# Impact of Weather Factors on Unmanned Aerial Vehicles' Wireless Communications
## 3 Results and Discussion
### Atmospheric Weather Variations
Although the humidity influences the attenuation peaks at 22 GHz, the peaks remain within 1 dB, a relatively insignificant value when compared with the free space path loss. For frequencies higher than 22 GHz, the attenuation decreases before exhibiting a subsequent peak at 60 GHz. However, since the 60 GHz absorption band is exclusively associated with atmospheric oxygen, the variation in water vapor density does not show any impact on the attenuation by the frequency band. Figure 8d illustrates the effect of temperature for the four values \(-\)40, \(-\)25, 25, and 40 \({}^{\circ}\)C. The propagation distance, height, water vapor density, and pressure were maintained at 2000 m, 400 m, 23.01 g/m\({}^{3}\), and 1013.25 hPa, respectively. The expected attenuation peaks at 22 GHz and 60 GHz show a marked difference in their characteristics. While the attenuation peaks corresponding to different temperatures converge at 22 GHz, at the 60 GHz oxygen absorption band, a higher attenuation was observed with lowering of temperature. Attenuations for the lowest temperature of \(-\)40 \({}^{\circ}\)C exceeds the attenuation at the highest temperature of 40 \({}^{\circ}\)C by approximately 25 dB. This is a significant increase attributed to increased absorption of electromagnetic energy by the oxygen molecules at lower temperatures. The attenuation in the lower frequency region i.e., [0, 22 GHz], shows an increasing trend along with frequency, the magnitude stays below 1 dB. This implies that for lower frequency UAV/GCS operation, the attenuation due to atmospheric gases is minimal. For higher frequencies, the attenuation peaks at 22 GHz and 60 GHz are due to the resonating electric and magnetic moments associated with water and oxygen molecules. Water molecules have a non-zero electric dipole moment, whereas oxygen molecules have a non-zero magnetic dipole moment. As a result, the electric and magnetic fields of the propagating electromagnetic waves interact with the water and oxygen molecules, respectively, leading to their transition to a higher energy state. Seeking a more stable state, these molecules subsequently give off a secondary emission and come back to a lower energy state. This self-perpetuating phenomenon is thus responsible for increased attenuation and secondary emission at the resonant frequencies. As the frequency increases, the energy of the EM wave increases, leading to increased energy absorption by the water and oxygen molecules within their respective absorption bands. The increased level of electromagnetic energy absorption in these frequency bands results in higher attenuation. Increasing the propagation distance always adds to attenuation. Variation in height for a given propagation distance has negligible impact on attenuation. Other than the distance, variation in water vapor density and temperature have a noticeable impact on attenuation in the 22 and 60 GHz bands, respectively. Our study shows that temperature variation has a bigger impact (up to 25 dB) on the 60 GHz oxygen absorption attenuation. Comparatively, at 1 dB maximum variation in attenuation, the 22 GHz water absorption attenuation has a much-diminished effect. It is imperative that link budgeting between the GCS and the UAV be performed with the worst-case scenario in mind to ensure reliable operations under all conditions. Figure 9a-c illustrate the impact of atmospheric clouds and fog on signal attenuation across the same frequencies as used in the case of gas-based attenuation. In this simulation, predetermined values for pertinent parameters associated with UAV flights were utilized. Given that clouds and fog contain larger micro-clusters of water molecules in liquid form, liquid water density was employed to quantify the amount present in a unit volume, rather than water vapor density. Figure 9a depicts the attenuation as a function of the carrier frequency for varying propagation distances (500, 1000, 1500, and 2000 m).