## 3 Statistical Analysis of the Collected Data for Different Seasons
### Pre-Summer and Summer Measurements  
In Qatar, May is considered to be as pre-summer season. Figure 5 illustrates the behavior of FSO link during this season. As shown from the plots, the PDR fluctuates significantly during one day where the average temperature exceeds 40 \({}^{\circ}\)C and the humidity oscillates in low levels (does not exceed 50%). The temperature has a strong negative relation with the PDR as it can be seen from the regression analysis and t-test statistics done in Table 2. In other word, if the temperature increases the PDR drops. The humidity does not affect the FSO communication performance as seen in Table 2, which can be attributed to the low humidity environmental parameter, which is not strong enough to affect its performance. Next, to validate this suggestion, FSO behavior during summer season is also presented. Figure 4: Behavior of FSO link vs weather factors during March 2015 (spring season). Figure 5: Behavior of FSO link vs weather factors during May 2015 (pre-summer season). Figure 6 illustrates the average PDR variations along with the average Temperature (T), Humidity (H) and Irradiance (I) for June, July, and August (i.e., summer season). It is evident from the plots, that the FSO performance is not steady in the summer and changes significantly during the course of the day. For the three months, the plots have the same behavior: healthier PDR by nightfall and lowest PDR in the afternoon. In June when summer starts (highest temperature and solar irradiance), the FSO showed the lowest performance. In July when the temperature and solar irradiance are highest, the FSO performance is the lowest during the afternoon. It is also evident that there was a steady rise in temperature and solar irradiance in the afternoon of July which contributed a large degradation of the FSO performance, i.e., from 90% to 73% PDR. In the other months of July and August, the increase in the solar irradiance and the temperature were not as steady as June, and thus the effect was also not that intense, as the PDR was in the range of 80% to 90%. It follows that for June to August, one can conclude that the degradation is recorded from 09:00 AM to 03:00 PM where the PDR falls below 80%. However, at night and early morning, the PDR achieves a maximum of 92%. The below results indicate that humidity varies opposite to temperature and that an increase in temperature can affect the FSO link more than an increase in humidity. In addition, the temperature increases with the solar irradiance. These two factors have a great impact on the FSO performance, which can also be validated from the regression analysis and t-test statistics shown in Table 3.  
High temperature and large amount of solar irradiance affect critically the FSO link and cause degradation of the link performance. From the results, a linear relation between temperature, humidity and irradiance can be seen. A strong relation is observed with the temperature and solar irradiance in all the 3 months compared to the humidity effect. The humidity is not high enough to cause degradation of the FSO link performance while the temperature was high enough to cause such degradation. Irradiance does not increase much with temperature, thus does not affect the performance much. However, from all the above results, it is evident that the temperature has a strong negative correlation with PDR, where it affects it highly in summer than in winter and spring. After continuous tests of the FSO performance and correlating its performance with the weather factors and studying the refractive index variations, it is found that the temperature has the biggest and noticeable effect on FSO behavior than any other factors. This adds a more insight on what really affect FSO performance as compared to the previous results of the authors [14].