Annular Solar Eclipse – 2017 February 26

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Figure 1: The central eclipse track across South America.

Climate of South America in February

Chile

This eclipse comes in the midst of the Southern Hemisphere summer, a time of low precipitation and pleasant temperatures across South America. In Chile, the eclipse track passes through the centre of the Zona Austral, a region of prevailing westerly winds where ocean meets mountain, giving a relatively small variation in annual temperatures and a strong gradient of cloudiness. Figure 2 shows that cloud cover is significantly higher along the coastal archipelago and the windward side of the Andes Mountains. Figure 3, derived from the same dataset as Figure 2, shows the variation in cloud cover along the central axis of the eclipse track in more detail. The satellite data, with a resolution of 100 km, are unable to reveal the full details of the complex cloud climatology in the mountains, but the surface-based cloud and sunshine measurements in Table 1 shows that mountain sites such as Coyhaique that lie behind the first line of coastal mountains have much less cloud than the ocean-facing locations.

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Figure 2: Average February fractional cloud amount derived from satellite imagery 1989-2009. Data: Patmos-x/CIMMS/University of Wisconsin-Madison.

Between Puerto Chacabuco and Coihaique, the Andes Mountains present a 2000-m barrier to moist Pacific flows. When the airflow descends into the broad Rio Simpson valley around Coihaique, it is warmed and dried by adiabatic compression, bringing a drier and warmer climate than on the coast. The eclipse track then climbs a more easterly branch of the Andes beyond Coihaique before descending onto the elevated plateau of Patagonia.

In Chile, good eclipse-viewing circumstances are to be found at Coyhaique and the surrounding countryside, which offers a small-town, relaxed experience within an attractive mountain plain. The city lies a little over 10 km from the centre line, which is easily reached by highways leading northward. Beyond the junction of Highways 7 and 240, 10 km north of Coihaique (by road) and 3 km south of the centre line (by crow), the landscape begins to close in, increasing the probability of hillside clouds that spread across the valley. The eclipsed Sun will lie at an altitude of 33 degrees, so interference from the terrain will not be a problem except within the narrowest valleys. Highway 240 to the northwest passes through one of these narrow valleys with impressive surrounding peaks that may present interesting photographic possibilities if the skies are suitable.

Argentina and Patagonia

The best skies for this eclipse come in western Argentina, where the full effect of the Andes barrier imposes its presence on the weather. The graph of satellite cloudiness shows a drop to less than 40 percent cloud cover Aldea and Rio Mayo. None of the three weather stations listed in Table 1 are particularly close to the eclipse track, and so we must rely on the satellite measurements to give us a reliable indication of the cloudiness under the shadow. Of all of the reporting stations, Balmaceda, in Chile, is probably the most representative of the cloud conditions to the lee of the mountains. Rio Mayo is acknowledged as one of the driest parts of Patagonia, but makes up for the lack of precipitation by an abundance of wind.

Figure 3: Average fractional cloud cover along the central axis of the eclipse track.
Figure 3: Average fractional cloud cover along the central axis of the eclipse track. This graph uses the same dataset as Figure 2.

From Rio Mayo, Highway 40 leads north to the cental axis of the eclipse, 70 km distant. Secondary roads that branch from 40 will get you there in 50 km, near the community of Facundo. This is flat country – billiard-table smooth on Highway 40 and only a little more interesting near Facundo, which borders a marshy, winding river. Rio Mayo, a community of 2800 souls, is notable for its national sheep-shearing competition (in January), but the community is largely a distribution centre and transportation stop, with a small tourist industry based on trout fishing. Lonely Planet calls it “a surprisingly humdrum place,” so the eclipse will bring an element of excitement to the region that may provide for a very pleasant eclipse experience. From a meteorological point of view, Rio Mayo (not nearby Centro Rio Mayo, though the weather there is probably just as promising) is the place to be for this eclipse.

Beyond Rio Mayo, the Moon’s track heads back into a modestly cloudier climate, though somewhat better than conditions at Coyhaique. At Trelew, a little beyond the north limit of the track, cloud and sunshine measurements offer conditions as good as those against the shoulders of the Andes at Balmaceda. The 70+ percent sunshine seen at Balmaceda and Trelew is probably at least 10 percent higher at Rio Mayo but there are no station reports in the area – the favourable weather is revealed only by the satellite measurements.wxtableTable 1: Cloud, sunshine, temperature, and precipitation data at selected locations along the eclipse track.

Climate of Southern Africa

In Africa, the eclipse comes ashore into a tropical climatology that is in the midst of its rainy season. Landfall is made just south of the small coastal community of Lucira and within 5 km of the coast, begins to climb the hills leading to the interior highlands. About 90 km inland, the centre line passes over the 2400-m heights of the Sierra de Neve and then descends onto the tableland that makes up the interior of the country. From Chongoroi to the end of the track, the land maintains an elevation of between 1000 and 1600 m, giving a pleasant, temperate climate to the tropical latitudes.

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Figure 4: Track of the central axis of the eclipse across southern Africa.

 

The lowest average cloud cover comes at the coast before the eclipse track begins its climb onto the interior plateau. Oceanfront regions are sunnier than those inland because of the cloud-suppressing effect of the cold Benguela Current that flows along the West African coast before turning westward near the position of the eclipse track. To the south lies the Namib Desert, whose influence may reach as far as the south limit of the eclipse at the coast (Figure 4). In spite of these forces, the percent of maximum sunshine, at 60 percent, is short of the prospects in South America by 10 or 20 percent.

Figure 5: Satellite-based map of average February cloud cover over southern Africa along the path of the eclipse.
Figure 5: Satellite-based map of average February cloud cover over southern Africa along the path of the eclipse.

Beyond the coast, sunshine declines and cloudiness rises with increasing eastward location so that seeing the eclipsed sun in the portion of the Moon’s track beyond Huambo has a one-third chance or less, according to sunshine measurements. Satellite measurements of cloud cover range between 80 and 90 percent across the region and surface measurements average between 70 and 80 percent.

Figure 5: Satellite-derived cloud amount in percent along the central axis of the eclipse in southern Africa.
Figure 5: Satellite-derived cloud amount in percent along the central axis of the eclipse in southern Africa. Station locations are not typically on the central line, but are plotted at their longitude. Data: NOAA/SSEC/ University of Wisconsin-Madison.

 

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