# Observing Meteorological Tides: Fifteen Years of Statistics in the Port of La Spezia (Italy)
## 2 Materials and Methods  
The starting point of this study is the monitoring of environmental parameters in the port of La Spezia, performed by means of the meteo-mareografic station working in the position 09\({}^{\circ}\)51\({}^{\prime}\)27.52\({}^{\prime\prime}\) E, 44\({}^{\circ}\)05\({}^{\prime}\)47.79\({}^{\prime\prime}\) N (see Figure 2) and belonging to the National Tidegauge Network managed by ISPRA [39; 40; 41; 42; 43; 44; 45]. The instrumentation used consists of a hydrometer and a barometer; the first measures the sea level on the basis of the round trip time taken by a sequence of radar pulses sent from the air towards the sea surface; since the speed of propagation of electromagnetic waves in the air, the round trip time of the pulses, and the position of the radar transducer are known, sea levels are calculated; a typical sampling interval to acquire tide data is at least 10 min. The barometer measures the atmospheric pressure by evaluating the deformations undergone by a silicon capacitive transducer; when the atmospheric pressure changes, the distance between the two plates and therefore the electrical capacitance also varies; measurements are typically made hourly (atmospheric pressure is characterized by very slow variations). The date and time are expressed in UTC, while the sea level refers to IGM's (Italian Military Geographic Institute) 0 level. The data used in this work and further information about the ISPRA's monitoring station are available on the website www.mareografico.it (data from 2006 to 2009 are not available on the website; they have been kindly provided by ISPRA) (accessed on 28 April 2022). Atmospheric pressure and sea level measurements (one sample every hour with resolutions equal to 10\({}^{-1}\) hPa and 1 cm, respectively) were compared with each other in  
Figure 2: The position of the monitoring station (pictures from Google Earth): (**a**) inside the port of La Spezia; (**b**) within the Eastern Ligurian Sea, Italy. order to evaluate the hydrobarometric transfer factor, a local parameter that represents the correlation between the two quantities. Figure 3 shows the signals acquired between 28 May and 7 June 2009 inside the port of La Spezia. The sea level is the overlapping of different contributions, among which the main ones are astronomical and meteorological tides, as shown in the power spectral densities plotted in Figure 4, as regards the measurements performed in 2009: Frequency components due to diurnal and semidiurnal components are at approximately at 1.2 and 2.3 \(\times\) 10\({}^{-5}\) Hz respectively, while meteorological components are characterized by lower frequencies (slower oscillations) related to atmospheric pressure spectrum (DC components are not plotted because they do not carry useful information). First of all, it was therefore necessary to filter acquired data in order to remove high-frequency components due to causes different than meteorological ones (there could also be a residual of the disturbance due to sea waves, although the hydrometer works inside a still-pipe): atmospheric pressure and sea level signals are subjected to low-pass filtering with  
Figure 4: Power Spectral Densities of measurements carried out in 2009 within the port of La Spezia: (**a**) atmospheric pressure; (**b**) sea level. Figure 3: Measurements carried out between 28 May and 7 June 2009 inside the port of La Spezia: (**a**) atmospheric pressure; (**b**) sea level. an appropriate cut frequency (\(10^{-5}\) Hz), so that only the contributions of meteorological origin survive. Figure 5 shows the result of the low-pass filtering applied to the data shown in Figure 3.  
By examining filtered signals, it is evident that a decrease \(\Delta p\) in atmospheric pressure equal to about 17.2 hPa causes an increase in low-frequency sea level \(\Delta h\) (low meteorological tide) equal to about 35.4 cm.