## 5 Results and Discussion
### Long Term Analysis
For a more reliable assessment of the long-term trends, the periodic component has been subtracted from the time series and a least square linear regression is performed (Figure 2b). The outcomes for all the analyzed stations are summarized in Table 3. The analysis of the longest time series resulted in an increase in precipitable water vapor ranging between 1.1 and 2.9 mm/decade, with an average value of 1.9 mm/decade. Figure 3: Spectral analysis of the low-pass-filtered daily PWV (like the red curve in Figure 2a) at five stations of the network; FFT amplitude spectrum (**a**) and a spectrogram of PWV time series at AGR1 (**b**); units of frequency are cycles per day (cpd). Figure 2: Time evolution of the PWV from 2006 to 2016 at AGR1 station (**a**,**b**) assessment of decadal trend (blue curve) after removal of an annual periodic component (red curve). It is well known that, as temperature rises, the air's capacity to hold moisture increases, by about 7% for every 1 \({}^{\circ}\)C, which is known as Clausius-Clapeyron rate; this has an obvious and paramount relevance in climatology. [PERSON] et al. [39], using the monthly average temperature of 54 Italian meteorological stations, in the period 1961-2016, found that the average temperature increase observed in recent decades in Italy is higher than the global average. They clearly state that Italian climate has shown rising mean temperatures on the last 30 years. [PERSON] et al. [40] analyzed the sea surface temperature (SST) over the Tyrrhenian Sea for the last 60 years. They found an oscillatory variability, with a cooling of about 0.5 \({}^{\circ}\)C in 1970-1990 and a warming of 0.9 \({}^{\circ}\)C in the last 10 years (2006-2016); the latter corresponds closely to the time window of our analysis. Unlike temperature, the precipitation regime shows neither pronounced nor unambiguous trends over the Italian regions. Accounting for the previous considerations, we guess that the trends observed in GPS-retrieved PWV might be ruled by a general temperature increase, as was even found by [PERSON] et al. [41] in Germany (Central Europe), but we cannot exclude that there could also be feedback with synoptic mode variability in the region, as found by [PERSON] et al. [42], over the Eastern Mediterranean area. The retrieved increment in PWV (about 2 mm/decade) around 7 \(\div\) 9% of the mean PWV is comparable to the theoretical rate of the Clausius-Clapeyron gradient. Since the Neapolitan area is near the Mediterranean Sea, we can compare our results with other studies that analyze PWV in this type of climate. [PERSON] et al. [42], in Israel, have shown that the PWV long-term trend for 1998-2019 is \(\sim\)0.5 mm decade\({}^{-1}\) while it is \(\sim\)1.1 mm decade\({}^{-1}\) for 2010-2019 interval. [PERSON] et al. [42] speculate that the PWV increase, in the last decade, is due to the response to an increasing temperature signal. Based on the analyses of time series collected on globally distributed 150 IGS stations (1994-2006), [PERSON] et al. [43] investigated the secular trend and seasonal variation of zenith tropospheric delay (ZTD), as well as its implications for the climate. They found that secular variations are systematically increasing in most of the northern hemisphere and decreasing in the southern hemisphere, meaning that, at a global scale, the secular ZTD variation is in balance. Furthermore, seasonal ZTD cycles were recognized and interpreted as mainly due to the wet-component variations (ZWD). Figure 4: Monthly variability in different years of PWV at three stations of the NeVoCGPS, located at heights ranging between 70 m (AGR1) and 864 m (BKE1) a.s.l. Unfortunately, more actual meteorological observations would be needed to validate our results as well as longer data sets, which were not available for this research. As expected, the spatial distribution of PWV is clearly influenced by the topography of the volcanic edifice, the range of variations presents cylindrical symmetry, i.e., similar values for homogeneous altitude bands.