## 3 Results of Studying Atmospheric Propagation of subTHz Radiation
In astronomy, adequate surface coverage in the UV plane is essential for radio interferometry, which causes great interest in the EHT team working in the transparency window of 1.3 mm in northeastern Eurasia. In a giant interferometer, it is impossible to replace the complete coverage of the surface with a group of telescopes scattered far from one another. In communication and radar, the surface coverage is even more critical. The challenge of keeping in touch with deep-space missions as they disappear under the horizon is obvious. Looking for meteoroids and locating space debris generally requires a total coverage of the Earth's surface by tracking stations. Unfortunately, our last investigations demonstrate that the North Caucasus has not yet revealed any good places for the 0.8 mm window, and the 1.3 mm window appears only for a short time and only in the near-zenith directions. The reason for this is evident once you take a look at the map of rains: the atmosphere is humid due to the proximity of the Black Sea, while the predominant air currents bring in all the moisture from the Mediterranean area, which then condenses on the Caucasus, acting like a giant condenser and making the use of subTHz instruments problematic. Even the ascent to the heights of 2500-4000 m (Arkhyz, Terskol) did not result in any encouraging numbers. In the summer of 2021, a very adventurous attempt was made to instrumentally assess the future use of Mount Elbrus, the highest point in Europe (5642 m), see Figure 5. Long-term monitoring of satellite and meteorological data made it possible to choose the optimal time for ascent and measurements. The experiment was scheduled for the expected window of August 9 to 11. The expedition was successful, unlike the effort of a group of climberswho followed the same route two weeks later and whose endeavor ended fatally for five of its participants. However, as soon as the equipment was deployed on the Elbrus slope, the weather worsened sharply, the sky's transparency dropped, and it started to snow. The measurements yielded an atmospheric absorption of \(\sim\)0.25 Np with an error of \(\pm\)0.25 Np due to the horrible weather. One might think that this result is of no value at all. However, it has severe scientific significance: the western part of the North Caucasus, including its highest peaks, is too wet and has no prospects for the operation of subTHz instruments due to the limitations of the microwave astroclimate at least up to waves shorter than \(\Lambda\leq 1\) mm. Turning the leaf on this list of relatively negative results, we should speak of a number of optimistic and inspiring results for the Year 2021 and further plans. Using unified approaches and uniform methodology and instruments, we explored a vast territory of northeastern Eurasia to map the conditions for the propagation of subTHz radiation in the atmosphere to compare different sites for placing antenna systems for astronomy, telecommunication, and radar applications. Long-term studies on the Sufffa Plateau (Figure 6b) have shown good prospects for the completion of construction of the RT-70 antenna and the conditions sufficient to ensure its operability up to wavelengths of 3-2 mm, and revealed, at the same time, that it is unfeasible to erect a costly fine-tuned high-quality 70 m mirror with an operation range of 1.3 and 0.8 mm atmospheric windows due to the astroclimatic limitations of the site.