## 7 Other Temporary Atmospheric Attenuation Factors  
Although gaseous absorption is the most dominant impacting factor for signal transmission, other atmospheric factors provide severe signal attenuation such as rains, clouds, and fog. The occurrence of these tropospheric phenomena depends on the regional and seasonal climatic characteristics where rainy and humid regions suffer from these factors most of the year [26]. The rain rate, signal frequency, along with the path length of the signal passing through rain all determine the rain attenuation as recommended by ITU-R P.838-3 [28] which will be used for simple calculations in this paper, while other rain attenuation models can be also applied [26]. For a stratospheric 5G aircraft network, frequencies below 10 GHz suffer slightly at light and moderate rain conditions as shown in Figure 16 for an aircraft located at the airport and communicating with SB. The curves in this figure assume a worst case of 300 km length of the rainy region that the signal passes through. At 6 GHz, the very heavy rain conditions result in 4.4 dB loss while at 10 GHz it becomes as high as 21.7 dB. For frequencies above 10 GHz, the rain attenuation becomes very impacting which almost results in signal loss especially at very high frequencies. Only light rain conditions can be tolerable while other rain rates almost shadow the signal with more than 100 dB loss at frequencies above 30 GHz during very heavy rain conditions. For a network of SB with handover operation, an aircraft-SB maximum LOS passes through the troposphere and the rain path may be 500 km long. The rain attenuation performance for this case is shown in Figure 17 where a higher attenuation occurs, especially at higher frequencies.  
The other main affecting temporary factor is the fog/cloud attenuation. This attenuation is insignificant for frequencies below 10 GHz. However, as a worst case, the clouds and fog may be extended over the whole horizon from the aircraft towards SB during airport-climbing/descending phases and maximum LOS distance such as in tropical regions or over oceans near equator circle. The air temperature and the cloud water density along with the communication distance will determine the fog/cloud attenuation according to the ITU recommendations ITU-R P.840-8 [29]. As the LOS from an aircraft to SB passes through different tropospheric layers, then the temperature profile should be inserted in the fog/cloud attenuation equation for accurate calculation.  
Figure 16: Airport maximum rain attenuation for aircraft–SB communication.  
As most cloud peaks do not increase more than 12 km, then the total fog/cloud attenuation, \(L_{FC}\), in dB can be given by:  
\[L_{FC}=\sum_{n=1}^{710}\Upsilon_{fc}\rho_{n} \tag{32}\]  
where \(\Upsilon_{fc}\) is the fog/cloud specific attenuation in dB/km [29] and \(\rho_{n}\) is the nth air layer length generated by Equation (20) with layer thickness \(\delta_{n}\) calculated from Equation (16) and the upper limit of \(n\) is 710 calculated to achieve the total height of 12 km as follows:  
\[\sum_{n=1}^{710}\delta_{n}=12\text{ km} \tag{33}\]  
The atmospheric temperature of the \(n\)th layer used in the calculation of \(\Upsilon_{fc}\) can be given by:  
\[T_{n}=15-6.5h_{n}\text{ Celsius} \tag{34}\]  
where \(h_{n}\) is the height of the \(n\)th layer in km and can be calculated as from the following summation:  
\[h_{n}=\sum_{m=1}^{n}\delta_{m} \tag{35}\]  
The fog/cloud attenuation performance with frequency is shown in Figure 18 where approximately 13 dB of minimum attenuation level occur at 10 GHz for an aircraft located at the airport and increases to approximately 32 dB for the maximum LOS case. Severe fog/cloud attenuation occurs at higher frequencies which indicate a total loss of the communication link at these frequencies. For example, at 28 GHz, the loss will be approximately 80 dB for airport communication while it increases to 200 dB for maximum LOS communication.  
Figure 17: Rain attenuation at maximum aircraft–SB LOS distance communication.