## 3 Results  
### Monitoring of the Millimeter-Wave Atmospheric Opacity at Selected Sites  
To date, atmospheric absorption studies with the MIAP-2 radiometers have been carried out at several sites in Russia and Uzbekistan [33; 62; 63; 64; 65; 66; 67]. However, a long-term monitoring covering all seasons is limited to the RT-70 radio telescope construction site on the Suffa plateau in Uzbekistan (altitude 2400 m), the site of the BTA telescope of the Special Astrophysical Observatory (SAO) in the North Caucasus (altitude 2040 m) and the Badary observatory in the Baikal region (altitude 813 m). Among these sites, the best atmospheric transparency has been observed on the Suffa plateau [33; 62; 63]. From these measurements, the PWV values were derived, which are in good agreement with the satellite and aerosonde data [33]. Then, the expected optical depth in other atmospheric transparency windows was evaluated. The expected monthly averaged values of the optical depth in zenith at 1.3 mm are \(\sim\)0.3 in winter and \(\sim\)0.8 in summer (Figure 3, left panel). The corresponding monthly averaged PWV values are \(\sim\)4 mm in winter and \(\sim\)14 mm in summer [33] (Figure 3, right panel).  
In addition to long-term monitoring, short-term measurements of atmospheric absorption have been carried out in several locations: the Muus-Khaya peak in Yakutia (altitude 1950 m), the Terskol peak in Caucasus (altitude 3150 m), Svalbard (altitude 36 m), Caucasian Mountain Observatory (altitude 2112 m), Monday, Sayans in Buryatia (altitude 2006 m) and Karadag in Crimea (altitude 105 m). Measurements have recently begun in the eastern Caucas [67]: in Dagestan (Mount Mayak, 2700 m and Mount Shalbuzdag, 4142 m) and in North Ossetia (Stolovaya Mountain, \(\sim\)3000 m).  
Figure 1: (**Left panel**): the scatter diagram between the WVR and MERRA-2. (**Right panel**): correlation between the target PWV values (MERRA-2) and the predicted values on the validation dataset.  
The Muus-Khaya peak is located in the region of the \"Pole of Cold\". The measurements were taken in July and gave quite good results for this season: PWV\(\sim\)5 mm [62], which is significantly better than at the RT-70 site on the Suffa plateau in summer. The expected PWV value in winter is 1-2 mm, which makes this peak a promising site for millimeter and submillimeter astronomy. The short measurement on the Terskol peak did not show good results. The one-day measurements on Elbrus in August 2021 gave rather high opacity, too. Promising preliminary data were obtained for the eastern Caucasus. In May 2023, a long (over half a year) expedition ended in the vicinity of Kurapdag (Chirag village), which basically confirmed the previously formulated hypothesis about the lower humidity of the eastern Caucasus compared with the western one. An interesting feature was observed on Karadag (located near the Black Sea shore). The millimeter-wave absorption there strongly depends on the wind direction and can drop to quite low values when dry air enters from the Steppe Crimea [30]. This effect is apparently due to the specific conditions at this site and hardly can be expected at most other locations.  
All the data collected during numerous expeditions, including the semiannual expedition to Dagestan that ended in May 2023, where the results have not yet been processed or published, are posted in the open-access archive [68].