Laguna Seca: where summer meets winter (1/3)

Intro: lack of oxygen, excess of radiation

Bolivia is entirely situated in the tropical belt, but due to the huge differences in elevation has a diverse and complex climate. While the eastern part is dominated by wet lowlands and savannas, the western sector is mostly a high and dry plateau named the Altiplano. With an average elevation of around 3800 meters, this barren highland which is shared with three other South-American countries (Peru, Chile and Argentina) is the second largest and elevated in the world after the Tibetan Plateau, concomitantly the air contains only about 60% of the oxygen found at sea level.

The driest part is represented by the westernmost stretch, which gradually descends into the hyper-arid Atacama desert towards the coast of the Pacific Ocean. In Bolivia it can be found in the south-western corner of the country. In the middle of this region there is an extensive flat area covered with thick salt crust named Salar de Uyuni, which represents the closed bottom of the highland’s interior. As such, it has no outlet and all the rivers (mostly intermittent) are collected by the huge, shallow bowl where the water rapidly evaporates in the thin and dry air.

With more than 10.000 square km surface area, Salar de Uyuni (3660 m) is the largest salt flat in the world

The climate of the plateau is extreme. Especially notable are the daily temperature fluctuations, which can exceed 30 degrees in the dry season, which has its peak in the south hemisphere’s winter months (June-August). Beside the huge thermal amplitudes the other peculiar weather aspects are the strong westerly winds and the intense UV radiation. Regarding the latter, the highest value ever recorded on the surface of the Earth – an incredible 43.3! – was measured on the peak of Licancabur volcano (5916 m) situated right on the border between Bolivia and Chile on the fringes of the Atacama. Just imagine that the level 11 is considered “extremely high” on the UV chart and most European countries never reach that even in the summer.

Choosing the target

While already knowing that the Altiplano likely provides the best general context regarding the biggest daily temperature fluctuations, I was of course interested in the “finer details” which can maximalise this potential. Regarding the timing I concluded that the best period must be somewhere near the equinox, when the heating and cooling has similar chances of manifestation. As with both very short nights (think of polar summers) or very short daylight (think of polar winters) the thermal amplitudes are pretty small (constantly cool or constantly frigid), the biggest ones have to be equally far from both.

The location of the research area (black dot) inside the Altiplano (gray spectrum)

Concomitantly, much better is to have an abrupt sun-path, where the irradiance reaches the highest possible angle at zenit (namely tropical latitudes). This is of course influenced also by other factors such as the distribution of moisture (clouds and precipitation) between the seasons.

Taken all into account the month of September stood out as according to the available climatological statistics (Uyuni in Bolivia, Calama and San Pedro de Atacama in Chile) in the south-western part of the Altiplano it is the same dry as the mid-winter period, receiving almost no precipitations. Now let’s move to the topographical aspects.

Topo-map with the unnamed depression (lower-right center) situated east of the Alota – Villamar Mallcu dirt road

As the thinner air has less heat-retaining capacity, the general rule to follow is: the higher the better. In the simplified version the limitation is given by the highest possible elevation where closed basins can be found as in those places the colder air can be gravitationally collected. But there are also other influencing elements like the wind pattern, which instead is inversely proportional with the targeted amplitude potential, making the real picture more complex. Best should be (especially for shorter term observations like mine) to make a wise compromise, founding the “golden mean” between elevation and stability.

As such, I concluded that around 4000 meters elevation must be a well grounded decision, sufficiently high to have a serious advantage compared to the low elevations, but still relatively stable compared to the protruding tall ridges and peaks where the wind is certainly more active.

Satellite view of the endorheic basin with the contour levels 30, 20, 10 and 5 meters marked with red

Meticulously scanning the map I found out that there are only a few closed basins with sufficient depth which doesn’t have lakes on their bottom and of course exactly these are the best for my purpose. Finally I’ve chosen an unnamed depression south of Villa Alota settlement, whose floor is situated around 4030 meters, while the outflow point is some 30 meters higher. The bottom looks completely dry, lacking even the characteristic white salt crust.

Brief summary of the research

The Bolivian journey started on 13th September, landing in Santa Cruz de la Sierra, the biggest city of the country. Continuing with an internal flight to Sucre, the next day I reached an intermediate elevation between the hot lowlands and the arid high plateau. After two other days of acclimatization I arrived in Uyuni on 16th by bus via Potosí. On 17th I traveled to Villa Alota with a previously hired private jeep to begin the research as soon as possible, approaching the chosen spot a little more on the dirt track towards Villamar Mallcu. I started the hike with the equipment around 1 PM.

View of Laguna Seca from the west

After around 1.5 hours I reached the bottom of the nameless endorheic basin where the instruments were mounted on the tripod, which was also stabilized with an attached bag filled with rocks. The coordinates are 21.498 S, 67.547 W, the elevation around 4030 m and the distance of the sensor from the ground is 170-180 cm. The weather station started its operation at 2:56 PM. First reading: 14.4 degrees Celsius.

I did not waited long here, but returned from Alota the next day to spend the following two nights near the equipment. In all three cases the temperature dropped below -15 degrees, the coldest being the third morning with -16.5 degrees Celsius. The sky was generally clear with some altocumulus invasions during the evening and the first part of the second night. The wind pattern has a pretty obvious course, the afternoons being dominated by strong westerlies, while the nights and mornings are much calmer. I left the research spot in the morning of 20th and spent the following days in and around Uyuni.

The weather station on the bottom of the basin

After six days I returned to the endorheic basin for one last night after which the equipment was collected. That morning (27th) was the coldest of the entire research (-19.0 degrees Celsius at 6:23 AM) and it was also the period when the biggest thermal excursion happened: 38.1 degrees in under 24 hours, starting the huge drop from the previous day’s positive 19.1 degrees.

Six out of ten days the temperature fluctuation exceeded 30 degrees (dropping every time below -10) with a very likely seventh (the day before the last) when the strong afternoon wind overturned the tripod disconnecting the sensor. The middle days were somewhat milder, when at times I observed partly covered sky in the Uyuni area.

The coldest morning of the research was the last one

During my solitary hikes I saw many llamas on the outer slopes of the basin, where the locals have started the spring work on the quinoa fields and only vinunas inside, on the bottom. I spotted also some eagles, a viscacha and observed Darwin’s rhea (suri) tracks in the sand. Near my camp which was situated a little higher on the northern slope I identified signs of smaller scale mining activity. Except the relatively busy dirt track where the tourist filled 4wd cars circulated the surroundings lacked human presence, meeting only once two llama herders on the periphery of the basin.

The instruments used in the field

-One LogTag UTRED30-16 data logger with the measuring range between -40 and +99 degrees Celsius, an accuracy of 0.5 degrees Celsius and a resolution of 0.1 degrees Celsius

-One Greisinger G1710 thermometer with the measuring range between -70 and +250 degrees Celsius, an accuracy and resolution of 0.1 degrees, used for instant hand measurements

-One photo camera tripod serving as the support for the instruments

-One helical solar radiation shield from Barani Design Technologies: https://www.baranidesign.com/

The Barani Pro shield fighting against the vicious UV radiation

To be continued…

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