## 3 Results and Discussion
### Spatial and Seasonal Variations  
The comparison of global averaged PW between IGRA and COSMIC in four seasons during 2007-2016 is presented in Figure 4. The colored dots denote the results from IGRA radiosonde observations. The radiosonde stations are distributed unevenly in the Northern and Southern Hemispheres, and over 80% of radiosonde stations are in the Northern Hemisphere. Meanwhile, in the Southern Hemisphere, most stations are located in the continent of Australia and equatorial regions  
Figure 3: Scatterplot of latitudinal monthly mean COSMIC and IGRA PW. to the north of Australia. The dominant seasonal variations of PW derived from two observations are similar. PW peaks in the tropical area with average values of 40 mm. The larger differences shown in Figure 3 are assumed to be in this region, where PWs are generally larger than 30 mm. The scatterplot in Figure 3 shows the latitudinal averaged results. The COSMIC PWs contain the information of both ocean and lands, while the IGRA PWs mainly show the results on land. According to Figure 4, the IGRA stations usually located in the hot spots in the tropical region. This may be the reason for the IGRA PWs being larger than the COSMIC PWs in the region with PWs larger than 30 mm. There are 3 PW maxima regions in the tropical area. One is located in the Amazon Plain, the second in Southeast Asia and the last in Congo basin. For the first peak region, the river Amazon and the large cover of tropical rainforests supply the upper atmosphere with sufficient water vapor. In addition, under the control of the intertropical convergence zone (ITCZ), the continual upward flow makes the supply stronger. The second region is Southeast Asia, surrounded by ocean, having a great amount of water vapor sources to the upper atmosphere. The last and smallest region of peak PW value is Congo basin, which also has a large area of rainforest, providing abundant water vapor to the upper level. The pattern of variations in PW is similar to the distribution of annual mean precipitation presented by [PERSON] et al. (2015) and [PERSON] et al. 's (2016), and all PW maxima regions have larger precipitation. In [PERSON] and [PERSON]'s (2017) research of the temporal relations of column water vapor and precipitation in the tropical area, they found an autocorrelation time of PW compared to precipitation and that PW increases consistently with mesoscale convective activity. This indicates that changes in PW can be used as a predictor of precipitation. In addition, in [PERSON] et al. 's (2016) research, the two-way interaction between PW and convection is revealed. Most published results indicated the complicated relationships between PW and rainfall. The regions with large PWs may have a few amounts of rainfall or even no rain. The dynamical processes also play a role in determining whether it will rain. On the other side, the local PWs may be not enough to support one rainstorm and the water vapor transport is also important to the precipitation events. For mid-latitudes, PW is much smaller than in the tropics and it varies obviously in four seasons. Summer PWs are generally larger than other seasons. However, there is a consistent low PW region in all four seasons, which is in the southwest of China. The elevations of this region are relatively high, at about 3000 m or even higher. Stations of Figure 2c with high elevation are only in this  
Figure 4: Global maps of mean seasonal PW from IGRA radiosonde observations (colored dots) and from COSMIC observations in the last 10 years. region. This implies that the elevation also has some impact on the PWs and that higher elevation has smaller PW. In the polar regions, PW values decrease to less than 10 mm and do not show any seasonal variations. However, the Northern Hemisphere has a much brighter color in summer over the mid-latitude area, even over the Arctic, than that in the Southern Hemisphere summer. This might be associated with higher temperatures or active circulations in the summer of the Northern Hemisphere.