## 3 Results and Discussion
### Trends in PW  
Since the surface temperature has changed globally in recent decades, it is reasonable to ask whether PW has also changed in the past decades. Some earlier studies have reported PWs to have some significant changes regionally [1; 6; 7; 8; 9; 13; 16]. We will pay more attention to the changes in global PWs, especially for the most recent decade. We firstly choose four stations from different countries (locations shown as blue triangles in Figure 1) to investigate their PW trends in the last 60 years and in the most recent 10 years (Figure 7). Different changes are found in the long-term periods and in the recent decade. For example, PWs at the station Fuzhou China (Figure 7a) present an insignificant decreasing for the past several decades. However, it exhibits a statistically significant increasing trend in the past 10 years. The opposite situation is found at station Jackson/Allen C. Thompson Fiel, US, although both trends are not statistically significant. PWs in Australia show a consistent decrease in both periods, while PWs at the Finland station have not changed much in the long trend, but experienced a significant increasing trend in the most recent decade. Since there are more than 60-year measurements from IGRA radiosonde observations, long-term changes in global PWs can be obtained. Stations with 60 complete years of PW observations are plotted and stations with significant trends are marked with bold black circles in Figure 8. It can be found that most significant trends occur at those stations of the United States (US), Europe and Australia. Figure 7: Trends in PW at four stations: (**a**) Fuzhou, China; (**b**) Macquarie Island, Australia; (**c**) Jackson/Allen C. Thompson Fiel, US and (**d**) Sodankyla Arctic Research Cent, Finland. Blue linear lines represent trends during 1957–2016 and red linear lines represent trends for last decade (2007–2016). k1 and k2 represent the slopes of the linear lines. S represents trend significant at the 95% confidence level or greater; I denote the insignificant trend. PWs at all Australian stations present statistically significant decreasing trends in the last 60 years, while PWs at most stations in the US have increasing trends. [PERSON] and [PERSON] (2007); [PERSON] et al. (2010) and [PERSON] et al. (2010) both reported the increasing trends of PW in the US region during 1973-2006, which is consistent with our results. Besides the long-term trend in PW, we are more focused on PW changes in the recent 10 years. Since COSMIC observations perform quite well in comparison with radiosonde data and, more importantly, are covered globally and evenly, they can complement the limitation of IGRA observations. Based on COSMIC and radiosonde observations from 2007-2016, Figure 9 presents the global changes in PW during the most recent decade. It shows an overall similar trend pattern in PW. For most ocean regions, PW has been decreasing in the last 10 years, especially in the tropical area. Except for the Antarctic and inland of the Eurasian continent lands, most land areas show a statistically significant increasing trend in PW. In addition, for North America and European region, PWs have been increasing for the last 10 years and also for last 60 years. On the other side, in Australia, Eastern Asia, and Mediterranean region, PW has been decreasing for last 60 years, but with increasing trend in the recent decade. Due to the lack of data accumulations, most Asian stations are not suitable for long-term trend analysis but show statistically significant increasing trends in the last decade. [PERSON] et al. (2018) analyzed the global PWs from 1988 to 2011 and found widespread positive trends of PW all over the world, including lands and oceans. However, according to Figure 9, for the most recent 10 years, PWs have shown uptends over land and downtrends over oceans, which means the widespread increase of water vapor in the troposphere over land.