The tripod with the minimum thermometer was set around 5 PM at the bottom of the basin. The sky was clear, no air movement, still sunshine at that time. The alcohol column of the instrument showed -28 degrees Celsius. Shortly after the installation became shadowed and until 17:40 (when I left the hollow) the temperature decreased to -33.5 degrees. Near the snow surface I measured -36 degrees.
The first part of the night was completely clear and calm at my tent. After 3 AM (it was almost full moon) I observed some cirrus, than cirrostratus clouds invading the sky. At dawn I measured -17 degrees at head level and -23 close to the surface near my tent. In the hollow was -35 degrees in the early morning (-37 close to the surface), while the minimum of the night reached -41.9 degrees Celsius. This will remain the lowest temperature captured in the entire research period. In the same night Tsetsen Uul (the most representative weather station of the area) reached -38.4 degrees.
The 13-17 January interval
I left the research area in the morning of 13th January. As I had no logger to record the temperature curve, the single certain data what I will have after my return is the minimum value for the entire missing period. I visited the hollow for the second time in the afternoon of 17th January, when I found the index of the instrument stopped at -38.5 degrees Celsius. I can’t be sure which night gave this value, can’t even exclude the same morning when I left the basin. However most likely happened on the night from 14 to 15 January, when Tsetsen Uul reached -37.2 degrees.
Second research night (17-18 January)
The afternoon was changing from full sunshine to partly cloudy (cirrus-cirrostratus with some altocumulus lenticularis) and was warmer than on 12th, with the temperature around -21 degrees. The evening started well with clear sky and still air, but later in the night it became windy. No clouds till the morning, but the intensified wind completely destroyed the inversion of the basin. The temperature raised from the -40.9 minimum value to above -20 degrees before the sunrise. The same pattern can be observed in the statistics of Tsetsen Uul, which recorded -36 somewhere in the early night hours and warmed up to -19 degrees till the morning.
The 18-22 January interval
I left the research area in the morning of 18th and visited the hollow again in the afternoon of 22th January, when I found the thermometer’s index stopped at -39.6 degrees Celsius. This almost certainly happened in the morning of 19th January, when Tsetsen Uul recorded -35.6 degrees.
Third (last) research night (22-23 January)
The entire day was generally fine, also much warmer, with the afternoon temperature of -16 degrees. There were only intermittent light air movements, but thinner cirrus and cirrostratus clouds were partially covering the sky constantly. This setting continued also in the night, when the sky was starry, but slightly blurred. No wind till the morning. I’ve measured -17 degrees near my tent at dawn, which was a surprisingly high value, as I saw 2 degrees colder in the early evening hours in the same place (!) In the basin the minimum thermometer’s index was stopped at -36.8 degrees, while the temperature at the moment was also close to this value (around -36 degrees). This was a good night, very likely presented the characteristic temperature curve, but because of the too high starting temperature couldn’t reach too low, despite the 20 degrees of decrease.
But the most impressive happening was when I descended in a hollow between the dunes, just beside my tent. Certainly no more than 10 meters deep and with the drainage area not bigger than a soccer field. To my great surprise at the bottom of this banal concavity the head level temperature was -37 degrees Celsius at 8 AM (already -35 in the earlier part of the night), a full 20 degrees lower than on the nearby ridge and even slightly colder than the minimum for the entire night in the big basin with hundreds of times more drainage area! Meanwhile the near surface temperature was -39 degrees.
Compared with Tsetsen Uul the Burgastyn Els station reached lower minimums in all the 5 described intervals. The smallest difference is 1.1 degrees, the biggest 4.9 degrees, the average 3.0 degrees Celsius. In three of the five cases the difference was bigger than 3 degrees. According to the statistics on Ogimet Tsetsen Uul had only 8 cm of snow cover that time, while the researched basin had around 20 cm. In my opinion this could give an advantage of 1-2 degrees, but not above 3 degrees.
It can be noticed that the three bigger differences coincide with the three coldest measurements in the frost hollow, while the remaining two with less impressive discrepancies are the ones, when the cold was less severe. Tsetsen Uul’s lowest reading was also on 13th January, but was 3.5 degrees milder than in the high desert.
The most radical conclusion results from the comparison between the small hollow near my tent and the much bigger and deeper researched basin. It is true that it was only a single night, but as the two places are very close, the snow cover similar and the general conditions were characteristic, I am strongly inclined to believe that approximately the same thing would happen during any nights with good potential for strong thermal inversions. I also think that most of the same sized and shaped hollows (which are very common in this desert) will cool the same way as the observed one.
This case is especially interesting as the place is situated right on the saddle, not in an already cooled air mass as it would be somewhere inside the bigger basin’s endorheic sector. Most likely all that impressive “thermal plunge” came from the potential of the small scale topography. Beside these this hollow don’t even have a particularly good sky view factor, as a decent percentage of the slopes have the inclination above 20 degrees.
I have three main conclusions related to the above mentioned facts:
-First is that (regarding the drainage area) size doesn’t really matter at all.
-Second is that a 8-10 meters of depth is enough to approach the maximum potential of a frost hollow. The thermal drop will happen in a more abrupt way than in the deeper basins.
-Third is that relatively steep slopes are suitable (maybe even necessary?) for fast and efficient cold air pooling, despite the altered sky view factor.
As a summary this season was a modest one without a single really good night in the researched period, but the last one’s teaching compensated me for the lack of minuses. A return to the area (hopefully with all the necessary devices this time) is likely.