## 1 Introduction  
Radio astronomical observations at millimeter and submillimeter wavelengths are a very important tool for astrophysical research (e.g., [1]). They provide a unique opportunity for detailed investigations of the interiors of the cold dense interstellar clouds of gas and dust, which represent cradles of new stars. The emission peak of these clouds lies in this band. Millimeter and submillimeter waves are very rich in the spectral lines of various molecules, atoms and ions, which can serve as diagnostic tools of physical conditions and chemical content. At these wavelengths, the highest angular resolution can be achieved, which is very important for studies of compact objects, in particular active galactic nuclei. These studies are facilitated by a lower interstellar scattering in comparison with longer radio wavelengths. Bright examples of such a study are recent images of the \"shadows\" of supermassive black holes in the centers of M87 and our Milky Way galaxies [2, 3]. These results were obtained with the Event Horizon Telescope (EHT), which is a global VLBI network of sufficiently large millimeter-wave observatories operating at the 1.3 mm wavelength. Nowadays, there are about 10 such observatories in the world. The success of the EHT stimulates the project of its extension, known as the next-generation EHT (ngEHT) [4]. The concept of ngEHT includes observations at 3 and 0.8 mm wavelengths, in addition to the 1.3 mm band. This emphasizes the importance of all these bands for astronomy. Many locations in the world are considered as candidate sites for new EHT telescopes (e.g., [5]). There is a huge area in northeastern Eurasia, including the whole Russian territory, which lacks such facilities, although many years ago construction of a 70 m radio telescope intended for operation at short millimeter wavelengths started on the Suffia plateau in Uzbekistan [6; 7] (it was frozen after the USSR collapse), and now there are new relevant projects and proposals [8; 9; 10; 11]. The main obstacle to ground-based radio astronomy observations at short millimeter and submillimeter wavelengths, in addition to technical challenges, is the atmospheric opacity, caused primarily by water and molecular oxygen. The observations are possible only in the so-called \"atmospheric transparency windows\"--the bands of relatively high transparency between the strong spectral lines of these molecules. The main windows discussed here are those centered at \(\sim\)90 GHz, \(\sim\)140 GHz, \(\sim\)225 GHz and \(\sim\)350 GHz. They are usually referred to as the 3 mm, 2 mm, 1.3 mm and 0.8 mm windows, respectively. However, even in these windows, the opacity can be quite high. Radio astronomical observations at the sea level are possible only in the 3 mm and 2 mm windows. At higher frequencies, high-altitude locations should be used. While the oxygen absorption is stable and can be rather easily evaluated, the water content is highly variable. Water in the atmosphere is present in two forms--water vapor and liquid water (the latter one mainly in clouds). The amount of water vapor is usually characterized by the PWV (Precipitable Water Vapor) parameter, which is the vertically integrated amount of water vapor in the atmosphere. It is usually measured in millimeters. The amount of liquid water is parameterized by the Liquid Water Path (LWP), measured in g m\({}^{-2}\) or in \(\upmu\)m (e.g., [5]). The zenith opacity (optical depth) of the atmosphere in dependence on frequency (\(\
u\)) is related to these parameters by the following expression:  
\[\tau(\
u)=\tau_{\text{O2}}(\
u)+\beta(\
u)\text{PWV}+\gamma(\
u)\text{LWP}\,, \tag{1}\]  
where \(\tau_{\text{O2}}(\
u)\) is the molecular oxygen contribution to this opacity, \(\beta(\
u)\) is the specific absorption coefficient per PWV unit and \(\gamma(\
u)\) is the specific absorption coefficient per LWP unit.