## 6 Conclusions  
Tropospheric water vapour content is a crucially important meteorological parameter because, besides being one of the main greenhouse gases, it is involved in the energy exchanges that take place in most of the meteorological and climatic phenomena. However, due to its extreme variability in space and time, water vapour is one of the most poorly understood weather components. We have conducted a study on the potential use of dense GPS networks, designed for geodetic monitoring of active volcanoes in Naples. Our study demonstrates that the retrieval of GNSS tropospheric delay can provide important contributions, both for long-term trend analyses and for the prediction and study of the evolution of intense thunderstorm phenomena. In the first case, it is possible to obtain information of fundamental importance for the long-term evolution of the climate and, therefore, all the problems related, for instance, to global warming, as well as sea-level rise. From the analysis of the decadal time series, we estimated increases in precipitable water at an average rate of 1.9 mm/decade. This value is in agreement with estimates obtained in Europe by other authors, e.g., [58]. This rise, as per the Clausius-Clapeyron process, might be associated with a climatological trend of increasing air temperature and moisture content in the troposphere, leading to increased levels of rainfall, greenhouse effect and associated climatic feedbacks. Indeed, compared to the scale involved in routine climate studies, our analysis was conducted on a very small scale (a few tens of km). The involved time scale is not climatological, either;, in fact, our study covers a relatively long time span, of about 11 years, that is nonetheless too short to draw definitive conclusions of any climatological relevance. Consequently, our interpretation of the trends observed in the PWV can only remain at a speculative level. It has also become clear from our study that tropospheric delay is strongly controlled by orography. From the analysis of GPS data collected in the Vesuvius sector of the NeVoCGPS network, it is clear that the amplitude of the tropospheric delay is inversely correlated with altitude and, coherently with the climate in Vesuvius area, emerges as larger in summertime, with negligible differences for homogeneous altitude bands. An accurate knowledge of the 4D (space-time) distribution of tropospheric delay can certainly improve the signal-to-noise ratio of geodetic data. All the space geodesy techniques based on the study of the satellite-to-target electromagnetic signal propagation would benefit from this. As a result, our ability to solve problems typical of applied geodetic monitoring--for example, the monitoring of volcanic or landslide areas--would improve. Dense networks of permanent GNSS stations could even provide a fundamental contribution to the tomographic modelling of tropospheric delay functions. In the short-term studies, it was possible to highlight the importance of tropospheric delay in tracking the evolution of the examined phenomena and, especially, the presence of an increase in precipitable water in the hours immediately preceding the meteoric event. In other words, the analysis of time transience and trends in precipitable water vapour can shed light on the preparatory stage of extreme rains. These outcomes, in particular, bode well for the possibility of using these results to set up an early warning system that could play a fundamental role in terms of civil protection and risk mitigation. Such a high density of stations is not necessary for warning purposes; to create an efficient early warning system, it would be more appropriate to select points to be equipped with permanent GNSS and weather stations close to areas exposed to landslide risk, especially of rapid flow, such as the Apennine slopes, which are covered with layers of unconsolidated pyrodastic sand primarily deriving from the explosive activities of Mt.