## 5 Conclusions  
In this review, we described methods used to study atmospheric transparency at millimeter and submillimeter wavelengths and summarized the main results of such studies in northeastern Eurasia, with an emphasis on the results of our work. The main agents responsible for the absorption of millimeter and submillimeter waves in the atmosphere are water in various physical states and oxygen. Under clear sky conditions, variations in atmospheric opacity are related to variations in the amount of the precipitable water vapor (PWV). The direct determination of the atmosphere optical depth is usually based on measurements of sky brightness. It can be also evaluated from the PWV value. PWV estimates can be obtained from measurements of the atmosphere emission with water vapor radiometers. Nowadays, the delays of the GNSS (Global Navigation Satellite System) signals are frequently used for this purpose. We proposed the statistical approach for the determination of the PWV values from radiometric measurements based on machine learning algorithms, without addressing the physical model of the atmosphere. Atmospheric absorption at any frequency can be calculated for any selected location using the global atmospheric models now available. We estimate degradation of the telescope sensitivity due to atmospheric opacity.  
Our studies of the atmospheric transparency at the candidate sites for millimeter and submillimeter telescopes have been ongoing for about 10 years already by various methods. The considered sites are located in Russia and in the nearby surroundings (in particular, in Uzbekistan). The atmospheric optical depth has been measured with the dual-band tipping radiometer and evaluated with global atmospheric models. Our results, as well as other investigations, show that there are several sites in this area which can be effectively used for astronomical observations in at least the major atmospheric transparency windows at 1.3 mm and 0.85 mm. In general, the results show that northeastern Eurasia is a promising area for submillimeter astronomy.  
The investigations continue. More detailed studies of several recently suggested sites are planned, as well as an extension of the measurements to the 1.3 mm atmospheric window.  
Conceptualization, I.I.Z.; methodology, I.I.Z., A.V.L., V.F.V. and P.M.Z.; formal analysis, I.I.Z., A.V.L., V.F.V., P.M.Z. and T.A.K.; writing--original draft preparation, I.I.Z.; writing--review and editing, I.I.Z., A.V.L., V.F.V. and P.M.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the IAP RAS state program FFUF-2021-0005.  
Not applicable.  
No new data were created or analyzed in this study. Data sharing is not applicable to this article.  
We are grateful to everyone who supported these works and took part in the measurements and estimates of atmospheric opacity considered here, as well as in discussions of the results obtained.  
The authors declare no conflict of interest. The funders had no role in the design of the study.  
The following abbreviations are used in this manuscript:  
Multidisciplinary Digital Publishing Institute  
GNSSGlobal Navigation Satellite Systems  
NASANational Aeronautics and Space Administration  
VLBIVery Long Baseline Interferometry  
EHTEvent Horizon Telescope  
LSTMLong Short-Term Memory  
LWP Liquid Water Path  
PWVPrecipitable Water Vapor  
WVRWater Vapor Radiometer