Total Solar Eclipse – July 2, 2019

Overview

figure1
Figure 1: The eclipse track across the Pacific to South America.

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.

Average July sea-level pattern with the eclipse track.
Figure 2: Average July sea-level pattern with the eclipse track. Note the two high-pressure centres; these are semi-permanent structures in the Earth’s weather patterns.

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 that gradually erode the  clouds 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.

Average July cloud cover along the eclipse track.
Figure 3: Average July cloud cover derived from satellite observations. Click on image for a larger view. Source: NASA/Patmos-X.

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 limit 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.

Cloud amount graph along the eclipse central axis.
Graph 1: Average July cloud amount along the central axis of the eclipse across South America. The locations of communities along the track are indicated, but the places themselves are not usually on the central line. Topographic features are also noted.

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 5.

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.

 Average July afternoon fractional cloudiness along the eclipse track over South America
Figure 4: Average July afternoon fractional cloudiness along the eclipse track over South America. Data are extracted from 14 years (2002-15) of observations from the Aqua satellite. Click for a larger display. Data: NASA.

 

East of the Cuyo, Argentine topography settles onto a broad and fertile Pampas — a flat terrain that angles slowly downward, from about 1600 metres at Mendoza to sea level near Buenos Aires some 1000 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, and they come with great abundance.

Cloudiness over the Cuyo and the Pampas comes 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 can reach well past 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.

Chile

Map of eclipse track over Chile
Figure 5: Topographic features along the eclipse track over Chile. Click on the map for a larger version.

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 and astronomers. Observations from La Florida airport, about 7 km inland from the coast, show that the city receives only about 54 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 (Figure 6) shows a large number of clear or mostly clear days.

Table of Chilean weather statistics.
Table 1: Weather statistics for selected sites in Chile along the eclipse track. Cloud category data are for the time of the eclipse. “Few” denotes 2/8ths cloud cover or less, scattered 2/8 to 4/8, broken 4/8 to 7/8, and overcast 8/8ths. “Days with” cloudiness come from Chilean observations and are not defined, but are believed to refer to daytime cloud estimates. XTmax is the extreme high temperature (°C) for the period of record; XTmin is the extreme minimum. Precipitation (pcpn) is measured in mm.

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, and so under-represent the afternoon sunshine.

Figure 6: Frequency distribution of hours of sunshine at La Serena in July
Figure 6: Frequency distribution of hours of sunshine at La Serena in July. La Serena has a daylength of about 10 hours in mid July.

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 15 percent drop in average cloudiness compared to La Serena. If the day should turn cloudy, the region is famous for its  pisco brandy to mollify the disappointment.

Figure 6: Satellite images of cloud patterns along the eclipse track over Chile on eclipse day
Figure 7: Satellite images of cloud patterns along the eclipse track over Chile on eclipse day from 2010 to 2014. The left column shows the cloud cover at about 14-15 UTC (late morning); the right column shows images from about 18 – 19 UTC (mid afternoon). Note the overall cloudiness along the coast and the dissipation of low-level coastal cloud over land as the afternoon progresses. La Serena is approximately in the centre of the image. Click for a larger display. Source: NASA.

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. There are no regular climate statistics for these sites, but 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. These results suggest that eclipse plans should favour the Elqui Valley, particularly at Vicuña or where the valley turns southward toward Pisco Elqui. 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.

As the Sun warms the ground, marine stratus erodes first along its landward edge and then gradually back toward the coast. Even a small distance inland can make a significant difference to the amount of daily sunshine: solar radiation is 14 percent higher in the eastern suburbs of La Serena compared to the city’s waterfront.

Argentina

Figure 7: opographic features along the eclipse track over Argentina
Figure 7: Topographic features along the eclipse track over Argentina. Click on the map for a larger display.

The Andes Mountains form an impenetrable barrier to Pacific moisture except at the upper levels of the atmosphere, but the flat plains of central Argentina do not pose much of an obstacle to Atlantic moisture and so weather systems approach 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 an 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 Iglesias (north of Bella Vista) lie on an open plain where satellite imagery shows the lowest average cloud amount anywhere along the track. Bella Vista is an 80 km trip from San José de Jáchal through rising terrain, but the village itself is on an open plain with good visibility west toward the lowering Sun (11° high at mid-eclipse). The highway is paved to Bella Vista.

Weather statistics for selected sites in Argentina along the eclipse track
Table 2: Weather statistics for selected sites in Argentina along the eclipse track. Column headers are explained in Table 1. Data: various.

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 to 36 percent at Jáchal and then declines a bit to 32 percent at San Juan. 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.

The rise in cloudiness at Jáchal is related to the rough 3400-m Precordillera terrain in which the community is embedded, but the overall dryness afforded by the nearby Andes limits the effect of these mountains, and average cloud cover only increases by about 8 percent. San Jose, east of the Precordillera, and in a downslope location, has a small decrease in cloudiness compared to Jáchal. Beyond San Juan, however, cloudiness begins to rise steadily, jumping up sharply at Marayes because of the 3100-m barrier of the Sierra de Pie de Palo. Cloudiness then drops a modest amount past Chepes and Calendaria, only to begin a steady rise near Merlo on the slopes of the Sierras de Cordoba. On the east side of the Sierra, the track is exposed to the full effects of Atlantic moisture and average July cloud cover rises from 47 percent to 67 percent (Graph 1), remaining near that value until the shadow leaves the Earth south of Buenos Aires.  Cloud trends are 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.

In the Cuyo region, a significant part of the cloudiness that does occur comes from wind-blown clouds that form atop the nearby Andes and are 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.

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 2016

Climatology and weather for celestial events