A mean-sea-level pressure time series for Trieste, Italy (1841–2018)

. A time series of mean-sea-level pressure was built from observations performed in Trieste from 1 January 1841 to 31 December 2018. Original historical documents provided information on the instruments and on the observation sites. Mercury barometers have been always available. Until 1877 they represented the only instruments in operation, while from 1878 onwards barograph records became available. The time series consists of mean daily values, that were computed from 24 hourly data, when possible, or otherwise adjusted to 24-hr means on the basis of climatological daily pressure cycles. The 10 time series was homogenized on the basis of the available metadata, reducing pressure to 0° C and to mean sea level. Basic quality checks were applied, including the comparison with pressure time series from nearby stations. As a result, the data prior to 1865 turned out to be suspect. A mean-sea-level pressure trend of 0.5±0.2 hPa per century was estimated for the 1865– 2018 period. The data are available through PANGAEA (https://doi.pangaea.de/10.1594/PANGAEA.926896; Raicich and Colucci, 2021).


Introduction
Long time series of environmental observations represent key elements for climate studies.
In the last decades, significant efforts have been made in Europe to recover long meteorological time series among which those of the atmospheric pressure.Quality-checked and homogenized time series of daily pressure data were produced for several European cities in the framework of the IMPROVE project, extending from the mid-18 th century to the 1990's (Camuffo and Jones, 2002).Pressure data for various sites of the Po Plain, in Italy, were recovered by Maugeri et al. (2004).More recently, Cornes et al. (2012aCornes et al. ( , 2012b) ) reconstructed over 300-yr long time series for London and Paris, respectively.A comprehensive global inventory of pressure time series that started before 1850 can be found in Brönnimann et al. (2019).
In Trieste regular observations of the atmospheric pressure and the air temperature were reported to start in 1788 (Rossetti, 1829), however, only sparse data are available before 1802.Since then the observations have been performed regularly.The earlier observations are only available from local newspapers, while data from 1841 onwards can be found in original manuscripts that are held in the archives of the Institute of Marine Sciences of CNR (CNR-ISMAR) in Trieste (Italy).This paper focusses on the reconstruction of a daily mean-sea-level pressure time series from 1841 to 2018 using the original observations and the information on the instruments and their heights above mean sea level.
In the next section the data used in this work will be described together with their sources and the instruments.The methods used to derive the daily mean-sea-level pressure will be outlined in Sect.3. Section 4 will include basic information on the data availability.Concluding remarks will be summarized in Sect. 5. Technical details on the barometer corrections and the trend estimates will be presented in Appendices A and B, respectively.

Overview
Pressure observations were initially performed under the responsibility of the local institution committed to the education of future seamen.It operated under different denominations until 1898, namely the Nautical School (Scuola di Nautica) until Until 1812 the observations were performed in the Public Library situated in the Grand Square (piazza Grande, GRA, Fig. 1c).A second-order meteorological station was established in 1817 (Osnaghi, 1886) on the premises of the Royal and Nautical Academy in Leipzig Square (piazza Lipsia, LIP, Fig. 1c).In 1841 a new observatory was set up in the same building.In 1868 the meteorological station and service were reorganized.In the first half of that year a new temporary station, located in another room and with new instruments, was operated in parallel with the old one.A fully equipped observatory was finally established on the first floor in July 1868 and the old one was discontinued.Probably in 1868, but certainly before 1884, the observatory became a first-order station (Osnaghi, 1886).In 1870 the station was relocated on to the third floor and, finally, in 1876 on to a new room on the roof.In 1903 the whole observatory was moved from Leipzig Square, not far from the harbour, to Villa Basevi (BAS, Fig. 1c), on a nearby hill, and it was equipped with new instruments.In February 1920 the station was relocated to Sant'Andrea (AND, Fig. 1c), again in the low-lying part of the city near the harbour.On 10 June 1944, the building hosting the station partially collapsed due to an aerial bombing and, as a consequence, the barograph and the control barometers were Table 1: Summary of the observation sites shown in Fig. 1c, their geographical positions (from Google Earth) and the time intervals when the observations were made.

Site
Lat N (°) Long E (°)   1c).In August 1950 the instruments were relocated to the previous site (AND) in the rebuilt institute, and they remained there until the beginning of January 2019, when the station was moved to Molo (Pier) Sartorio (SAR, Fig. 1c).The positions of the observation sites are summarized in Table 1.
For the sake of completeness, we should mention other meteorological stations where atmospheric pressure is being measured in Trieste, namely: Molo (Pier) Fratelli Bandiera (BAN, Fig. 1c), managed by the Civil Protection of Friuli Venezia Giulia Region (since 1994); piazza Hortis (Hortis Square, present name of Leipzig Square, HOR, in Fig. 1c), managed by the Nautical Technical Institute and the University of Trieste (since 1993); Molo (Jetty) Lega Navale (NAV, Fig. 1c), managed by the Istituto Superiore per la Protezione e la Ricerca Ambientale (Superior Institute for the Environmental Protection and Research) since 1998.
Data prior to 1841 can only be found in the local newspaper L'Osservatore Triestino, which in January 1802 started publishing the observations of pressure, temperature and the 'state of the sky', namely cloudiness and precipitation.The publication of meteorological data was rather regular until 1808, then increasingly irregular until September 1812 when it ceased; it was resumed in January 1819 and continued regularly except for occasional interruptions.
Original meteorological records and summaries are only available since January 1841; they are held in the archives of CNR-ISMAR.The data from 1 January 1841 to 30 June 1868 were collected in seven handwritten volumes, named Repertori (Gallo, 1841(Gallo, -1868)).Subsequently, the direct meteorological observations were summarized on monthly data sheets.When automatic recording instruments became available, hourly data were also tabulated on a monthly basis.
Note that the unavailability of any data before 1802 and of the original observations until 1840 was already stated by Gallo (1846).The second part of this statement is surprising, because he took over the responsibility for the observations on 1 January 1841 but he seemed unaware of the activity performed in the very same place until 31 December 1840.Moreover, he explicitly told said that the barometer used in 1841 had also been used previously (Gallo 1841(Gallo -1868;;1846).

The instruments and the observations
Mercury barometers were available during the whole 1802-2018 period, while barographs were used in 1868-1870 and from 1878 onwards.The station was always equipped with at least one control mercury barometer that was checked against a standard barometer (also named Normal Barometer).Barograph records were always calibrated by means of the observations made with a control barometer.Three types of mercury barometer were used, namely siphon (Gay-Lussac type), with adjustable cistern (Fortin type) and with fixed cistern (Kappeller type).In order to obtain the atmospheric pressure in standard conditions, that is 0° C temperature and mean sea level (MSL), the observations require appropriate corrections, described in Appendix A.
The original data sheets and summaries generally reported the essential information on the barometers in use, namely model and manufacturer, and height above MSL.Table 2 summarizes the instruments from which the pressure data used in this study were obtained, as retrieved from the original documents and from review works (Mazelle, 1889;1905;Polli, 1951Polli, -1952;;Stravisi, 2006).Unfortunately, the 1802-1840 period lacks of metadata, namely the barometer location, height and temperature, therefore, those data were excluded from further analyses.
Three direct measurements of the atmospheric pressure were made with the control barometer (Table 2) in the morning, in the early afternoon and in the evening, namely at 7, 14 and 22 h from 1 January 1841 to 30 June 1868, then at 7, 14, 21 h until 31 December 1918, at 9, 15, 21 until 31 March 1920, at 8, 13, 18 h until 7 April 1927, and at 8, 14, 19 h approximately until 1980.
More recently only one daily observation was performed, usually in the morning.The 1841-1868 pressure data of the Repertori (Gallo, 1841(Gallo, -1868) ) are generally raw measurements, that is without any corrections and normalizations.The observations were made at 'true time', that is the time relative to the Trieste meridian.
The difference with modern standard time is negligible, as the Trieste longitude of 13.75° E corresponds to a time lag of only -5' relative to GMT+1 time, which is defined for 15° E. According to Gallo (1846) the observations were made at 'true time', that is the local apparent time.On 8 September 1852 the local astronomical observatory began signalling the 'medium Noon', corresponding to 55'3" ahead of Greenwich Mean Time (I.R. Accademia di Commercio e Nautica, 1853).As astronomers and meteorologists were in close contact, perhaps Gallo adopted such a 'medium time' when it became available.Based on the equation of time (Meeus, 1998), the difference between apparent and mean times is between -15 min, on 3 November, and +16 min, on 11 February.Except during extreme events, pressure changes in 15 min are usually small compared to the daily pressure range, therefore, we disregarded that time difference.We also neglected the 5-min difference corresponding to the difference between the station longitude (Table 1) and the 15 °E meridian, where modern standard time is defined.
The barometer temperature is also available making it possible to correct the raw data to 0 °C.The instrumental correction for the Kappeller N. 17 is available from the Repertorio for 1864-1866 (Gallo, 1841(Gallo, -1868) ) and from Central-Anstalt (1866).
The three daily observations from 1868 to 1902, made with the Kappeller N. 1004 and 710 barometers (Table 2), include uncorrected pressure, barometer temperature and pressure corrected to 0 °C.The instrumental corrections and the constants of normalization to the standard barometer can be found in Central-Anstalt (1884;1885).In some years, pressure reduced to MSL is also included.
According to the original documents all the direct observations from 1 January 1903 onwards include the correction to account for the barometer temperature and the normalization to a standard barometer, but, unlike the previous data, they are not reduced to MSL.With regard to instrumental corrections and performance, we only know that the Wild-Fuess N. 462 control barometer (Table 2) was checked by the manufacturer in 1938 and 1952, and that all the control barometers were cross-checked several times (Polli, 1951(Polli, -1952;;Stravisi, 2006).
No information was found about possible instrumental drifts,; however, the calibrations and cross-checks have probably allowed to keep them under control.Besides the three daily observations, pressure was measured every hour on the solstices and equinoxes from 1843 to 1864, and, from 7 to 22 h only, during the whole year 1845.The 1845 data were obtained with a barometer manufactured by Hanaczik (Table 2); the original observations are missing but the daily means were published in a conference communication (Gallo, 1846).Barographs were operated from 1868 to 1870 and from 1878 onwards.The Kreil, Sprung-Fuess and Fuess 87M barographs were used as main instruments, while the Richard barograph worked as a back-up (Table 2).

The estimate of daily means
The time series consists of daily mean pressures reduced to 0 °C and to MSL.
First, the data underwent a preliminary basic quality control in order to recognize and correct evident errors.At that stage We we only corrected those errors that could be easily justified, for example, by a writing or printing mistake or missing conversions to metric units.The latter problem occurred in a limited number of cases from 1871 to 1902, when pressure and temperature were reported in metric units but were still measured with instruments having scales in Paris inches and degrees Réaumur, respectively.Subsequently, pressure changes (p) between adjacent observation times were used to detect suspect values.A visual inspection was carried out when three daily observations were available.By contrast, suspect hourly data were identified when |p| > 3 hPa (the threshold value was chosen arbitrarily).When possible these data were checked in comparison with the original barograph charts.Erroneous values that could not be corrected were removed from the data set.
Instead of adopting the pressure data reduced to 0 °C reported in the manuscripts for 1868-1902, we recomputed them taking advantage of the availability of observed pressure, barometer temperature, barometer corrections and constants of normalization to the standard barometer.
Thanks to the instruments redundancy of several barometers it was possible to fill almost all the gaps caused by instrumental failures.Nevertheless, interruptions still exist, particularly in the original hourly record.By contrast, the direct observations at 7, 14 and 21 or 22 h are generally available.Table 3 summarizes the data used to build the 1841-2018 time series.
We assumed the 'true' daily mean pressure to be obtained by averaging 24 hourly observations, in order to account for the daily cycle.When at least one observation was available a provisional daily mean was computed; if less than 24 data was available, a correction was subsequently applied to adjust it to the 24-hr mean.
Mean corrections were estimated following an approach similar to the one adopted in Raicich and Colucci (2019).For each calendar day (1 January-31 December), climatological values were obtained by averaging hourly (0-23) pressures and mean daily pressures.This was done when all the 24 hourly observations were available, in order to have the full daily cycle represented.
If h is the hour, d the day, m the month, and y the year, let Po(h,d,m,y) be the observed pressure, Pc (h,d,m) the climatological hourly pressure and P24c(d,m) the climatological daily pressure: w being a weighting factor, equal to 1 if Po is available and 0 if it is not; y1 and y2 are the first and last year of the period over which the sums are made.A 91-day running mean is subsequently applied to Pc and P24c in order to smooth out the effect of outliers.The values for 29 February are interpolated using those of 28 February and 1 March.
Each observation site is characterized by a peculiar mean daily cycle, therefore, we adopted different climatologies to adjust the provisional daily means.In principle the corrections for 1841-1870 could be estimated using the climatology computed from the 1868-1870 hourly data, but this 3-year period is too short to obtain a smooth climatology and this option was ruled out.
The MSL pressure at each observation site is characterized by a peculiar daily cycle, mainly related to the air temperature used to estimate the correction to the mean sea level.In fact, the air temperature cycle depends on the exposure of the outdoor thermometer, its distance from buildings and its height above the ground, all of which changed a few times (Table 3).The effect of such issues in historical temperature time series are discussed, for instance, by Cocheo and Camuffo (2002) and Maugeri et al. (2002b).Therefore, we adopted different climatologies to adjust the provisional daily means.
The data for 1841-1870 were adjusted using the 1878-1902 climatology, although the 1868-1870 climatology was also available.We compared the adjustments obtained with both climatologies and we found that the daily differences were mostly less than 0.1 hPa, in absolute value; only in 4% of the cases absolute differences were larger than 0.1 hPa and never larger than 0.2 hPa.Although either period could be adopted, we chose the climatology based on the longer time series, which makes it less sensitive to outliers.
As a consequence, the climatology computed from the 1878-1902 data was used to adjust all the 1841-1902 daily means.
Potentially where w = 1 if the observation is available and w = 0 if it is not.Padj (adjustment) is represented by the daily climatological pressure plus a correction consisting of the mean difference between observed and climatological pressures, computed using the hourly values available on the relevant day.Clearly, if Po is available for all hours from 0 to 23, then Padj is not used in Eq.
3 and P is the arithmetic average of the 24 observations.
The error on P, namely σ, is computed from those on the observation, σo, and on the climatologies, σ24c and σc, respectively.
(, , ) = ] 2 ⁄ (6 where σo is the observational error and σc and σ24c the errors on the hourly and daily climatological values, respectively.
These errors were assessed semi-empirically as explained in the following.Due to the often uncertain information on the instrumental performances, we did not aim at accurate error estimates but rather at obtaining reasonable representative values.
An observation is basically affected by an instrumental error and an environmental error.The instrumental error can be estimated on the basis of the uncertainty on instrumental corrections and normalizations and the nominal reading precision.
The environmental error is caused by pressure fluctuations occurring at sub-hourly time scales, that may affect the (nominally) instantaneous measurement.
The accuracy of digital instruments is about 0.2 hPa.
The environmental error was estimated as the average hourly pressure range.These were computed using the hourly pressure extremes, available for 2008-2017.The mean hourly pressure range turns out to be 0.13 hPa and on 90% of the days it is lower than 0.24 hPa.
Thus, taking all errors into account, we cautiously assumed the observational error on an individual reading (σo) to be 0.3 hPa, independent of time.The errors on the climatological values, σc and σ24c, were obtained as the standard deviations of Pc and P24c, respectively (Eq. 1 and 2), to account for the interannual variability of pressure.For both climatologies (1878-1902 and   [1903][1904][1905][1906][1907][1908][1909][1910][1911][1912][1913][1914][1915][1916][1917][1918][1919] σc varies approximately between 4.0 hPa in July and 9.5 hPa in January, while σ24c varies between 0.8 and 2.0 hPa, in the same months.These errors are much greater than σo but they only contribute when the hourly observation is missing. Figure 3 shows the daily corrections (c, panel a), to be added to the provisional daily means to adjust them to 24-hr means, 10 and the related errors (σ, panel b).Only the 1841-1920 period is displayed because, from February 1920 onwards, 24 observations per day are always available, therefore c = 0 hPa and σ = 24 -1/2 σo = 0.06 hPa.Two data sampling schemes can be distinguished, namely in 1841-1867 and 1871-1877, when pressure was observed three times per day, and in 1868-1870 and from 1878 onwards, when barograph records were available.
When three daily observations are available (1841-1867 and 1871-1877) marked annual cycles of both corrections and errors can be seen due to the seasonal variations of the daily cycle.In 1841-1867 corrections normally range between -0.09 and +0.09 hPa with a mean value of -0.01 hPa, while in 1871-1877 the mean correction is +0.03 hPa, ranging between -0.05 and +0.12 hPa.The difference is the result of changing the time of the evening observation from 22 h to 21 h.Note that observing at 7, 14 and 22 h makes the mean annual bias almost negligible.The spike corresponds to 19 march 1862, when only one observation is available.In 1841-1867 the mean error is 1.30 hPa and it normally ranges between 0.76 and 1.85 hPa.On solstices and equinoxes of 1843-1864 the error drops to 0.4 hPa.In 1868-1870 and from 1878 onwards the magnitudes of corrections and errors reflect the abundance of the observations.In the few cases when less than three observations are available, |c| 0.2 hPa and σ attains 1.8 hPa.
The daily means were used to compute monthly (Fig. 4) and annual (Fig. 5) mean pressures and errors.In 1841-1867 and 1871-1877 the monthly error generally varies between 0.15 and 0.35 hPa, and the annual error is than 0.015 hPa.In 1868-1870 and from 1878 onwards monthly errors can be as low as 0.01 hPa (Fig 4b) and annual errors smaller than 0.005 hPa, depending on the amount of observations (Fig 5b).

Comparison with other stations
In order to detect suspect or erroneous data, we compared the daily means, reduced to 0 °C and to MSL, with those of nearby stations, namely the homogenized time series of Padua (1725-1999;Camuffo et al., 2006) and Milan (1763-1998;Maugeri et al., 2002a, b).These time series were selected as they include most of the period of our interest, as only the last 20 years are missing, and because of the relatively short distances from Trieste, namely about 160 km and 350 km, respectively.Another reference was used, namely the daily means of mean-sea-level pressure available on a 2°×2° grid from the 20 th Century Reanalysis, version 3 (Compo et al., 2011;Giese et al., 2016;Slivinski et al., 2019), interpolated onto the stations positions.This data set was not used to check the actual pressure values, but rather as a help to detect possible persistent anomalies and drifts in the individual local time series.
Note that anomalous pressure differences can temporarily occur between Trieste and Padua or Milan, situated in the Po Plain, that are not related to the barometer performance.Particularly in autumn and winter the weather in Trieste is often characterized by Bora, a katabatic northeasterly wind with mean hourly speeds that can be higher than 20 m s -1 and gusts that can frequently exceed 30 m s -1 (Raicich et al., 2013).Gusty wind causes pressure fluctuations that alter the barometer readings (Liu and Darkow, 1989;WMO, 2014), and, therefore, affect the comparison of daily pressure between Trieste and Padua and/or Milan.
Typically, strong Bora can last from a couple of days to a week, occasionally up to a couple of weeks (e.g.Raicich et al., 2013).
The daily pressure difference between Trieste and Padua and between Trieste and Milan shows an anomalous behaviours from 10 March 1844 to 23 May 1846.The 91-day running means, used to smooth day-to-day variability, is are displayed in Fig. 6 (only the 1841-1855 period is shown).A sSeasonal cycles, retained by the running mean, is are clearly visible, with higher values in the early months of the year and lower values in the later months.The blue curve represents the pressure data of the Repertori reduced to 0° C, while the red curve represents the 10 March 1844-23 May 1846 as they were reported.It is evident that the seasonal cycle amplitude is more coherent if the data in the anomalous period are not reduced to 0° C (Fig. 6, red curve).The analogous comparison with Padua data leads to the same conclusion.Initially, all the Trieste data reported in the Repertori were reduced to 0 °C (Fig. 6, thin lines).However, the amplitudes of the seasonal cycles of the pressure differences appear much larger in March 1844-May 1846.We attributed the anomaly to the fact that, unlike the previous and following observations, the Trieste pressures were written after being reduced to 0 °C, and, therefore, were erroneously reduced again.The anomalous behaviour of the March 1844-May 1846 period was confirmed by comparing the monthly means with those reported in Kreil (1854), Jelinek (1867), Osnaghi (1874) and Mazelle (1886), which were also computed from Gallo's data and explicitly reported to be reduced to 0 °C.Perhaps the pressure measured with the Schlosser barometer was reduced for 5   comparison with the data measured with the Hanaczik, mentioned in Sect.2.2, which were also reduced to 0 °C and MSL (Gallo, 1846).The annual differences between pressure at Trieste, Padua and Milan are displayed in Fig. 7.In the earlier decades Trieste seems to be lower than expected by at least 1 hPa until the mid-1850's, relative to all the reference time series (Fig. 7a).
Another anomaly may occur in the early 1860's but not relative to the reanalysis.We recall that in late December 1856 the barometer was moved to a higher position (Table 3) and in April 1865 a new instrument was introduced (Tables 2, 3).Possible reasons of anomalous behaviours of Trieste data are represented by missing instrumental corrections of the Schlosser barometer, used until March 1865, and inaccurate information about instrument heights.The year 1921 also seems anomalous; note that two instrument height changes took place in 1920 (Table 3).
In general, the differences between station data and the respective reanalysis decrease.The average rates from 1861 (after the increase observed at Trieste) onwards are -1.1 hPa per century (Trieste), -1.9 hPa per century (Padua), and -1.2 hPa per century (Milan).This might be related to the reanalysis behaviour in the area of interest, however, for our purpose, it indicates that the Trieste behaviour cannot be attributed to local observations.Another anomaly in the reanalyses is observed in the 1910's, probably connected with lack of good observations during WWI.
In general, sSystematic problems are not easy to detect because, when compared to each other, all the time series seem affected by anomalous behaviours.For instance, Padua data seems too high with respect to both Trieste and Milan from 1920 to 1960 (Fig. 7a, 7c) and in the early 1990's (Fig. 7b).Milan itself seems affected by a systematic decrease in the early 1960's, when compared to the reanalysis; Maugeri et al. (2004) pointed out that the Milan station was relocated in 1951 (Fig. 7c).
We can conclude that Trieste pressure of the 1841-1864 period should be considered suspect, while no other major problems can be detected.We recall that in late December 1856 the barometer was moved to a higher position (Table 3) and in April 1865 a new instrument was introduced (Tables 2, 3).Possible reasons of anomalous behaviours of Trieste data are represented  3. Commission, 1869;Mazelle, 1889).On the basis of the available information we corrected the 1841-1868 data.The data for 1868-1902 were already corrected by the observers, but we re-computed the corrections.The data from 1903 onwards were corrected by the observers (Sect.2.2).

A.4 Reduction to MSL
The equation used to reduce the station pressure to MSL is: Where p0 is the reduced pressure, ps is the station pressure, g is the gravity acceleration, z is the barometer height above MSL, R = 287.053J kg -1 K -1 is the dry air constant, and T'v is the adjusted virtual temperature in kelvin.The formula was taken from Stravisi (1988) which includes the details on the involved variables and the calculation, which is based on Eq. 16 in WMO (1968), having rewritten some variables.T'v is computed using the observed air temperature and a monthly climatological relative humidity.The adoption of climatological values is justified because the effect of humidity variations on pressure reduction is much smaller than the effect of temperature variations (e.g WMO, 1954).
Figure A1 displays the daily time series of the reductions to MSL, as a function of the barometer heights of Table 3.The seasonal cycle of air temperature causes the high-frequency oscillations.

Appendix B: Trend with autocorrelation
We estimated the linear trend of the atmospheric pressure time series by linear regression.In order to properly estimate the associated error, the data autocorrelation in time was taken into account.The effect of time autocorrelation is essentially a lower number of degrees of freedom and, if neglected, the consequent underestimation of the standard error.
To account for autocorrelation, we followed Zervas (2001).A time series consisting of n data points is modelled according to: =   +  1 ( −1 −  −1 ) +   (B1) where   is the detrended mean annual pressure (k runs from 1 to n), obtained by subtracting the linear trend from the original data, b is the slope of the fitting line, tk represents time in years, 1 is the lag-1 autoregressive coefficient, and  k is the residual.
As a result of autocorrelation, the standard error of the trend increases from b to b AR (AutoRegressive) by an amount that can be approximated by the square root of the Variance Inflation Factor (VIF): where

Figure 1 :
Figure 1: a) The Adriatic and Po Plain regions; b) the Gulf of Trieste; c) aerial image of Trieste.The observation sites whose data are relevant for this study are displayed in red; other sites cited in the text are shown in light blue.(Images extracted from © Google Earth; © 2020 Landsat/Copernicus, © 2020 CNES/Airbus, © 2020 Digital Globe, © 2020 TerraMetric.)

Until 31
May 1974 pressure was expressed as the height of the mercury column, the units being the Paris line (1 line = 1/12 inch = 1/144 foot, 1 Paris foot = 324.845mm; Martini, 1883) until 1870 and the millimetre from 1871 onwards.On 1 June 1975 the atmospheric pressure started being expressed in hectopascals.

Figure 6 (
Figure 6 (thick curves) shows that the seasonal cycles appear more coherent when the Trieste data from 10 March 1844 to 23 May 1846 are retained as reported in the Repertori.

Figure 6 :
Figure 6: 91-day running means of daily mean-sea-level pressure differences between Trieste and Milan (hPa).Trieste pressure reduced to 0° C is shown in blue, and the 10 March 1844-23 May 1846 as appears in the Repertori is shown in red.

Figure 7 :
Figure 7: Comparisons of annual mean pressures (hPa): a) differences involving Trieste (TS); b) differences involving Padua (PD); c) differences involving Milan (MI).Black curves represent the comparisons between each station and the corresponding time series from the 20 th Century Reanalysis (20c); the comparison between Trieste and Milan is shown in red, that between Trieste and Padua in blue and that between Padua and Milan in green.

Figure A1 :
Figure A1: Daily corrections to reduce pressure to MSL (MSLC) in hPa.The dashed vertical lines correspond to the barometer height changes summarized in Table3.

Table 3 : Chronology of the atmospheric pressure observations composing the 1841-2018 time series. Heights are above mean sea level.
, 25 values of Po are available for a given(h,d,m)but, because of data gaps, the actual number varies between 21   (, , ) =  24 (, ) + ∑ [  (ℎ, , , ) −   (ℎ, , )] • (ℎ, , , ) 8 days are missing inSeptember 1845, 20 in October 1845, 6 in March 1862, 1 in September 1862, 3 in June 1915, 4 in July   1915and 2 in December 1915, making a total of 44 missing days out of 65013.The data of September-October 1845 are missing because the barometer temperature is unavailable, and those of 20-25 March 1862 due to the observer's illness(Gallo,   1841(Gallo,    -1868)).In the other cases the lack of observations occurs for unknown reasons.