Jáchal.
Though the climate is dry, terrain still has a modest effect on the cloud statistics as seen in the graph of satellite cloudiness. From a minimum of about 28 percent at Bella Vista, the cloud amount rises a fraction at San Jose de Jáchal and then climbs more distinctly to 44 percent at San Juan (Graph 3). Bella Vista (along with Iglesias and Rodeo) lies in a deep north-south tectonic valley (sometimes called the Iglesias Valley) with the 6000-m Andes Mountains to the west and the 3400-m Precordillera to the east. The valley is narrow – only about 40 km wide – with a climate and geography that is comparable to that in Death Valley, California, though without the dunes. Because it is protected by terrain on the east and west, it lies in a particularly effective rain shadow and so has one of the driest climates in Argentina. It is also one of the sunniest regions in the country, making it a prime wine-growing area.
A significant part of the cloudiness that does occur over the eastern slopes of the Andes comes from wind-blown clouds that cross the nearby peaks, carried over the region by the upper-level flow. Because they form at very cold temperatures, there is usually little moisture in the air and the high clouds are often semi-transparent. If not too thick, the high cloud would interfere with an eclipse but not entirely obscure it.
From the slopes of the Andes to as far east as Merlo, the eclipse track crosses a region with a sunny disposition except for a modest increase in average cloud amount near Marayes, where the Sierra Pie de Palo has an influence. Merlo lies on the western slopes of the Sierra de Cordoba, a prominent 2200-m mountain chain that acts as a barrier to moisture flowing inland from the Atlantic. From the Sierra to the coast near Buenos Aires, average cloud amount jumps by nearly 30 percent (Graph 1). That cloud trends is mirrored in the percent of possible sunshine (Table 2), which drops from a decent 75 percent or more along the Andes foothills to less than 50 percent (at Eziza, to 30 percent) near Buenos Aires.
Though the eclipse comes in the winter season, temperatures are not particularly cold by North American and European experience. Below-freezing temperatures can be expected at higher elevations, but a perusal of Table 2 reveals some curious features of the climate. Extreme high-temperature records at San Jose de Jachal, San Juan, Chamical and others reach above 30°C, a value that is considerably at odds with the season. The cause is the Zonda – a downslope wind that dries and warms by compression as it descends the Andean slopes, bringing the exceptionally high temperatures when it reaches the lower elevation of the Cuyo. The Zonda is the equivalent of the North American chinook.
A typical Zonda has winds of 40 km/h but has been known to reach above 120 km/h, with stronger storms spreading eastward beyond Cordoba into central Argentina. Along the Andean slopes, the wind begins in the morning hours; at San Juan, it is usually about noon or shortly thereafter. Ordinarily, they last about 8 to 12 hours, but may continue intermittently for several days. The Zonda can make the Sun turn brown with blowing dust and sand and bring extensive damage to the local infrastructure. The dessication that comes with these downslope winds often damage the region’s wine grapes (the area around San Juan is Argentina’s second-largest wine-growing area), but at the same time, the wind makes Rodeo famous for its windsurfing competitions. Eclipse seekers should be prepared for at least a modest amount of wind on the critical day.
Updated: September 2017