## 7 Summary and Conclusions  
Terrestrial fog characteristics are different in terms of distribution of droplet diameter depending on the source of the cloud. For example, night radiation (convection) fog has a lower size distribution \(<\)5 \(\upmu\)m; then, advection maritime clouds are \(\sim\)10 \(\upmu\)m [29]. Both are common in the Mediterranean basin areas and may perturb electromagnetic radiation differently because of the effect of Mie scattering. However, in the case of mmW propagation, the size parameter allows for the assumption of Rayleigh approximation and hence the most relevant attenuation effect in the atmosphere is due to absorption rather than scattering. Therefore, in this experiment, a greater concern was given to carefully stabilize the droplet size distribution of the cloud, from which the total water concentration could be derived, rather than simulating a particular cloud distribution scenario. The reliability of determining the concentration of this extremely high liquid water content sources (\(\sim\)24 gr/cm\({}^{3}\)) allows us to examine the mmW phase retardation at a relatively short distance (18 m), which simulates a cumulative potential effect of mmW propagation in the open atmosphere. Propagation of millimeter wave radiation in the atmospheric medium is characterized via the complex refractivity of the particle composing the air. In foggy conditions, the water droplet suspended in the air causes attenuation and phase shifts, which are determined by the imaginary and real parts of the refraction index, respectively. Experiments performed in a controlled chamber for different fog densities show comparable results to those estimated by the MPM model. It is demonstrated that attenuation is growing as the water droplet concentration increases (the visibility goes down). In Figure 15a, the attenuation coefficient \(\alpha\) is described as a function of the fog visibility, revealing an attenuation range up to two orders of magnitude above that of clear air. Consequently, in mmW links operating in the presence of fog, the transmission power should be increased in order to accomplish the same link budget as that of a link operating in clear weather conditions. The graph of Figure 11 shows that there is a decrease of 1.5 dB in the received power intensity for a propagation distance of 36 m as in the experiment. Thus, attenuation of 0.042 dB per meter is expected in similar outdoor foggy conditions. For a distance of 100 m, an attenuation of 4.2 dB is anticipated, while for 1 km it will grow to 42 dB. During fog, the attenuation becomes a significant factor to be considered for maintaining the required link budget necessary for reliable mmW communications even in relatively short distances. The water suspension also leads to an increase in the real part of the refractivity, slowing down the group velocity of the mmW signal. This effect causes a dispersive phase shift that was measured during the experiment for different visibilities (see Figure 15b). The study can be used for estimating the link budget of wireless links operating in the millimeter wave regime and may also contribute to the understanding of the observations by liquid water clouds radar in the Ka-band. It is focused on the frequency band recently allocated for the 5G cellular network. Heavy fog results in a severe attenuation that may cause a reduction in the signal-to-noise ratio to levels that may disrupt the availability of the communications. Phase dispersion may cause interference in high symbol rate transmissions. **Author Contributions:** Conceptualization, Y.P. and G.A.P. ; methodology, Y.P., G.A.P., A.E. and O.S. ; software, G.A.P., A.E. and Y.G. ; validation, Y.P., G.A.P., A.E. O.S. and Y.G. ; formal analysis, Y.P., G.A.P., A.E. and Y.G. ; investigation, A.E. O.S. and Y.G. ; resources, A.E. O.S. and Y.G. ; data curation, A.E. O.S. and Y.G. ; writing--original draft preparation, A.E. O.S. and Y.G. ; writing--review and editing, Y.P. and G.A.P. ; visualization,, Y.P. and G.A.P. ; supervision,, Y.P. and G.A.P.