## 4 Discussion
### Comparison of the Candidate Sites for Millimeter-Wave Telescopes in Northeastern Eurasia  
Long-term monitoring of atmospheric opacity with the MIAP-2 radiometers has been performed at several sites in Russia and Uzbekistan, as described in Section 3.1 (the RT-70 radio telescope construction site on the Suffa plateau, the site of the BTA telescope in Caucasus and the Badary observatory). Among these sites, the best conditions for millimeter-wave astronomy have been observed on the Suffa plateau. However, these conditions are far from being excellent and hardly allow for regular observations at wavelengths \(\lambda\lesssim\) 2 mm. At 1.3 mm, only episodic observations are possible in winter, when the monthly averaged value of zenith opacity at this wavelength drops to \(\sim\)0.3 (somewhat lower estimates of the opacity for this site are obtained in [5]).  
Promising results have been obtained in the short summer measurements on the Muus-Khaya peak in Yakutia (Section 3.1). However, long-term monitoring at this site is needed. Short measurements on the Terskol peak are not conclusive.  
Investigations with various methods (e.g., [9,41,67,72,73,74,75,76]) reveal other promising sites in Eurasia with better conditions in comparison with the Suffa plateau. In the paper in [9], based on the NASA GEOS-FPIT model, the eastern Pamirs and Tibet are shown to be the best places. According to [73], based on the ERA5 reanalysis, very good conditions  
Figure 5: The degradation factor \(R_{\rm D}\) (gray scale) in dependence on the receiver noise temperature \(T_{\rm RX}\) and zenith opacity \(\tau\) for the zenith angle \(\theta=45^{\circ}\), assuming \(T_{\rm a}=250\) K. The curves correspond to \(R_{\rm D}=1.2\), 1.5 and 2 (from bottom to top).  
exist at the Ali 1 site in Tibet (PWV \(\sim\)0.4 mm in winter) and at Muztag-Ata in the Chinese Pamirs (PWV \(\sim\)0.7 mm in winter). Comparable conditions exist in the Sayan Mountains. The Khulugaisha peak, in terms of its characteristics, is close to the sites of Tibet and Pamirs (PWV \(\sim\)0.6 mm in winter). There are promising sites in Altai and Dagestan, in particular the Khorai and Kurapdag mountains [73]. The Terskol peak is also rather good in terms of PWV [41; 77] but not so good concerning cloudiness. The Aktashtau peak in Uzbekistan (3383 m) located near the RT-70 construction site has PWV statistics similar to Terskol [73]. With PWV \(\lesssim\) 2 mm, astronomical observations at least in the 1.3 mm and 0.85 mm atmospheric windows can be quite efficient. Even lower PWV values of \(\sim\)0.5 mm make observations in the higher-frequency windows possible.  
It is worth noting that in rugged terrains, the spatial resolution of the global models can be insufficient to characterize atmospheric conditions on certain sites (e.g., on local peaks). This emphasizes the importance of local measurements with radiometric systems or GNSS devices. High-resolution weather prediction and recording on the cloudiness near the observation site could be promising in this respect, too.  
So far, our measurements have been performed at 3 and 2 mm. Measurements in the 1.3 mm window would be very important.  
In this consideration of the candidate sites for new millimeter- and sub-millimeter-wave telescopes, we have taken into account only the atmospheric opacity. However, there are other criteria which should also be considered. One of them is the stability of the atmosphere [78; 79]. Enhanced instability can lead to strong phase fluctuations [80] and anomalous refraction (e.g., [81]). This factor is still poorly investigated.  
A new large millimeter-wave telescope would be an efficient part of the global VLBI network (EHT). The estimates show that from this point of view, the Caucasus region is the most effective with the existing EHT configuration ([PERSON], private communication). However, the situation will change if a large millimeter-wave telescope is built in eastern Asia. In this case, places like the Pamirs and Tibet will have an advantage.