## 2 Characterization of Cloud Types and Their Impairments
### THz Propagation Properties and Models  
Most THz measurements performed so far focus on indoor short-range scenarios that are limited in terms of propagation distance and usually require perfect synchronization between the transmitter (Tx) and receiver (Rx). The latter is due to the lack of fully functional communications equipment available for frequencies higher than 100 GHz. Specifically, there are very few outdoor measurement campaigns at frequencies above 100 GHz, and they are primarily focused on LoS propagation using either reflected materials or VNA-based extensions of the RF-over-fiber system [18].  
While the principal THz-specific propagation effects have been characterized in detail so far [14; 19], limited THz channel measurements and models are available for mobile THz systems. These results are scarce and target very specific scenarios, e.g., vehicular-to-vehicular [20; 21], train-to-infrastructure [22], indoor [23; 24], as well as static use-cases [25]. Specifically, the detailed THz measurements campaign for vehicle-to-vehicle (V2V) communications environment performed in [21] has revealed that the major channel parameters are drastically different from state-of-the-art mmWave measurements at 28, 60, and 72 GHz [26; 27], suggesting that the same will hold for THz systems. The human body blockage THz measurements performed in [28] also qualitatively agree with these conclusions. The studies in [29; 30] further confirmed these observations with a measurements campaign performed in the office and in microcell environments and proposed a 3 GPP-style model fitting the measurement data. Ref. [31] used measurements in an indoor conference room at 140 GHz to develop a mixed ray-tracing and stochastic channel. Several channel models for the THz band were compared and evaluated in [32]. The penetration losses in an office environment have been reported in [33]. In general, the information in studies on outdoor signal propagation in the THz range is sufficient to analyze the influence of water vapor, oxygen, and other atmospheric components.  
The impact of atmospheric effects such as rain, fog, snow, and clouds is significantly less studied in the THz band. Specifically, rain attenuation is the most severe effect at frequencies below 90 GHz [34]. At frequencies above 100 GHz, the effects of rain tend  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Characteristic** & **Description** \\ \hline \(\gamma_{c}\) & Cloud attenuation per kilometer, dB/km \\ \hline \(\sigma_{e}\),\(\sigma_{s}\),\(\sigma_{a}\) & Extinction, scattering and absorption coefficients \\ \hline \(N(r)\) & Particle size distribution of clouds \\ \hline \(K_{l}\) & Attenuation coefficient, (dB/km)/(g/m\({}^{3}\)) \\ \hline \(\omega\) & Liquid water content, g/m\({}^{3}\) \\ \hline \(p_{u}(t)\) & Cloud liquid water density \\ \hline \(L\) & Cloud attenuation for particular distance, km \\ \hline \(P_{out}(f,d)\) & Received power aircraft antenna, dB \\ \hline \(K(f\),\(x)\) & Overall absorption coefficient \\ \hline \(L_{C}(f,d)\) & Clouds overall attenuation, dBm \\ \hline \(L_{A}(f,d)\) & Absorption loss, dBm \\ \hline \(p_{C}\) & Cloud blockage probability \\ \hline \(p_{k}(f,d)\) & Cloud blockage probability on layer k \\ \hline \end{tabular}
\end{table}
Table 1: The main notation utilized in this paper.  
to be constant, while attenuation due to water vapor and clouds becomes more relevant. To improve knowledge of atmospheric propagation at millimeter and THz frequencies, the work should be focused on conducting experimental measurement studies aimed at confirming the validity of existing attenuation models, and further studying the interaction between radiation and the atmosphere, especially in the presence of water particles [35; 36]. The most important factor is that it is water particles and liquid crystals in the clouds that will be the catalyst for signal dispersion, and accordingly, under such conditions, power losses will be extremely high.