Interpretation of the logger’s graph (using LogTag Analyzer 3 software)
First day and night (17-18th September)
I installed the measuring equipment on the bottom of the endorheic basin in the early afternoon during moderately windy conditions and almost clear sky. The logger set for 2 minute intervals was started at 2:56 PM and the first reading showed 14.4 degrees Celsius. Concluding from both the time of the day and the temperature curve it was around the hottest part of the day, reaching the slightly higher maximum of 14.7 degrees at 3:25 PM. I left the equipment before 4 PM and hiked back to Alota under generally clear sky with some altocumulus lenticularis clouds over the higher mountains. The moderate wind persisted also in the evening.
After a smoother afternoon start the temperature drop is slightly disturbed in the evening and night but generally is pretty straightforward. Descending below 0 for first time at 10:35 PM, the continuous negative values started right around midnight. The minimum of -16.2 C was reached at 5:11 AM and was approached again with -16.1 C at 6:25 AM before sunrise. I spent that night in Alota and measured -7 degrees outside in the early morning during clear and calm conditions.
Second day and night (18-19th September)
After sunrise the temperature rise is fast and clean, already passing over 0 degrees around 8 AM. Later the curve becomes less and less abrupt but has a general constancy until around 3 PM, reaching the peak of 17.1 C at 3:49 PM. Together with the -16.2 C morning minimum this means a 33.3 degree temperature excursion in the same day. The obvious drop starts around 4:30 PM becoming more pronounced after 5:20 PM. Like in the previous night’s case, the afternoon drop is the cleanest while the evening and first part of the night somewhat disturbed, still without significant changes on the higher scale. The 0 degree mark was passed for the first time at 11:21 PM, while the continuous negative values started from 0:45 AM. The minimum of -15.2 C was reached at 6:17 AM.
I spent the night in the basin at my camp situated a little higher, outside the inversion layer on the southern slope of Jubina mountain. Despite the optimistic clear forecast, from the late afternoon well into the night there were constant altocumulus (also cirrostratus) invasions, passing from west to east and partially covering the sky between 1/8 and 5/8 ratio. The wind was generally weak, reaching the lowest speed at dawn when the sky finally became completely clear. I was present at the research spot during the coldest period before sunrise and observed calm conditions on the bottom.
Third day and night (19-20th September)
I continued to stay in the basin observing an all day long clear sky. The very rapid temperature rise passed again above 0 degrees around 8 AM, the curve becoming less abrupt before 9 AM. This time the maximum of 17.8 C was reached a little sooner at 2:15 PM, which together with the -15.2 C morning minimum represents a 33.0 degree daily amplitude. The warmest early afternoon period is characterized by pretty big 1.5-2 degrees small scale oscillations in short periods of time. The general drop started after 4 PM and again the cleanest part was the late afternoon. The evening and especially the night was less stable, where a 5 degree warming happened around 2 AM and only after that started to drop rapidly again. It reached below 0 for the first time at 11:17 PM, while the continuous negative temperatures started from 0:35 AM. The minimum of -16.5 degrees Celsius was reached at 6:03 AM, thus slightly exceeding the first nights low.
I was present in the area, spending the night at my camp and checking the logger in the coldest early morning period. During the dark hours I observed no clouds at all and only weak, sometimes moderate wind on the slope. Regarding the earlier mentioned abrupt night warming, as I can’t recall any clouds on this night, the disturbance was likely caused by wind alone. On the bottom was calm again at dawn and morning.
Fourth day and night (20-21th September)
I left the research area after sunrise and hiked back to Alota from where traveled to Uyuni by bus. In Alota was still completely clear and close to calm around midday. The diagram shows the characteristic undisturbed morning warming, passing the 0 degree mark again around 8 AM and continuing the rapid increase above 6 degrees Celsius at 8:50 AM. With a more moderate but still generally constant ascending slope, the peak of 18.4 degrees was reached at 3:09 PM, which together with the morning’s -16.5 C represents an impressive 34.9 degree daily amplitude, the biggest until now.
The obvious drop started after 5:30 PM, again cleaner in the late afternoon and more fragmented in the evening and night, but keeping the big lines. This pattern seems to be a generality here. The continuous negative values appeared early on at 9:57 PM. This time the lowpoint of -14.6 degrees was reached earlier, around 4:30 AM as after that was a significant increase to -6.8 C at 5:30 AM, dropping afterwards back to -14 around 6 AM. Wasn’t present, but probably wind disturbance again.
Fifth day and night (21-22th September)
I was in Uyuni on this day where observed a partially cirrus covered sky, spotting a few undeveloped cumulus clouds over the mountains. From the diagram the conditions in the Alota area are still stable, this time the fast morning warming raised the temperature above 0 already around 7:45 AM.
The curve follows a very similar pattern as in the previous days, reaching a little higher peak (19.5 degrees) shortly before 4 PM. Then comes the cleaner afternoon drop and the more disturbed evening and night. Reaching 0 degrees at 10:13 PM, the continuous negative values started from 11:11 PM. With a low of -11.7 C around 6 AM is still a cold morning, though less frigid than the past ones. Huge 34.1 degree daily amplitude, second biggest until now.
Sixth day and night (22-23th September)
I don’t remember the sky’s state on this day as was resting inside my hotel room most of the time (flu). The day part of the temperature curve is similar to the previous one, exceeding 0 degrees after 7:30 AM and reaching the peak of 19.9 at 2:45 PM.
The night cooling is somewhat similar (more disturbed after the evening) but weaker, reaching only -6.8 degrees at 5:35 AM, preceded by some 3-5 degree ups and downs in shorter periods of time. First negative value appeared at 0:29 AM and became continuous from 1:37 AM. The 31.6 degree daily fluctuation is still solid.
Seventh day and night (23-24th September)
Except some more developed convective clouds above the mountains in the east the sky was still clear in Uyuni. It’s the warmest day of the research, the 0 degree mark was exceeded already around 7:15 AM, while the maximum climbed to 20.3 C at 1:17 PM.
No significant changes in the curve’s general pattern, but the drop is even less potent this time, becoming continuously negative only from 4:49 AM and reaching a mild -5.4 C low at 6:15 AM. The 27.1 degree amplitude is less pronounced than in the previous days when it was constantly well above 30 degree.
Eighth day and night (24-25th September)
On this day I made a longer hike to Salar de Uyuni where the sky was clear, observing developed convective clouds only above the eastern mountains. However, now the air was a little dusty with less visibility than in the previous days.
Again, no mentionable changes in the curve’s rising part, shortly after 7 AM reached 0 degrees and raised to 19.4 C at 1:09 PM following the general “convex pattern”. This was followed by the weakest night, the temperature becoming negative only from 5:35 AM and recording the warmest low of -4.1 degrees at 6:33 AM. This was preceded by more than 6 degree short term ups and downs, warming up to positive 9.4 degrees not long before midnight. Compared to the first days, the 24.8 degree amplitude is also modest.
Ninth day and night (25-26th September)
Another less memorable day regarding the ambience as was again mostly inside because of UV damage to my eyes (reflected rays on the saltpan), but I remember it was pretty bright in Uyuni, as it seriously disturbed my vision around midday.
The graph shows the same clean and strong morning rise with the temperature passing over the 0 degree mark not long after 7 AM and reaching 19.0 degrees at 1:55 PM. But at 3:13 PM the diagram is suddenly interrupted. I concluded that the tripod was overturned in this afternoon by the violent westerly wind and the sensor’s cable was disconnected due to the fall.
Tenth day and night (26-27th September)
At dawn I went back to Alota by bus, where the morning was quite chilly, likely below -5 degrees. After the 20 km hike I reached the research spot before 11 AM. Fortunately the trouble caused by the wind was only temporary and the logger could continue recording the temperature after reconnecting the sensor. The day was completely clear (only some convective clouds in the far horizon above the southern mountains) but windy again in the early afternoon when I made an ascent to the peak of Jubina mountain.
The day’s maximum was 19.1 degrees Celsius and was reached at 2:39 PM. I’m pretty sure that today the amplitude exceeded the 30 degree mark again, but having no concrete data from the night this is only an opinion. Nonetheless, I have data from the following one which was the coldest in the entire research period. The first freezing appeared around midnight, while the continuous negative values started from 0:37 AM. Despite being again less stable in the late evening and in the first part of the night when we can observe 3-4 degree short term ups and downs on the graph, the completely clear conditions favorized to descend very deep in the second part of the dark hours, reaching an impressive -19.0 degrees at 6:23 AM shortly before sunrise. This together with the previous day’s 19.1 degree maximum gives a staggering 38.1 degree amplitude in under 24 hours. The logger was stopped at 7:01 AM when the screen showed -11.6 degrees.
The average temperature of the 10 days research is 5.7 degrees Celsius. The mean minimum is -12.2 degrees, while the mean maximum 18.5 degrees, which give a mean daily amplitude of 30.7 degrees. We can observe that the mean temperature is clearly higher than we could expect solely from the minimums and maximums, which is caused by the fact that the positive part of the curve has a rounded peak, while the negative one is much sharper, technically V shaped.
General conclusions
- The sky is predominantly clear both day and night, altocumulus lenticularis being the most common cloud type.
- The wind follows an obvious pattern: in the night and morning is weak or calm, while the afternoon is much more windier, when the westerlies are dominating.
- The daily temperature fluctuation is huge, usually exceeding 30 degrees, possibly producing one of the biggest average amplitudes on Earth.
- The warming after sunrise is much faster than the cooling after sunset, regularly reaching or exceeding 10 degrees in one hour.
- Regarding the finer details, interestingly the short period disturbances in the temperature’s course are always bigger in the evening and even in the night than in the late afternoon when the drop is much smoother, but it seems that this doesn’t correlates with the wind speed.
- More than 1 degree fluctuations can appear often in very short periods of time even in the coldest, virtually windless mornings.
- There is a clear difference between the positive (related to the maximums) and negative waves (related to the minimums) of the temperature curve, the former having a rounded, while the latter a more angled shape. That means the cooling is pretty constant until the morning without approaching an equilibrium state (very low humidity), thus, before sunrise is often 15, but sometimes even 20 degrees colder than around midnight.
- Due to the elevation and the tropical setting the UV radiation is extremely high.
- There is striking difference between sun and shade, without wind the same temperature feels much warmer than at sea level.
Comparing my logger’s results with the official data of the local weather stations
After finishing the research I managed to obtain valuable information at Uyuni airport meteorological station, thanks to the helping nature of the hotel staff. Below is a chart with the minimums, maximums and amplitudes registered in the same time period:
The most obvious thing is the huge discrepancy between the minimum temperatures, Laguna Seca being colder with 6 degrees on average and 8-10 degrees during the frostier days, while in the milder (less stable) ones they are pretty close to each other. There is no doubt that this is caused by the much stronger temperature inversion in the closed basin, which manifests itself during the calmer periods. Even if Uyuni too is situated on the floor of a basin, the more extensive flat terrain doesn’t permit the same level of cold air pool formation.
On the other hand, the highest temperatures are very similar on average, the closed basin being even slightly warmer despite the higher elevation, which under the same circumstances should gave him a disadvantage of around 2-2.5 degrees. This is probably caused by the stronger föhn effect in the Alota region, where the western mountain ranges are much closer and the dominant wind is blowing exactly from their direction.
The partially missing Ogimet data shows that Potosí weather station which is situated on the eastern part of the Altiplano and higher than Uyuni (above 3900 m) had 19-22 degree maximums in this period (likely föhn effect too), but also much weaker minimums (-3, 2 degrees) certainly due to the more exposed topography.
Regarding the amplitudes, thanks to the big advantage on the low temperature front, Laguna Seca exceeds Uyuni with a good 6 degrees, but sometimes the discrepancy can be more than 10 degrees. While Uyuni’s biggest amplitude in under 24 hours was 28.6 degrees (from 19.4 to -9.2 degrees) in the period of my research, the closed basin reached 38.1 degrees (from 19.1 to -19 degrees). No doubt that this place can surpass 40 degrees in the best circumstances. Taking into account that Uyuni has one of the biggest average daily fluctuations of all the official weather stations I’ve encountered until now on the net, a place which is regularly 5-10 degrees ahead of this definitely can be considered worth of studying in the long term. The future is open as always, we’ll see.