Interpretation of the logger’s graph
As I found the tripod with the instruments fallen over and partially buried under the snow on June 27, I had to found out when this happened. Analyzing the graph I noticed a remarkably elevated maximum temperature of 16.8 degrees Celsius in the early afternoon of June 22. That’s definitively an impossible value at these altitudes. The certitude regarding this conclusion is confirmed by the extreme fluctuation what happened shortly afterwards, when in a matter of a single hour the temperature dropped more than 14 degrees than rised again more than 10 degrees.
This would certainly not happen if the sensor was properly sheltered in the radiation shield. Here the sensor was over-exposed and the extreme temperature change must be caused by alternating windy and calm periods. So the fall over happened before. But when?
The first two days (when I was present in the region) are excluded. At the time the weather was fine, without even moderate wind. The temperature curve of this period is also realistic. Going a little ahead the things started to make sense. In the night of 21-22 the temperature was suspiciously high and constant (between -1 and -5 degrees). This can happen here only during very cloudy or windy weather (or both). And in the next day was the dubious 16.8 degrees maximum. Puzzle solved.
On the other hand in the last 4 days the maximums didn’t climbed above 0 degrees. This is probably because the sensor became snow covered. As the minimums of these days are quite low (approaching -20 degrees) and the temperature curves are looking regular I suppose the weather was again clear and not very windy (at least in the night).
After this short detective work let’s go back to the scientific evaluation. We have only 2 days to take in consideration as having valid datas:
June 20: This first day was the luckiest with both the lowest (-26.6 degrees Celsius) and highest (4.0 degrees Celsius) temperatures, respectively the biggest daily thermal amplitude (30.6 degrees).
During this interval happened a temperature rise of 12.7 degrees in 60 minutes and 6.7 degrees in 15 minutes.
Even if the weather station started its activity more than an hour after the astronomical sunrise, because the crater’s bottom was still in shade the inversion wasn’t affected before the sun reached the lowest elevations. This fact can be clearly seen in the graph, where the temperature remains almost uncheanched between 8:21 and 8:51. The minimum was registered at 8:36 AM.
Then the temperature started to rise very fast and in less than 3 hours and a half climbed to +4 degrees, the maximum of the day. What’s interesting this happened pretty soon, just after midday. In the early afternoon the curve became close to an isotherma, remaining mainly around 2-2.5 degrees Celsius. In the latter part of the afternoon the temperature started to plummet and the vigorous fall continued during the evening hours. I was not present in the crater in this warmest period, but the extreme temperature rise was obvious also outside. It was absolutely no wind even at noon. A light breeze started after 1 PM. This explains why the maximum was registered at 12:16 PM.
June 21: The temperature dropped to -22.1 degrees Celsius in the morning at 7:46 AM. The sharp rise started again just before 9 AM and continued to the noon when reached 3.7 degrees, the days maximum. You can observe an outstanding upward curve during the first part of the night when the temperature rised from -16 to -5 degrees before starting to plummet again. It means even light winds can seriously disturb the night inversions in the basin. The amplitude for this day is 25.8 degrees, quite big, but significantly less than the first days’s 30.6 degrees. The night was clear again, but sometimes I noticed a light breeze at my camp (the same place, around 6 km’s from the crater). The near surface temperature was also less low than in the first night (more than 5 degrees milder). There were some cirrus clouds in the morning. I was present in the surroundings to around 3 PM and the weather was comparable with the day before: sunny, calm or light winds and quite warm for these elevations.
-If the weather is clear and calm the inversion layer in the crater is thick (filling completely the endorheic basin).
-During ideal conditions the lowest temperatures occur around 8-8:30 AM. The inversion is destroyed only well after the astronomical sunrise, when the sun reaches the bottom of the caldera.
-When is partial snow cover the head level air temperature above the snowy part is much closer to the ground level temperature above the uncovered part than to the snow surface temperature just below. The minimum temperatures above the snow surface are much lower than in the air above (up to 8-9 degrees difference).
-Because the first air movements usually start after midday the maximums are registered around noon when the heating of the ground is at peak levels.
-Clear skies are prevalent in the area during winter, but the calm conditions are much rarer. Strong winds (mainly westerly) blow often even at the crater’s bottom.
-The nights and mornings have more calm periods, the afternoon beeing the windiest part of the day.
-During windy weather the night minimums are significantly warmer (up to 20 degrees Celsius).
-The cooling period has a much longer curve than the warming period. The steepest parts of the graph are the ones just after the sun reaches the depression’s bottom, but it continues quite abrupt to the noon without major changes.
-The daily thermal amplitude can exceed 30 degrees, producing probably one of the biggest fluctuations in the entire world. *
-The minimums can go down well below -25 degrees (very likely also below -30 in certain conditions), lower than the current bolivian national record (-31 degrees Celsius) and maybe than any other official temperature measured between the tropics.
*The actual world record for the highest amplitude in 24 hours is more than 55 degrees Celsius and it was observed in Browning, Montana-USA. However this is a completely different kind of temperature change as the big difference was caused by a sudden change in the weather conditions and not by the pure physical potential of the local air.
Comparing my results with the local and global statistics
After the return to La Paz I visited the National Meteorological & Hydrological Service of Bolivia. There I found out some important datas:
-The lowest official temperature ever measured in a bolivian settlement is -25.7 degrees and it was registered in Uyuni (around 3700 meters elevation).
-The lowest official temperature ever measured in Bolivia is -31 degrees and it was registered at Laguna Colorada Weather Station in May 1992, close to the lake with the same name (around 4300 meters elevation).
-On 20 June 2018, when my logger recorded -26.6 in the crater of Aguita Brava volcano the minimum temperature in Uyuni was -9.4 degrees Celsius. Unfortunately they have no infos about Laguna Colorada for the same day. They only sometimes get datas from there. My guess is when those -25.7 and -31 degrees happened it was continuous snow cover in both locations. The difference between a -9.4 and a -25.7 degrees is enormous and can’t be explained otherwise, taking in consideration the weather was constantly clear and even windless on 20 June 2018.
In the period of my staying it was no snow cover in Uyuni nor in any other place in the country situated below 4000 meters and the satellite datas showed the immediate surroundings of Laguna Colorada it was also free of snow. Definitely out of question to go even close to -31 degrees under these circumstances. During my visit the caldera was only partially snow covered and still went down to -26.6 degrees Celsius and close to the snow surface even much lower (-35 degrees). Interestingly the temperature close to the uncovered surface was about the same as the air at head level above the snow. I have no doubt if the same atmospheric conditions are present and there is continuous snow cover in the crater the minimum would go well below -30 degrees, surpassing the lowest official bolivian temperature.
Coldest place between the tropics?
To my knowledge beside Laguna Colorada there is a single place situated on tropical latitudes where the temperature plummeted below -30 degrees. It’s Ollagüe in Chile with -37 degrees Celsius. This is the lowest temperature measured between the tropics what I found on the net. Ollagüe settlement is situated right on the border with Bolivia, but it is also a high volcano named Ollagüe nearby. The village’s elevation is similar with Uyuni’s and much lower than Laguna Colorada’s, so a -37 degrees there is a little dubious for me. Maybe it was registered somewhere on the volcano? Is this data reliable at all? Maybe it was only -27? It looks much more realistic for a place situated at 3700 meters elevation at these latitudes. However, until solid facts reveal the truth this remains an open question.
In my opinion the crater of Aguita Brava is a strong canditate for both the titles “the tropical pole of cold” and “the pole of diurnal thermal amplitudes”.