## 5 Conclusions  
In this work, an empirical experimental model for the attenuation coefficient estimation of an FSO system, which operates in low height over the sea, during night-time, was presented as a function of the atmospheric temperature, the wind speed, and the relative humidity. According to the obtained results that were presented, the model achieved very high accuracy in cases with low wind speed, low relative humidity, high temperature, and without precipitations where the attenuation coefficient was relatively low. Such an empirical model is a very useful tool in order to predict the performance and reliability of an FSO link, in a specific environment with known characteristics and has a great impact in the designing of high demanding modern communication networks.  
Conceptualization, A.N.S., G.A.P., H.E.N., A.D.T. and N.A.A.; methodology, A.N.S., G.A.P. and H.E.N.; software, A.N.S. and G.A.P.; validation, A.N.S., G.A.P., H.E.N., A.D.T., N.A.A. and G.S.T.; formal analysis, A.N.S. and G.A.P.; investigation, A.N.S., G.A.P. and N.A.A.; resources, A.N.S., G.A.P., H.E.N., A.D.T., N.A.A. and G.S.T.; writing--original draft preparation, A.N.S. and G.A.P.; writing--review and editing, A.N.S., G.A.P, H.E.N., A.D.T., N.A.A. and G.S.T.; visualization, A.N.S. and G.A.P.; supervision, A.N.S., H.E.N., A.D.T. and G.S.T.; project administration, A.N.S., H.E.N., A.D.T. and G.S.T.; funding acquisition, H.E.N., G.S.T. and N.A.A. All authors have read and agreed to the published version of the manuscript.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Day** & **Time** & **\(\mathbf{R^{2}}\)** \\ \hline \multirow{2}{*}{6 April 2020 (Figure 9)} & 00:00–01:00 & \textless{}0.5 \\ \cline{2-3}  & 20:00–23:59 & 0.783 \\ \hline \multirow{2}{*}{16 November 2019 (Figure 10)} & 00:00–05:00 & 0.667 \\ \cline{2-3}  & 20:00–23:59 & 0.752 \\ \hline
17 November 2019 (Figure 11) & 00:00–05:00 & 0.791 \\ \hline \hline \end{tabular}
\end{table}
Table 6: \(R^{2}\) values for several days that were not included in the regression method.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Temperature (\({}^{\circ}\)C)** & **Relative Humidity (\%)** & **Wind Speed (m/s)** & **Attenuation Coefficient** & **Attenuation Coefficient** \\  & & & **Mean Value** & **Standard Deviation** \\ \hline
11–12 & 60–61 & 1.0–1.5 & 0.3371 & 0.0058 \\ \hline
13–14 & 74–75 & 1.5–2.0 & 0.3821 & 0.0241 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Mean value and standard deviation of experimental attenuation coefficient.  
**Funding:** H.E.N. and G.S.T. acknowledge that this work received funding from the European Union's Horizon 2020 research and innovation program, under grant agreement No: 871900. N.A.A. acknowledges that this research was co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Program \"Human Resources Development, Education and Lifelong Learning\", in the context of the project \"Strengthening Human Resources Research Potential via Doctorate Research\" (MIS-5000432), implemented by the State Scholarships Foundation (IKY).  
**Data Availability Statement:** Data available on request due to restrictions eg privacy or ethical. The data presented in this study are available on request from the corresponding author. The data are not publicly available as the experiment was held in a military base.  
**Conflicts of Interest:** The authors declare no conflict of interest.