## Results
### Effects of Water Vapor  
The water vapor importantly decreases the incident solar radiation at the surface due to its absorption of solar radiation [12]. Figure 6a,c exhibit the vertical relative humidity (RH) in all-sky and clear conditions, which consistently and gradually decreased with a maximum decrement of approximately 10% during the monsoon season in South Asia from 2006 to 2015. The spatial distributions of the temporal change of the PW are also highly consistent between all-sky and clear-sky conditions (Figure 6b,d). PW decreased in space with a maximum decrement of 6.6 mm in South Asia, especially in the western and central regions. Although a small increase in water vapor (less than 2 mm) occurred in the southern part of South Asia, the increment was considerably lower than the decrease in water vapor in most regions of South Asia. The decrease in atmospheric water vapor may be attributed to the weakened monsoon intensity in the recent decade during the monsoon season in South Asia [51]. Due to the intrinsically positive association of water vapor and clouds, we focus mainly on the effect of water vapor on solar radiation in clear-sky conditions to restrict the disturbance from clouds. Figure 7a depicts that radiation is highly and negatively correlated with PW from the ECMWF-AUX. Thus, water vapor is a substantial controlling factor in solar radiation variability. This control lies in the direct and indirect effects. The direct effect refers to the absorption of solar radiation in which the PW decreased by nearly 2.8 mm (\(p\) = 0.13) over the brightening period. Based on the evaluation of BUGSrad, solar brightening was greater than 2.5 W m\({}^{-2}\) (\(p\) = 0.14), given the weakened absorption caused by the decrease in water vapor. The SSR in clear-sky conditions from CloudSat increased by approximately 4.8 W m\({}^{-2}\) (\(p\) = 0.05). Except for the contribution of water vapor variations, this radiative change was possibly caused by the change in atmospheric conditions and the incoming solar radiation provided by ECMWF-AUX and CloudSat [52]. Figure 5: (**a**) Temporal variations in spatial average CRE at the TOA and surface, and (**b**) spatial distribution of the temporal changes in average SW CRE from CloudSat at the surface 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. The spatial variation in SSR in the clear-sky conditions was highly negative and consistent with that of PW (Figures 6d and 7b), especially in the western and central regions of South Asia, which partly consists of deserts. Furthermore, the indirect effects of water vapor occur in the all-sky conditions, and such effects are called water vapor-cloud interaction. Generally, a high concentration of water vapor favorably generates additional or robust convective clouds and results in decreased radiation at the surface, and vice versa [52]. Moreover, water vapor also regulates cloud amount by affecting the aerosol-cloud interaction, which generally shows a positive relationship under moist atmospheric conditions [53]. Therefore, the effect of water vapor is more complicated and uncertain in  
Figure 6: Temporal variations in spatial average vertical RH in **(a)** all-sky and **(c)** clear-sky conditions; spatial distribution of temporal changes in the average PW in **(b)** all-sky and **(d)** clear-sky conditions from the ECMWF-AUX during the monsoon season in South Asia from 2006 to 2015. Figure 7: Temporal variations in spatial average **(a)** PW (blue line) and SSR in clear-sky conditions from BUGSrad (red line) and CloudSat (orange line); spatial distribution of temporal changes in average **(b)** SSR from BUGSrad in clear-sky conditions 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.