Imagine a wide flat plain, 170 kilometers across, 500 kilometers long, surrounded by some of the highest mountains in the world, and lying at 3 to 4 thousand meters above sea level - as high as Mauna Kea. This is the altiplano or Puno of Peru and Bolivia, homeland of the Incas and magnificent mysterious ruins. What a place to watch an eclipse! Although the site has its problems (especially access) the weather is cooperative. Table 14 shows that Uyuni is the sunniest location in the Andes and Charaña ranks only a little behind Arequipa. Charaña lies on the western side of the altiplano and Uyuni on the east. The two straddle the eclipse line so their climatologies should give a reliable indication of the weather across the region. Figure 9 is even more encouraging. During the two years that these data were collected the altiplano was the sunniest location on the eclipse track. The weather prospects in this area are very good for two reasons. First, the altiplano is very high and the air contains less water vapor at 4 thousand meters. Second, the fortress of peaks to the east and west of the Puno wring most of the moisture from any breezes that approach from the tropical interior of the continent or from the Pacific. Winds blow downhill from all directions, drying and dissipating most of the low and middle level clouds that approach. High ice crystal cirrus is the most common cloud, blowing above and off the mountain tops, and often covering a large portion of the sky in a thin veil. Cirrus would not usually hide the eclipse unless it is very thick. Although the frequency of completely clear skies is less than the cloudy coast of Peru , the typical cloud is thin and appears as a transparent cirrus veil or as small patches hanging off mountains in the distance.
When bad weather does invade the plateau, it comes from strong weather systems approaching from Paraguay and Argentina. These systems are high enough to spill over mountain barricades and can fill the altiplano with cloud for a day or two. Some of these low pressure disturbances continue right on to the Pacific, bringing deep layers of cloud and even a little rain to Arequipa and other parts of the Peruvian coast. Because these storms contain plenty of high level cloud, they can be seen approaching from a long distance. Unfortunately, they are also very large systems and can be very difficult to avoid.
Thunderstorms are legendary on the Puno with intense convection forming on nearly every afternoon in the summer. Rain, hail, and strong gusty winds come with the build-ups, often bringing dust and blowing salt from the large salt flats. Fortunately, summer is in its earliest stages in November, and the eclipse track crosses the driest part of the altiplano. Moreover, the thunderstorms are mostly confined to the afternoons and the eclipse is over before 9 AM. Since the source of the moisture that feeds the build-ups comes from the east, sites on the west side of the Puno are more likely to be free of interference. Sucre reports thunderstorms on 2 days of the month on average, but at Charaña it is only once every three years. All-in-all it is difficult to pick the best location on the altiplano. Figure 9 suggests that it might be on the west side near Charaña while Table 14 votes for the east near Uyuni. From the flow of the weather and sources of moisture, the west side appears more promising but in any case the difference is small.
Travel on the altiplano is not difficult though access is not convenient to most of the eclipse track. Potosí is the closest large city to the path, but it lies within the eastern branch of the Andes some distance below the altiplano. To reach the center line on the altiplano is probably easier from Oruro, though the distance is in the neighborhood of 150 kilometers. This is adventure traveling, and the 3800 meter altitude should not be taken lightly. Acclimatization is required. The brain doesn't want to think clearly in an environment with only 60% the oxygen content of sea level. Eclipse observers should plan on several days at altitude beforehand and not a quick visit from sea level. It is probably a good idea to limit the tasks you want to do during the 3 minutes of totality because of the chance of altitude-induced befuddlement.
As the Moon's shadow leaves the altiplano, the weather becomes increasingly cloudy as the path descends to lower pressures on the Atlantic side of the Andes. The increase is gradual at first, so locations around and east of Potosí are quite promising. One good site, where the Pan American Highway crosses the center line south of Potosí, had more than an 80% frequency of sunny skies in 1991 and 1992 according to Figure 9. However this site could have been sunnier than usual over those two years. La Quiaca in Argentina, the closest climatological station, shows a frequency of clear skies (13.7) much lower than Uyuni in Table 14, though still good for the region.
Cloud cover is extremely variable within this region, depending on the exposure of each valley to the east and south winds that bring the moisture. The mountain slopes heat quickly during the morning, drawing in moist air from the lowlands and foothills. These winds are forced to travel uphill, gradually cooling and becoming more humid until cloud begins to form. The clouds grow through the afternoon until deep enough to bring showers and thundershowers.
Cloud-making processes also occur at night. As long as winds are blowing uphill from the east, clouds will form along the Paraguay-Bolivia border. Figure 9 shows this process very well. Longitudes 63 deg W and 64 deg W are the cloudiest of the whole track except along the Peruvian coast. In Table 14, Tarija and Yacuiba have one of the lowest frequencies of sunny skies. The area along the Bolivia-Paraguay border should be avoided for this eclipse.
Before leaving Bolivia and the altiplano, it is worthwhile looking at the temperatures. Though November is the warmest month, the altiplano is a very high plain and the mercury can drop sharply overnight. Mean daytime highs at Charaña reach a pleasant 21 C (72 F) but nighttime lows average -5 C (22 F) and the record low is a frigid -15 C (5 F). Travel with warm clothing. Since the eclipse occurs in the early morning on the altiplano, it is entirely possible that temperatures will be below the freezing point. Make sure your equipment and clothing can handle it.
