## 4 Extended Discussion of Seasonal Differences  
Furthermore, we discuss the seasonal difference of aerosol, cloud, and water vapor variations and the correlated brightening/dimming in South Asia from 2006 to 2015. Figure 9 illustrates the recent changes in aerosol loading and SSR during the different seasons, namely, pre-monsoon, monsoon, and dry seasons, respectively, where the three seasons occur in March-May, June-September, and October-February, respectively. The AOD consistently increased around the entire year (Figure 9a), whereas the increments of the AOD were greater during the pre-monsoon and dry seasons, by 0.08 and 0.11, respectively, than during the monsoon season from 2006 to 2015. The significance level of the AOD variation detected by MODIS was higher during the non-monsoon season (\(p\) < 0.01) than during the monsoon season (\(p\) < 0.22), primarily because of the disturbance of widespread clouds during the monsoon season [(16)]. The AOD detected by MODIS is inevitably affected by cloud contaminations caused by thin and mixed-phase clouds undetected by MODIS, broken cloud layers, and precipitating clouds [(53; 54)]. Moreover, the accuracy and significance of MODIS observations are considerably lower during monsoon season with widespread clouds than during the pre-monsoon and dry seasons with insufficient clouds in South Asia [(55)].  
With the resultant change of aerosol, cloud, water vapor, and others factors, the average SSR (red line) in the all-sky conditions from CERES increased by 16.2 W m\({}^{-2}\) increments during the monsoon season in South Asia from 2006 to 2015. These phenomena indicate that the attenuating effect of the atmosphere on solar radiation gradually weakened, thereby increasing the incident solar radiation at the surface (brightening). Moreover, the significance level of 0.40 during the monsoon season also indicates the instability in the recent transition from dimming to brightening [(9)]. However,  
Figure 8: Temporal variations in spatial average PW in (**a**) all-sky and (**b**) clear-sky conditions at different ranges of altitude during the monsoon season in South Asia from 2006 to 2015. The dashed lines indicate the linear fit line of the solid line with the same color; \(p\) is the significance level.  
Figure 9b displays no distinct change in the SSR during the pre-monsoon season, while even the significant reverse trend of SSR during the dry season (dimming) compared with that of during the monsoon season.  
Many reasons can contribute to the seasonal difference of SSR variations. Aerosols dominate the change in the solar radiation that reaches the surface in clear-sky conditions [9]. Therefore, aerosols exhibit a greater weight effect on solar radiation during the dry season than during the monsoon season. The increase in aerosol loading directly contributes to the decrease in the SSR, thereby resulting in the dimming during the dry season with low cloud cover, which is consistent with the study of [PERSON] and [PERSON] [15]. In addition, the resultant of the impacts of 3D aerosol, cloud, and water vapor are considerably complicated and varied given the intensive atmospheric activities during the monsoon season, and hence, this observation possibly contributes to the fluctuation of SSR variations during the monsoon season. The pre-monsoon season is the transition period from the dry to the monsoon seasons and is associated with establishing the southwestern wind regime over South Asia [44]. The atmospheric condition and radiation variations are indistinct compared with the trend of the SSR between the dry and monsoon seasons.