G. Asteriadis

Atmospheric tides in the area of Thessaloniki

Abstract The tidal changes of the barometric pressure in the area of Thessaloniki, Greece were investigated by analyzing a sample of 10.75 years of hourly measurements. The tidal analysis showed that the signal-to-noise ratio varies from 16.8 to 1 for the long period waves (Sa, Ssa, Mm, Mf). For the short period constituents, the corresponding quantity was negligible with the exception of the diurnal P1, S1, K1 and semidiurnal waves S2, K2 where values of 14, 93, 15 and 61 and 7 were found respectively. The amplitude and the phase difference of the above tidal waves were determined with an accuracy of 0.008–0.18 hPa and 1–3 ° correspondingly. There is a considerable correlation of the surface temperature to the changes of the pressure affecting mainly the Ssa, S1 and S2 waves. The barometric changes computed in the area of Thessaloniki using the amplitude and phase difference determined through the tidal analysis reach a maximum amplitude equal to 10 hPa per year. It is also shown that the tidal changes of the barometric pressure in the test area correspond to about 37% of the total changes. A Fourier analysis of the daily mean values of the barometric pressure for the last 44 years showed significant amplitudes of the power spectral density (PSD) at the non-tidal periods of 8 and 52.6 days.

Long-Term Changes of the Flare Stars EV Lac and by Dra

The long term changes of the quiet-state luminosity and the flare activity of the flare stars EV Lac and BY Dra during the decade 1971–80 have been studied using the photoelectric observations in the International UBV System carried out with the 30-inch Cassegrain reflector of the Department of Geodetic Astronomy, University of Thessaloniki installed at the Stephanion Observatory, as well as all other relevant data available. The quiet-state luminosity of the star EV Lac shows long-term fluctuations with an amplitude of 0 . m 3 and a period of about 5 years. The flare activity of this star shows similar variations, which in most of the cases run parallel to the fluctuations of the quiet-state luminosity. The quiet-state luminosity of the star BY Dra shows also longterm fluctuations. The current cycle has a duration of at least 14 years and an amplitude of more than 0 . m 3. Long-term changes are also indicated for the flare activity of this star, although the observational data available are rather limited.

Hydrologic Changes as Possible Earthquake Precursor in Greece

Although reliable reports on pre-, co- andpost-seismic variations of the hydrologiccharacteristics of a seismic active area were knownfrom the antiquity in Greece, it was only the last twodecades that systematic research focused its intereston the hydrologic precursory phenomena. This researchis summarized in this paper.On the basis of the results of the research forearthquake precursory phenomena on the undergroundwater level and temperature at selected areas of highseismic risk, which were performed the last fifteenyears in Greece, it can be supported that monitoringthe shallow underground water level and temperaturefor detecting earthquake precursory phenomena mayprove a useful method in the frame of aninterdisciplinary research for earthquake prediction.

Assessment of recent tidal models in the Mediterranean Sea

Ocean tides especially in closed sea areas can deviate considerably from the theoretical values due to unequal water depths and to the fact that the continents impede the movement of water. Satellite altimetry enabled the development of improved tidal models event in closed sea areas, by assimilating altimeter data into hydrodynamic models. The Mediterranean Sea due to its morphology is an ideal test field for the assessment of tidal models, based on this technique. An attempt to assess the recent tidal models TPXO.6, GOT00.2 and NAO.99b was based on (a) an inter-comparison of tidal heights computed from the three models at different time moments on the same 15′ × 15′ grid covering the Mediterranean and (b) on the comparison of the statistics of a crossover analysis of nearly 2.3 years of JASON-1 altimeter data (Cycle1-86), before and after the tidal correction, using the three tidal models. The inter-comparison in terms of mean value and standard deviation of the differences between the tidal heights on the 15′ × 15′ grid resulted in mean values up to 3 mm and standard deviations ranging from 16 to 26 mm. However, maximum values of differences exceed several dm. On the other hand, the statistics of the crossover analysis showed a 17% decrease of the standard deviation of the JASON-1 crossover differences after the tidal correction. These results show a good agreement between the three tidal models.