Free-space optics (FSO), initially studied in the 60s, is attracting attention in the frame of 5G+ systems, which demands wireless back-hauling technologies with extremely high data rates over distances up to a few kilometers. The current lack of a global path loss model for FSO motivates studying the optical link s behavior under different atmospheric conditions. This thesis deals with the development of models for predicting the attenuation due to fog and rain for designing FSO links in 5G and beyond technologies. We study extensive surface measurement databases worldwide as a benchmark for analyzing FSO responses considering available long-term data. We find that the highest impact on visibility worldwide is caused by fog, snow, and rain. Thus, we select fog and rain as the focus of this study. Several path loss models based on visibility are assessed. We propose lower and upper-bound models to estimate the specific attenuation, gamma, due to fog which considers uncertainties as the sensitivity of gamma to the visibility at 1.550 µm and a micro-physical model of fog. Regarding attenuation caused by rain, we study models available in the literature for microwave links and modify them for application to FSO. When the multiple scattering effects are considered, a significant reduction in the rain attenuation has been found. From the findings, we build a method to statistically combine the attenuation effects on FSO caused by different weather conditions. The performance of FSO links is assessed in terms of the maximum link range against availability under four weather conditions: (i) rain only, (ii) fog only, (iii) rain and fog, and (iv) all conditions. Finally, experimental events for FSO and mmWave links implemented at Leonardo Campus of Politecnico di Milano are classified and analyzed depending on the studied atmospheric effects: stratiform rain, convective rain, light rain, wind, heavy fog, snow, and others.