Extreme temperature events in NW Greece.

In this work, the extreme temperature events in, NW Greece are studied for winter and summer. The data used are the maximum and minimum daily temperatures recorded at the meteorological station of Ioannina University for the 33-year period 1970-2002. Days with temperature above 35.8 C in summer (JJA) and below -7.6 C for winter (DJF) are only considered. These thresholds are the 95% upper and lower limits respectively (quintiles). It is seen that, in winter, the maximum frequency of minimum temperature values below the above threshold appears in the 5-year period 1989-93, a period characterized by a high positive North Atlantic Oscillation index. In summer, the maximum frequency of maximum temperatures in the upper quintile appears the recent years, in agreement with the general warming trend in eastern Mediterranean. For these extreme temperature days (approximately 150 days for each season), at first, the pressure patterns over Europe are constructed (from 40W to 50E and from 30N to 70N - NCEP-NCAR data). Then, using Factor Analysis, the 150 charts (days) are grouped objectively in order to reveal the main pressure patterns associated with extreme temperature values in NW Greece. In winter, 4 factors were selected, explaining 79% of the total variance. In all of them the main feature of the synoptic conditions is an anticyclone centered either north or northwest or southwest of Greece, and extended over Greece. Thus, clear nights prevail, favouring extremely low minimum temperatures in the morning. In one case, the position of the anticyclone favours a northeasterly flow over NW Greece, which transfers cold air masses from eastern continental Europe. In summer, 5 factors were selected, explaining 81% of the total variance. Here, also, the extremely high maximum temperatures are associated with an anticyclone. The anticyclone is either located over NW Africa (extension of the Azores subtropical anticyclone), transferring warm air masses from the Sahara desert, or it is located over Greece causing sunshine and very low wind or no winds at all, conditions obviously related to heat waves.

































A study on evaporation in Ioannina, NW Greece.

Evaporation in Ioannina, NW Greece is studied for the period 1988-2003. The data basis comes from the readings of the Wild evaporigraph charts of the meteorological station of Ioannina University. The total number of data consists of 70,080 bi-hourly values. The intra-annual variation of evaporation resembles to a simple sinusoidal curve with one maximum (July, 153mm) and one minimum (December, 16mm). The maximum inter-monthly evaporation change is found from August to September (-56mm) and the minimum from December to January (+6mm). The diurnal variation is also simple with a maximum at 14:00-16:00 hour (0.15mm in winter and 0.90mm in summer) and a minimum at 4:00-6:00 hour (0.03mm in winter and 0.05mm in summer). During winter, daily evaporation, in 90% of the cases, is less than 1.80mm. The maximum frequency appears for daily values less than 0.20mm, while the maximum values recorded are around 5mm. In summer, the curve is closer to the Gaussian distribution with maximum frequency of daily evaporation at around 4-6mm and maximum values above 10mm. A long-term study revealed that there is a statistically significant negative trend during winter and autumn. For spring, summer and the year as a whole, trends were not found statistically significant. Examination of the number of days with evaporation above or below specific thresholds showed that in winter and autumn, the number of days with low evaporation increases while that with high evaporation decreases, in agreement with the general winter trend. The lower winter evaporation values during the recent years are related to the increase of precipitation in Ioannina and NW Greece after 1990 due to the increase of cyclonic circulation in C Mediterranean. Finally, the synoptic patterns over Europe and the Mediterranean, associated with cases of extreme evaporation values in NW Greece, are investigated. Factor Analysis was applied on the pressure space-series of the days of the upper decile for winter and lower decile for summer. The approximately 145 days (pressure patterns) of each category were grouped objectively to 4 factors in winter and 5 factors in summer, explaining around 85% of the total variance. In winter, it appears that the wind plays the major role since in the first 3 factors the main feature of the pressure pattern in the area of NW Greece is a dry and katabatic easterly or north-easterly flow. In summer, in the 4 stronger factors, a cyclonic flow appears over the Ionian sea.
































An objective classification system of air mass types for Szeged, Hungary with special interest to air pollution levels.

This paper determines the characteristic air mass types over the Carpathian Basin for the winter (December, January, and February) and summer (June, July and August) months with the levels of the main air pollutants. Based on the ECMWF data set, daily sea-level pressure fields analysed at 00 UTC were prepared for each air mass type (cluster) in order to relate sea-level pressure patterns with the levels of air pollutants in Szeged. The data basis comprises daily values of twelve meteorological and eight pollutant parameters for the period 1997-2001. Objective definition of the characteristic air mass types occurred by using the methods of Factor Analysis and Cluster Analysis. According to the results, during the winter months five air mass types (clusters) were detected corresponding to levels of the primary pollutants that appear with higher concentrations when irradiance is high and wind speed is low. This is the case when an anticyclone is found over the Carpathian Basin and when an anticyclone prevails over the region south of Hungary, influencing the weather of the country. Low levels of pollutants occur when zonal currents exert influence over Hungary. During the summer months, anticyclones and anticyclone ridge situations are found over the Carpathian Basin. As a result of high irradiance and very low NO levels, secondary pollutants are highly enriched.

































Comparison between winter and summer sky-luminance distribution in Central Europe and in the Eastern Mediterranean.

This work studies the illuminance levels at a city in Central Europe (Bratislava) and another in the Eastern Mediterranean (Athens) and defines the prevailing sky-luminance distributions for winter and summer. The data used are five-minute values of global and diffuse horizontal irradiance and illuminance along with zenith luminance from the IDMP stations at the two cities for a period of five years. The study is based on the new set of the sky types recently adopted by CIE. These sky standards correspond to 15 theoretical diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. The theoretical curves converge above 35� of solar elevation and in many cases they intersect each other. For this reason each observation is classified into one of the 15 sky standards, only when the value of the ratio of zenith luminance to diffuse horizontal illuminance lies in a zone of �2.5% around the theoretical curve describing the specific category. If any observation is classified into more than one sky standards, the corresponding ratio of global horizontal illuminance to extraterrestrial horizontal illuminance is compared to the average ratio of the sky types implicated, for the same solar altitude, and the observation is classified in the sky type with the nearest value. It is found that the most frequent sky types for Bratislava are the overcast sky with steep luminance gradation and slight brightening towards the sun (winter) and the white-blue sky with a distinct solar corona (summer), while for Athens the cloudless polluted sky prevails in both seasons. It is also found that during a summer day, a noticeable shift appears in the sky-luminance distribution at both sites.

































Relation between sensible and latent heat fluxes in the Mediterranean and precipitation in the Greek area during winter.

Sensible and latent heat fluxes from sea surface to the atmosphere are two parameters with an important contribution in the supply of energy to the Mediterranean air masses during the cold period of the year. In this work the relation of these parameters with precipitation in the Greek area is examined for winter (December-January-February) and for the period 1959-1997. The investigation of statistical significant relations between the above parameters is carried out with the use of Factor Analysis (FA) and Canonical Correlation Analysis (CCA). FA was applied separately on the time series of each of the parameters in order to reduce the dimensionality of the original data set. Then, CCA was applied on the FA results for the pairs: precipitation-sensible heat flux and precipitation-latent heat flux in order to detect the statistically significant cases of covariability. The application of CCA on the pair precipitation-sensible heat flux led to one only statistically significant pair (W,V) which explains 50% of the common variance of the two fields with a canonical correlation r=0.70. The canonical variate W is strongly correlated with precipitation in west Greece and the east Aegean, while the canonical variate V corresponds to sensible heat flux over the west Mediterranean. Similar results were found for the pair precipitation-latent heat flux (one canonical pair, 54%, r=0.71). It is then revealed a statistically significant relation between heat fluxes (sensible and latent) in the west Mediterranean and precipitation at the windward areas in west Greece and the east Aegean. High values of sensible and latent heat fluxes in the west Mediterranean imply an enrichment in energy of the air masses traveling over the area because of the Westerlies and consequently more cases of cyclogenesis. Also, the depressions formed cause cold and dry NW flow at their western parts, which contributes to a further enhancement of these fluxes. The same depressions traveling to the east, cause precipitation at the west winward areas of Greece, due to the S-SW flow along with the presence of potential instability in the atmosphere. According to the variations of the values of the corresponding canonical variates, a slight decrease is observed in all the parameters (especially in precipitation) from early 70�s until early 90�s.

































Sky type classification for the region of Athens, Greece, during a typical daylight and solar radiation year.

In this work, three typical meteorological years (TMYs), based on daylight and solar radiation measurements, are designed for the area of Athens, Greece, using: a) the Danish method, b) the Festa-Ratto method and c) the Sandia National Laboratories method. The data basis of the study consists of 5-minute values of global and diffuse horizontal illuminance, global and diffuse horizontal irradiance, zenith luminance and solar altitude as well as daily values of sunshine duration for the period 1992-1996. Moreover, Linke�s turbidity factor, luminous turbidity factor and relative sunshine duration are calculated and utilized. Then, for each TMY, the predominant sky luminance distribution over Athens is identified, by using the methodology developed by Kittler, who has presented 15 theoretical standard sky types in diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. The TMYs so generated are not identical. The ones derived by the Festa-Ratto and the Sandia National Laboratories methods resemble more, as they have most months in common. Despite the differences, the sky types defined for the region of Athens seem not to depend on the choice of TMY. Also the predominant standard sky, for all the cases examined, is type 13, a cloudless, polluted sky with a broad solar corona.

































Frequency probabilities of daylight illuminance courses due to sunshine duration.

Lighting controls in daylit interiors have to correspond to exterior illuminance changes in various seasons associated with local climate conditions. Typical half-daylight illuminance courses indicated by long-term 5-minute regular measurements in Athens and Bratislava are related to monthly mean relative sunshine duration. During a prevailing clear half day the relative sunshine duration is very high; the higher its monthly value is more clear days have are expected. Cloudy sky conditions are associated with shorter sunshine duration while highest probabilities are related to monthly relative sunshine duration in the range 0.2-0.6. Overcast days have usually no sunshine data and, therefore, the higher the monthly sunshine duration is, the less the frequency for overcast half-days becomes. The broken and sparse cloudiness causes dynamic situations with frequent sunshine spells; thus maximum dynamic sky conditions are related to monthly sunshine duration in the range 0.5-0.75. After Athens and Bratislava IDMP measurements these frequency probabilities were analysed in relation to measured sunshine duration and this interrelation was determined by simple formulas. Frequency probabilities of these four typical daylight situations are important for modelling monthly differences derived from monthly average sunshine duration especially in locations where no other climatic parameters are available.

































Sky type classification in south England during the winter period.

In this work, we study the conditions of illuminance in south England (Garston area 51.71��, 0.38�W) during the winter period. This study is based on the 15 categories of sky luminance distribution defined by Kittler, Perez and Darula. These 15 sky types are represented in diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. The data used are diffuse horizontal irradiance and illuminance, global horizontal irradiance and illuminance, zenith luminance and solar altitude, for the period 1/10/1991-31/3/1992. The classification of the observations into the 15 sky types was performed for the following situations: a) For solar altitude greater than 5� and less than or equal 35�, where the theoretical curves are not intersected. In this case, the ratio of zenith luminance to diffuse horizontal illuminance of each observation was compared with the corresponding ratios of the 15 theoretical sky types, for the same solar altitude and then the observation was classified in the sky type with the nearest value. b) For all solar altitude greater than 5�. In this case, due to the fact that the theoretical curves converge and above the 35� solar altitude they are intersected, each observation is classified in one of the 15 sky types, only when the value of the ratio of zenith luminance to diffuse horizontal illuminance lies in a zone of �2.5% around the theoretical curve, which describes a specific category. In case that, an observation is classified in more than one sky types, the ratio of global horizontal illuminance to extraterrestrial horizontal illuminance is compared with the average ratios of the various sky types (estimated from all observations), for the same solar altitude and the observation is classified in the sky type with the nearest value. The above study has shown that the predominant sky type in south England and probable in regions with similar geographic and climatic conditions is the sky type 2 (I.2), which is an overcast sky with steep gradation and slight brightening toward the sun.

































Reconstruction of missing measured illuminance values in regular daylight data recordings

During routine daylight measurements disruptions due to equipment failure may occur resulting in data losses. These data losses can be attributed to power failure or recording of incorrect values due to lightning, snow or sensor interference by birds. After the appropriate quality control some data can be rejected too. Such irregularities cause gaps in data sets, which must be mended when fluent data sequences have to simulate illuminance courses or time availability. Therefore, in some evaluation studies the problem of missing data reconstruction has to be faced. There are several methods to deal with this task. The simplest is a linear interpolation between marginal values in narrow gaps assuming no interference of sunshine. However, in wider gaps, when fluent curves follow solar altitude influences, the quadratic or cubic interpolation is more suitable to express the nonlinearity of illuminance variations. Another more appropriate approach based on the identification of diffuse and global illuminance descriptors is proposed here. If only illuminance gaps are present but irradiance measurements are correct and fluent, then sunshine periods even under dynamic conditions can be followed. Thus using luminous efficacy ratios the illuminance values can be approximated satisfactorily. When no measurements are available during the gap, then the marginal measured diffuse and global levels are applied as descriptors for the fluent courses assuming a homogeneous case within the gap. This method can be recommended because it offers a new approach in filling gaps in global and diffuse illuminance data sets.

































Sky type classification in Central England during winter

In the present work there are studied the exterior daylight conditions in the area of Sheffield in central England (53.38��, 1.50�W) during the winter period. The study makes use of the fifteen standard sky types defined by Kittler et al. These sky types are presented in diagrams of the ratio of the zenith luminance (Lz) to the diffuse horizontal illuminance (Dv) against solar altitude (�). The illuminance parameters used are time series of half-hourly values of Dv, Lz, global (Ge) and diffuse (De) horizontal irradiance and � for the period November 1st, 1993 to February 10th, 1994. This study shows that the most frequent winter sky types in central England are: (i) I.2 (overcast with the steep gradation and slight brightening toward sun), with frequency of occurrence of 19.5%, (ii) I.1 (overcast with the steep gradation and with azimuthal uniformity) (10.5%) and (iii) II.1 (overcast moderately graded with azimuthal uniformity) (9.7%). The most predominant cloudless sky is that of V.5 (cloudless polluted with a broader solar corona) with frequency of occurrence of 7.6%.

































Sky luminance distribution in central Europe and the Mediterranean during summer period.

In this work the conditions of illuminance in Central Europe and the Mediterranean are studied and the prevailing sky luminance distributions are defined for the summer period. The data used are five-minute values of global and diffuse horizontal irradiance and illuminance along with zenith luminance from Bratislava and Athens for a period of five years. The study is based on the 15 theoretical sky types defined by Kittler et al., which are represented in diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. Due to the fact that the theoretical curves converge and above 35� they are intersected, each observation is classified in one of the 15 sky types, only when the value of the ratio of zenith luminance to diffuse horizontal illuminance lies in a zone of �2.5% around the theoretical curve, which describes the specific category. If any observation is classified in more than one sky types, the corresponding ratio of global horizontal illuminance to extraterrestrial horizontal illuminance is compared to the average ratios of the various sky types, for the same solar altitude and the observation is classified in the sky type with the nearest value. It is found that the most frequent sky types in Bratislava are: the white-blue sky with a distinct solar corona and the very clear sky with low illuminance turbidity, while in Athens the cloudless polluted sky with a broad solar corona prevails.

































Dynamics of radiation and daylight climate linked with standard skies.

A system of twin radiometric and photometric descriptors defining the radiation and daylight climate at any location is based on the local extraterrestrial flow of solar radiation. Due to atmospheric transmission, redirection and diffusion these quantities on ground level have to be studied as direct solar and diffuse sky components forming together the so called global values. The latter are usually measured either within the world-wide meteorological network or at general IDMP stations. Thus by the help of these basic descriptors can be documented daily variations using ratios of global, direct and diffuse to extraterrestrial levels corresponding to time or sun altitude changes. Secondary descriptors as Linke or luminous turbidity factors, relative sunshine duration, clearness index etc. can be deduced either by further measurements or calculation. The importance of radiance and luminance distribution on various sky types is linked to the basic descriptor system by the ratio of zenith radiance/luminance values to diffuse irradiance/illuminance levels respectively. Both these ratios on the other hand are linked to the diffusion indicatrix and gradation functions characterising sky patterns. Applying data of the set of five-year recordings gathered in Athens and Bratislava seasonally typical changes of their radiation and daylight climate will be presented considering the possibilities of their simulation by general standard skies.

































On the relation between sea surface and lower troposphere temperature over the northern hemisphere.

The covariability of sea surface and lower troposphere temperatures (SST-AT) is investigated for winter (DJFM) and summer (JJAS), over the whole northern hemisphere, by utilizing monthly anomalies in grid boxes for a 22-year period. Air temperature variations in the lower troposphere are examined in terms of 1000-500hPa thickness. At first, factor analysis (FA) is applied on both sets of data in order to reduce their dimensionality and to group objectively grid points with a similar long term temperature variability. Also, FA revealed numerous teleconnections in both fields, most important being those governed by PNA in the Pacific Ocean and NAO in the Atlantic Ocean. Then, by using the FA results as a guide, correlation coefficients between the mean temperature time series of the subareas defined by FA in the two fields are estimated. The highest values, up to r=0.70, were found for subareas located one over/under the other. Finally, by introducing time lags, an attempt of forecasting either SST form AT or AT from SST is made. Although in some cases the coefficients appear increasing, test showed that the increase is not statistically significant. In any case, it appears that, on monthly time scale, it is the AT that controls SST and not the vice versa.

































The effect of meteorological and pollution parameters on the frequency of hospital admissions for cardiovascular and respiratory problems in Athens.

This study aims to investigate the influence of meteorological and pollution parameters on the frequency of hospital admissions for cardiovascular and respiratory diseases. The study includes 1,554 patients who were hospitalized in a major hospital in Athens during an 8-year period. The meteorological and pollution database consists of daily values of 12 original parameters and 24 constructed ones. The statistical methods used are simple linear correlation and linear stepwise regression analysis and they were applied on 10-day period data. The results showed that temperature, humidity and smoke concentrations are significantly correlated with the events of these diseases.

































The 850hPa relative vorticity centres of action for winter precipitation in the Greek area.

In this work, the relationship between atmospheric circulation over Europe and precipitation in Greece is studied during high winter (January-February). Specifically, the 850hPa relative vorticity centres, affecting or related to precipitation in the various areas of Greece, are defined and the corresponding temporal variability of both parameters is examined. Factor Analysis is at first applied on both fields, in order to reduce the dimensionality of the original data sets. Canonical Correlation Analysis is then applied on the Factor Analysis results, in order to interrelate the two parameters. It is found that: (i) precipitation over west and north Greece, east Aegean islands and west Turkish coasts is mainly controlled by vorticity over Italy and the Gulf of Genoa, (ii) south Aegean Sea and Crete precipitation is controlled by relative vorticity west of Crete and (iii) east mainland areas precipitation is significantly affected by a vorticity seesaw between Tunisia and northern Aegean. The temporal variations of precipitation and relative vorticity parameters over the above areas show a precipitation reduction over most Greek stations, combined with enhanced anticyclonic activity over almost the whole Mediterranean during the late 1980s and the early 1990s, a period characterized by high North Atlantic Oscillation index values. Finally, Cluster Analysis is applied on the factor scores time series of precipitation, in order to classify the winter precipitation anomaly patterns into objectively defined clusters. Six main precipitation patterns were revealed and the corresponding relative vorticity mean anomaly regimes over southern Europe indicate the atmospheric circulation characteristics provoking these specific patterns.

































An objective definition of air mass types affecting Athens, Greece; the corresponding atmospheric pressure patterns and air pollution levels.

This work aims at defining characteristic air mass types that dominate in the region of Athens, Greece during the cold (November-March) and the warm (May-September) period of the year and also at evaluating the corresponding concentration levels of the main air pollutants. For each air mass type, the mean atmospheric pressure distribution (composite maps) over Europe and the Mediterranean is estimated in order to reveal the association of atmospheric circulation with air pollution levels in Athens. The data basis for this work consists of daily values of thirteen meteorological and six pollutant parameters covering the period 1993-97. The definition of the characteristic air mass types is attempted objectively by using the methods of Factor Analysis and Cluster Analysis. The results show that during the cold period of the year there are six prevailing air mass types (at least 3 % of the total number of days) and six infrequent ones. The examination of the corresponding air pollution concentration levels shows that the primary air pollutants appear with increased concentrations when light or southerly winds prevail. This is usually the case when a high pressure system is located over central Mediterranean or a low pressure system lays over south Italy, respectively. Low levels of the primary pollutants are recorded under northeasterly winds, mainly caused by a high pressure system over Ukraine. During the warm period of the year, the southwestern Asia thermal low and the subtropical anticyclone of the Atlantic Ocean affect Greece. Though these synoptic systems cause almost stagnant conditions, four main air mass types are dominant and ten others, associated with extreme weather, are infrequent. Despite of the large amounts of total solar radiation characterizing this period, ozone concentrations remain at low levels in central Athens because of its destruction by nitric oxide.

































A study on the intra-annual variation and the spatial distribution of precipitation amount and duration over Greece on a 10-day basis.

The intra-annual variation of precipitation amount and duration and their spatial distribution during the year are studied on a 10-day basis for the Greek region, using S-mode and T-mode Factor analysis. (i) For the intra-annual variation of precipitation amount, two modes were revealed: the first shows one broad maximum during the conventional winter in stations affected by the sea and the second presents two maxima, the first during late autumn � early winter and the second during late spring, corresponding to the northern mainland stations. (ii) For the spatial distribution of precipitation, three main patterns were revealed: the first one is the �winter� pattern with the maximum over the west windward area, the second is the �summer� pattern with a maximum over the north inland region and the third is the �autumn� pattern with the maximum over north western Greece. (iii) For precipitation duration, two types of intra-annual variation were revealed. The first one is similar to the first of the analysis for precipitation amount, while the second presents two maxima, the first during the beginning of December and the second during the middle of February, corresponding to the areas of north western and north eastern Greece. (iv) For the spatial distribution of precipitation duration three main patterns were revealed: the first is the �summer� pattern, which is similar to the second of the analysis for precipitation amount, the second is the �winter� pattern, with the spatial maximum located over the eastern mainland and western Crete and finally, the third one is the �autumn� pattern, with the maximum in north western Greece. During the third 10-day period of October and the second one of February, precipitation seems to present singularities, possibly due to fluctuations in atmospheric circulation. The above intra-annual variations and spatial distribution patterns are connected to: the seasonal variations of the depression trajectories, the atmospheric instability, the influence of sea surface temperature as a cyclogenessis factor and the windward or leeward character of the various areas (orographic effect).

































Sky luminance distribution in central Europe and the Mediterranean area during the winter period.

Five-minute data of global and diffuse irradiance and illuminance along with zenith luminance from Bratislava and Athens for a period of five years are used in order to define the prevailing sky luminance distributions at the two sites in the winter period. The classification into 15 sky types is mainly based on the value of the ratio of zenith luminance to diffuse illuminance with solar altitude as parameter (SSLD method). This is applied for the first time in a research work. It is found that the most frequent sky type in Bratislava is the overcast (with steep luminance gradation and slight brightening towards the sun); while in Athens it is the cloudless polluted sky (with a broad solar corona). That is, the winter daylight climate is quite different in Central Europe in comparison to the Mediterranean. An analysis for various solar altitudes reveals that the frequency distribution of the 15 sky types appears almost the same for every solar altitude.

































Weather conditions and sudden sensorineural hearing loss.

Climatic or meteorological condition changes have been implicated in the pathogenesis of Idiopathic Sudden Sensorineural Hearing Loss (ISSHL). We investigated the seasonal distribution of ISSHL and evaluated the influence of meteorological parameters (such as temperature, humidity, and atmospheric pressure), their variation and covariation on the incidence of the disease. A total of 82 cases of ISSHL, admitted to our University Hospital over a five-year period, were enrolled in the study. Seasonal distribution of the disease was investigated by dividing the year in four seasons. Meteorological data included daily values of 13 distinct parameters recorded at the meteorological station of the University of Ioannina during this period. A relationship between each meteorological variable and the incidence of ISSHL was investigated by applying (� 2) test on data from 13 contingency tables as well as by using logistic regression and t-test approaches. In addition, the influence of different weather types on the incidence of ISSHL was investigated using Cluster Analysis in order to create eight clusters (weather types) characteristic for the prefecture of Ioannina. The results of the study could not indicate any seasonal distribution of the disease. The incidence of ISSHL could not be significantly correlated either to any distinct meteorological parameter or to any specific weather type. Meteorological conditions, such as those dominating in the Northwestern Greece, and/or their changes have no proven effect on the incidence of ISSHL.

































Air pollution levels in Athens during winter weather types.

This work aims to define characteristic weather types over the region of Athens during winter period and then to evaluate the corresponding concentration levels of the main air pollutants. For each weather type, the mean atmospheric pressure distribution over Europe is estimated in order to reveal the association between atmospheric circulation and pollution levels in Athens. The data basis for this work consists of daily values of 13 meteorological parameters and 6 pollutants for the cold season of the year (November-March) and for the period 1993-97. The definition of the characteristic weather types is attempted objectively by using Factor Analysis and Cluster Analysis. Firstly, Factor Analysis is applied on the meteorological data in order to reduce their dimensionality by grouping objectively those parameters, which covary in time. Then, Cluster Analysis is applied on the factor scores time series in order to define clusters of days with similar weather. The results show that during the cold period of the year the prevailing weather types (at least 3% of the total number of days) are only 6. The examination of pollution concentration levels during these weather types showed that the primary pollutants appear with increased concentrations when light or southerly winds prevail. This is usually the case when a high pressure system is located over central Mediterranean or a low pressure system lays over south Italy, respectively. Low levels of the primary pollutants are recorded under northeasterly winds, mainly caused by a high pressure system over Ukraine. Very high concentrations of secondary photochemical pollutants in central Athens are rarely recorded since they are inhibited by primary pollutants.

































Effect of meteorological parameters on acute laryngitis in adults.

In this study we have investigated the monthly distribution of acute laryngitis and the influence of meteorological factors on the frequency of the disease on daily basis, in Ioannina prefecture, NW Greece. The study includes 825 cases referred to our department from 1995 until 1999. The meteorological parameters investigated were: temperature (minimum, mean, maximum, diurnal temperature range, etc), humidity (relative, water vapour pressure etc) and atmospheric pressure as well as their day-to-day changes. We examined the relationship between each meteorological parameter separately and the frequency of the disease, using Contingency Tables. Moreover, the 1825 days of the 5 years were divided into groups with common weather characteristics, using Factor Analysis and Cluster Analysis. For each group of days we calculated the number and the frequency of cases and then we searched for statistically significant influence of weather on the incidence of the disease. In the monthly distribution we observed maximum frequency of events in March and minimum in August. The detailed analysis revealed an increment in the frequency of cases when there was low temperature, low diurnal temperature range, low water vapour pressure, low atmospheric pressure, and when, in comparison to the previous day, we observed: a) great change of maximum temperature, and b) decrement in atmospheric pressure. Furthermore, we found a statistically significant influence of the weather type on the frequency of the disease. The highest frequency was observed during winter. Among the different winter weather conditions, peak frequency of cases was observed in those characterized by high humidity recordings.

































Seasonal water quality of shallow and eutrophic Lake Pamvotis, Greece: implications for restoration.

Lake Pamvotis is a moderately-sized (22 km**2) shallow (average depth=4 m) lake with a polymictic stratification regime located in northwest Greece. The lake has undergone cultural eutrophication over the past 40 years and is currently eutrophic (annual averages of FRP=0.07 mg P L**-1, TP=0.11 mg P L**-1, NH4+=0.25 mg N L**-1, NO3-=0.56 mg N L**-1). FRP and NH4+ levels are correlated to external loading from streams during the winter and spring, and to internal loading during multi-day periods of summer stratification. Algal blooms occurred in summer (Jul.-Aug. green algae, Aug.-Sep. blue-green algae), autumn (Oct. blue-green algae and diatoms), and winter (Feb. diatoms), but not in the spring (Mar.-Jun.). The phytoplankton underwent brief periods of N- and P-limitation, though persistent low transparency (secchi depth of 60-80 cm) also suggests periods of light limitation. Rotifers counts were highest from mid-summer to early autumn whereas copepods were high in the spring and cladocerans were low in the summer. Removal of industrial and sewage point sources a decade ago resulted in a rapid decrease in FRP. A phosphorus mass balance that examined external versus internal loading indicates further reductions to external loading from the predominately agricultural catchment will decreases phosphorus levels further. The commercial fishery and fish hatchery also provides opportunities to control algal biomass through biomanipulation measures.

































Weather conditions and Bell�s palsy: Five-year study and review of literature.

Background: Climatic or meteorological condition changes have been implicated in the pathogenesis of Bell�s palsy (BP).
Objective: To evaluate the influence of meteorological parameters, such as temperature, humidity, and atmospheric pressure, and their variation and covariation on the incidence of BP and to present a review of the literature on the effect of meteorological conditions on facial nerve function.
Materials and Methods: A total of 171 cases of BP admitted to our Department over a five-year period were studied. The meteorological database included daily values of 13 distinct parameters recorded at the meteorological station of the University of Ioannina during this period. A relationship between each meteorological variable and the incidence of BP was investigated by applying (�2) test on data from 13 contingency tables. In addition, the influence of different weather types on the incidence of BP was also investigated. For this purpose Cluster Analysis was used to create eight clusters (weather types) for the Ioannina prefecture and (�2) test was applied on the contingency tables consisting of the days of BP cases for each cluster.
Results: No significant correlation was found either between BP and each distinct meteorological parameter or between BP and any specific weather.
Conclusions: Meteorological condition changes have little effect on the incidence of BP. Multicenter studies taking into account atmospheric pollution, and climatic differences between countries, are necessary to scrutinize the environmental effects on facial nerve function.

































Spatial and temporal 850hPa air temperature and SST covariances in the Mediterranean region and their connection to atmospheric circulation.

The spatial and temporal covariability between the lower troposphere and sea surface temperatures are studied in the Mediterranean basin for the period 1958-98. Monthly air temperature anomalies for the 850hPa pressure level (T-850hPa) at grid points 2.5�x2.5� and sea surface temperature anomalies (SST) in grid boxes 5�x5� are utilized. As a first step, Factor Analysis (FA) is applied on both sets of data in order to reduce their dimensionality. Then, Canonical Correlation Analysis (CCA) is applied and leads to one statistically significant pair of canonical variates for winter and to two pairs for summer. In winter, a teleconnection (see-saw) between western Europe and eastern Mediterranean at 850hPa level is revealed, corresponding to a weaker one between the areas of central-west and eastern Mediterranean for SST. The correlation between T-850hPa and SST appears higher over the eastern part. In summer, the first pair of canonical variates reveals a covariability between T-850hPa and SST in the western Mediterranean, while the second one shows a covariability in the eastern Mediterranean, without an existence of any strong spatial teleconnection. The analysis is repeated, using time lags of one month, or longer, in order to detect any possible non-synchronous relation. Statistically significant results are found only when T-850hPa leads SST for one month. In particular, the results are statistically significant for winter only and the findings are similar to the ones of the first analysis. Therefore, the existence of an one-month time scale persistence in SST is detected for winter months.

































An objective assessment of the relation between meteorological parameters and the main air pollutants in Athens on a daily basis.

This work aims to reveal the relations between meteorological parameters and the concentrations of the main air pollutants in Athens. The data basis consists of daily values of 8 pollution and 14 meteorological parameters for the cold and the warm season of the year (November-March and May-September respectively) and for the period 1993-97. Factor Analysis (P-mode) was applied in order to group objectively the parameters showing common variation in time. In this way, the 22 initial variables were reduced to three Factors for the cold period and four for the warm. Consequently, the final three or four Factors can be considered as the main contributors to the formation of the weather and air pollution conditions in Athens basin. Wind speed is the crucial parameter for the accumulation of air pollutants in Athens basin throughout the year. The role of wind direction on air quality appears stronger during the warm period. The secondary pollutant of O3 is controlled by the primary ones and this is more apparent during summer.

































The influence of meteorological parameters on the frequency of epistaxis.

This retrospective study aims to search out the influence of temperature, atmospheric pressure and humidity parameters on the frequency of epistaxis events. The study includes 701 patients, who have suffered from epistaxis and they have been treated in the ENT department of the University of Ioannina Hospital, during the years 1995 and 1996. The statistical methods used are simple linear correlation and linear stepwise regression analysis. The results of simple correlation analysis showed that the daily number of epistaxis depends mainly on mean, maximum and minimum temperature and water vapour pressure. The corresponding correlation coefficients are statistically significant, indicating an influence of weather on epistaxis up to 9% of its total variance. By applying stepwise regression analysis, we managed to increase the linear correlation coefficient and the corresponding amount of variance of epistaxis explained by meteorological parameters. This percentage was found to approach 20% for the cold period and 10% for the warm period of the year.

































An evaluation of the nature and timing of summer human thermal discomfort in Athens, Greece.

An assessment of the summer human thermal climate over the period 1966 to 1995 for Athens, Greece was undertaken using the Predicted Mean Vote (PMV). At the seasonal (diurnal) time scale human thermal discomfort reaches a maximum over a 20 day (4-6 hour) period centred on the end of July (1400 hours LST). Not only are general levels of thermal discomfort variable at the intra- and inter-annual time scales but the timing of discomfort season onset and cessation is highly variable from year to year. Man-Kendall tests for trends of discomfort season diagnostics revealed that there has been a tendency towards an increase in the length of the discomfort season over the study period. The implications of Athens� harsh human thermal climate for staging athletic events are discussed and the concept of cool and warm synthetic summers is introduced as a basis for the planning of human activities.

































Daylight climate specification based on Athens and Bratislava data. Comparison of daylight conditions.

Available horizontal daylight illuminance and zenith luminance data gathered in 5-minute intervals during five years at IDMP general stations in Athens, Greece and Bratislava, Slovakia were analysed. Using the set of fifteen sky types of the proposed CIE General Sky Standard the seasonal and yearly occurrence frequency of sky types have shown relevant differences in daylight climates of Central Europe and the Mediterranean regions. The methods of statistical analysis can be applied in any location where long term measurement data are recorded and archived.

































Winter temperature covariances in the middle and the lower Troposphere over Europe and the north Atlantic ocean.

In this work, the variability and covariability of winter temperature in the middle and the lower troposphere are studied over Europe and the North Atlantic Ocean. Temperature variations of the middle and the lower troposphere are examined in terms of a) 500-700hPa and 700-1000hPa thickness and b) air temperature on the isobaric surfaces of 500hPa and 700hPa. At first, Factor Analysis, defined areas with characteristic temperature variability in each layer (and on each isobaric surface) and then, Canonical Correlation Analysis revealed areas in both layers (and on both isobaric surfaces) presenting common temperature variations. A temperature seesaw between North Europe and West Asia was revealed for both layers and isobaric surfaces implying that temperature changes in these areas are vertically spread. Another well defined area, appearing in both analyses, is the area of Labrador Sea and S Greenland. This region is also teleconnected to other regions, though not very clearly in every height. These temperature patterns are mainly attributed to the Eurasian (EU) and North Atlantic Oscillation (NAO) circulation patterns, which are responsible for large air mass exchanges in the area, being vertically extended in the middle and the lower troposphere.

































Sea surface temperature-850hPa relative vorticity relations in the Mediterranean region during winter.

In this work, a relation between 850hPa relative vorticity and sea surface temperature in the Mediterranean region was sought for winter during the last 40 years. For this reason, at first, Factor Analysis was applied on both sets of time series and then Canonical Correlation Analysis on the resultant factor scores. The results showed that there is only one statistically significant canonical pair which relates relative vorticity over central Mediterranean and Middle East with sea surface temperature over the Aegean and the Alboran sea. High relative vorticity over Middle East is connected with low relative vorticity over central Mediterranean and they imply high sea surface temperature over the Alboran sea and low sea surface temperature over the Aegean sea and vice versa. These seesaws have presented an one-sided tendency during the last 40 years corresponding to stronger or/and more frequent blocking anticyclones over central Mediterranean, being responsible for the reduction of precipitation observed over S Italy and the Balkans during the same period.

































A statistical study of precipitation in NW Greece.

In this work, using 20 year mean monthly precipitation totals from 54 stations, the general statistics for time and space distribution of precipitation in NW Greece, a very particular area, is at first described. Precipitation varies from about 1000mm at coasts to 2500mm on mountains, with a large variability. Then, the intra-annual variation is studied, using Fourier Analysis. This variation is described well by two harmonics, explaining about 90% of the total variance. The first harmonic, over 80% of the variance, shows a maximum from late December (coastal areas) to early January (continental areas) and is caused by the NW Mediterranean depression activity. The second harmonic exhibits maxima in May and November, also delayed in the continental areas, and is mainly caused by upper air troughs and instability. The spatial distribution of precipitation is then studied by using Factor Analysis. The 12 months of a year are classified in two parts corresponding to two statistically significant factors, explaining 88.5% of the total variance. Finally, the combination of positive and negative standardized scores of the two factors showed that NW Greece is characterized by four precipitation regimes, depending on the combination of depression activity, effect of sea and effect of topography.

































Temporal variations of the harmonic analysis parameters of the annual pressure march over the Atlantic, the Mediterranean and Asia.

In this study the temporal variations of the harmonic analysis parameters of the annual pressure march are studied over the Atlantic, the Mediterranean and Asia. Mean monthly pressure values have been used for the period 1873-1994 at grid points between 35N-50N and 40W-110E. Factor analysis revealed three representative areas inside which the above parameters covary in time. It was found that over the Atlantic the time of maximum of the first harmonic appears somewhat earlier the recent years. It was also found that the last years the amplitude of the first harmonic increases in the Mediterranean and decreases over Siberia. In most cases a turning point appears at about 1970. These changes are due to changes in mean monthly pressure values at specific months of the year. Finally it is especially mentioned that there exists a teleconnection (sea-saw) between the Atlantic and the Mediterranean in spring. During the last 50 years when pressure in spring is above normal over the Atlantic, it is below normal over the Mediterranean and vice versa.

































Analysis of the periodic variations of atmospheric pressure over Greece.

The annual variation of atmospheric pressure over Greece presents a double fluctuation with a primary maximum in October-November and a primary minimum in July. The diurnal variation presents a double fluctuation, too, with a maximum around 10:00, a primary minimum around 16:00 a secondary maximum around 22:00 and a secondary minimum around 05:00. The three harmonic terms reproduce the original curve well, as they explain 87% of the total variance. Most important, and almost identical to the final curve, is the 2nd harmonic term.




















Estimation of the eddy thermal diffusivity coefficient in water.

In this paper the eddy thermal coefficient in water is estimated. The experiment took place at Lake Washington, Seattle, USA. Water temperature in surface layer, wave frequency as well as all the other meteorological parameters were recorded simultaneously. After data processing, the value of the coefficient was found 20x10**(-5 ) m**2/s. Taking into account that the coefficient is depended on weather conditions the results are considered satisfactory.























Annual variation of pressure over the Mediterranean area.

This study deals with the annual variation of pressure over the Mediterranean area using monthly values from 24 stations, for a period of 22 years. The Azores and the Siberian anticyclones as well as the low pressure system of SW-Asia are responsible for the observed pressure pattern over the area and for the seasonal changes at each station. Fourier analysis shows that the reproduction of the annual variation of pressure is better at stations where the amplitudes of the successive harmonic terms converge faster. The strongest harmonic is the first one which presents its maximum from October (central Mediterranean) to January (east and west Mediterranean). Finally for central Mediterranean stations, which record many minima and maxima of pressure during the year, the amplitude of the second harmonic is smaller than the amplitude of the third.























The annual variation of pressure over the Northern Hemisphere.

The annual variation of the mean monthly surface pressure is studied over the Northern Hemisphere using the harmonic analysis. The study is based upon grid point data, 1900-1974. Eventually, the annual variation of mean pressure systems is studied, too. Two types of annual pressure variation are found, viz. one with a single harmonic and the other with two harmonics. The first is found around the middle of the large continents, with a pressure maximum in winter, and over the large oceans with a maximum in summer. In this type the first harmonic contributes about 90% of the total variance and the residual harmonics are negligible. The two-harmonics type is found in transitional zones between the continents and the oceans. The mean pressure systems are of thermal mostly nature, showing their maximum strength either in June-July (Asiatic and Californian Lows) or in December-January (Icelandic and Aleutian Lows and Siberian anticyclone).























Temperature fluctuations in the Mediterranean area during the last 120 years.

Based upon sea-surface temperature (SST) data, recently corrected for bucket-intake observational change, the SST fluctuations since 1873 in the Mediterranean are studied and compared with air temperature (AT) fluctuations at some land stations of this area and for the Northern Hemisphere (ATNH). Instead of the actual temperature, we used the frequency difference (per cent) of the number of the warm minus cold months, (Fw-Fc) annually and seasonally, smoothed using 10-year moving averages. This frequency difference was found to be a linear function of the actual temperature. For the whole Mediterranean, the trend of SST is simple enough, with a minimum in about 1910 and a double maximum in about 1940 and 1965. A secondary minimum follows in 1975-1980, very strong in the eastern Mediterranean, with a rise after that to the present. This trend when compared with ATNH presents some differences: in the ATNH a minimum appears earlier, in 1890, but this is shown only slightly in the Mediterranean SST. The second SST maximum, in 1965, is not shown in ATNH, and the general rise of recent years starts in the ATNH about 10 years earlier. When considering the AT of the Mediterranean stations, it can be seen that the rise, after the minimum of 1910, stops 15 years earlier than the SST and ATNH. Then, an anomalous and slow decrease follows until about 1975-1980, followed by a rise to the present, which is delayed in the eastern Mediterranean.























Sea surface temperature in the Mediterranean. Statistical properties.

Mediterranean Sea Surface Temperature is studied using various statistical properties. The whole area is divided in 5 long x 5 lat squares and the data used are mean monthly values for each year in these squares, since 1876. Harmonic analysis shows that one sinusoidal wave can represent well the annual variation of SST. The time series shows that there is a minimum in about 1910, a split maximum between 1947 and 1964, a secondary minimum in about late 70's, and a continuous increase since then. Spatial correlation, using the stepwise regression analysis shows that the SST of each square strongly depends on the SST of the West and East neighbouring ones. Finally, the ARIMA prediction models were applied and gave rather good results and better than the temporal multiple correlation coefficients, given by the stepwise regression analysis.























Atmospheric circulation anomalies in dry and wet winters in Greece.

The synoptic condition during the very dry winter of 1989 in Greece and most Europe is at first described. Similarly, the synoptic condition is investigated for the upper and lower decile of January rainfall, using Athens rainfall for its long record. The analysis has indicated that the winter of 1989 has been a very extreme case. In all cases, the persistence of a blocking anticyclone appeared to be the dominant synoptic feature. When this anticyclone covers Europe and the Mediterranean area, then dry conditions prevail over most Europe. On the other hand, when the anticyclone is located over the northeast Atlantic ocean west of the previous position, then wet conditions prevail over Greece and large part of Europe.























Covariability and climatic changes of the lower-Troposphere temperatures over the Northern Hemisphere.

The covariability and the long-term fluctuation of the lower-troposphere temperatures is studied, applying factor analysis on grid point (1000-500)hPa thickness data for the Northern Hemisphere. About 30 factors were retained and rotated for each season and year, but only about half of them were found to be significant. The grouping of the grid points according to the factor loadings has been mainly influenced by the land and sea distribution. Three teleconnections were found only, one in winter and two in summer. The winter one is a see-saw phenomenon between West Asia and North-West Scandinavia. In summer, the British Isles were found to covary with East Canada and the same is the case for almost all the subtropical deserts. The study of the factor scores revealed that the recent warming of the Earth's surface in the lower troposphere appears over the continents only.























Spatial and temporal sea-surface temperature covariances in the Mediterranean.

Using sea-surface temperature data for the post-war period, the Mediterranean Sea is divided objectively (principal components factor analysis) into subareas with similar covariances. This division is not the same in every season of the year. In summer, two subareas are found, the west and the east. In spring and autumn three, and in winter four significant subareas denote physical oceanographic and meteorological processes. The scores of the analyses show clearly the minimum of the late 1970s and then the warming of the recent years somewhat delayed in the eastern Mediterranean.























Pressure covariability over the Atlantic, Europe and N. Africa.
Application: Centers of action for temperature, winter precipitation and summer winds in Athens, Greece.

This paper deals with surface pressure covariability over the Atlantic/European sector of the Northern Hemisphere, using monthly grid point data for the 100 year period 1890-1989. Factor Analysis is applied to 90 grid point time series for January, February, July, and August. The initial 90 pressure variables can be reduced to 7-8 factors in winter and 10 in summer. A winter teleconnection was identified, known as the seesaw phenomenon, between the Icelandic low and the Azores subtropical anticyclone. In order to define the centers of action for temperature, winter precipitation and summer northerly wind frequency (etesian days) in Athens and in the Aegean sea, the variability of the factor scores and of these weather elements is compared. It is shown that the center of action for temperature in Athens is found to be in north and northwest Europe (centered over southern Scandinavia). For winter precipitation, the center of action is located in the west and southwest Mediterranean and northwest Africa. Finally, for the etesian winds frequency variability, this center of action is found over the northern Adriatic and northern former Yugoslavia, while there is no evidence of influence by the southwest Asia thermal low.























Temporal variations of atmospheric tides over Athens, Greece.

The mean daily march of pressure (hourly values) for each month of the 96 year period 1894-1989 in Athens, Greece is studied using Fourier analysis. The annual variation of the harmonic parameters (amplitude, time of maximum, x and y-components) is examined. The globally excited 2nd and 3rd harmonics show in general the same behaviour as in other places of the Earth. Similar features show the corresponding parameters of the daily solar radiation march. The first harmonic displays some local characteristics. The long period fluctuations study revealed statistically significant trends in most parameters of the 2nd harmonic, most strikingly the 0.5h shift in the time of its maximum during the 20th century. Power spectrum analysis showed the existence of a biennial oscillation in B1, the x-component of the first harmonic, but this oscillation is mainly confined to the second half of the entire period. A periodicity of about 6 years which could be associated to the Southern Oscillation was also found in B1 while in A1, the y-component, a periodicity close to the 11-year solar cycle and another one close to the nutation of the Earth's axis are present. The lunar tides are too small to be detected by the same methods. However the semilunar tide was revealed to be 10-20 times smaller than the semisolar.























Daily precipitation variability in semi-arid agricultural regions in Macedonia, Greece.

The spatio-temporal variability of daily precipitation series was investigated in a semiarid region of central Macedonia in northern Greece. Ten years of daily rainfall records for seven stations in the region constituted the data base. The spatial characteristics were examined by drawing composite correlation diagrams for the cool (October-March) season and the warm (April-September) season, and the results confirmed the regional homogeneity of the data sets. Furthermore, the temporal analysis indicated that the non-rainy days constituted the major portion of days throughout the year at all the stations. Similarly, light rainfall represented the majority of rainy days. Moreover the annual rainfall variation showed high values in March, April and November with low values occurring in the summer and autumn. A sharp increase of rainfall between the 185th and 195th day of the year must be taken into account when the harvest is scheduled. Harmonic and Power Spectrum analyses applied to the annual variation of rain depths using 5-day intervals revealed significant periodicities of 26, 122, 365 and 55 days. Finally the analysis of the annual variation of rain occurrences revealed periodicities of 365 and 122 days.























Factor Analysis of some climatological elements in Athens, 1931-1992: Covariability and Climatic Change.

The climatological parameters of a station are, in general, intercorrelated to various degrees (e.g. cloudiness with sunshine duration or possibly rainfall with temperature). In this work, 15 climatological parameters of the National Observatory of Athens are distributed in groups where the contained parameters are highly covariant. This classification is achieved using P-mode Factor Analysis, based on the monthly values for the period 1931-1992. Using several known criteria, we found up to four significant factors. The first two are the most important ones including the "temperature" and the "precipitation" parameters, respectively, with loading values above 0.85. The other two are weaker and variable, depending on the month. Finally, climatic trends of the grouped parameters (factors) are examined and the results are compared with those of studies using classical methods.























Northern Hemisphere gross circulation types. Climatic change and temperature distribution.

In this work the atmospheric circulation patterns over the N Hemisphere are objectively revealed and discussed, applying Factor Analysis, T-mode, on monthly grid point data of surface pressure. The analysis has been effected for January and February on the one hand and for July and August on the other. The results were similar in adjacent months in almost every respect. Using Monte-Carlo technique, up to 6 circulation types (factors) were found statistically significant (at 0.05 level) during the high winter months, in the 100 year period 1890-1989, but 2 of them comprise very few years only. The first pattern refers mainly to Europe and Asia, the second to the Atlantic, the Pacific and the adjacent coasts, the third to the whole N Hemisphere and the fourth (5 cases only) mainly to the N Pacific. For the high summer months, only two factors were found, affecting mainly the polar regions. In the first, a polar low appears and in the second a polar high. For the remaining regions worth mentioning pressure anomalies appear over NW Europe only. The time series of patterns indicates a climatic change in atmospheric summer circulation, leading to corresponding changes also in other meteorological parameters (e.g., precipitation). Finally, using 1000-500hPa thickness data for the post-war years, the temperature distribution of the revealed patterns is shown to be in accordance with the prevailing wind field.























Variability and covariability of basic air pollutants in Athens.

The mean annual variability and covariability of the basic air pollutants (Smoke, O3, SO2, NO2) monitored in 9 stations of the Greater Athens area, is studied for the period 1991-94. On the time series of each pollutant Factor Analysis has been applied in order to define the main modes of variation and to group the stations in subgroups with a particular annual variation. The results showed that NO2 is the only one exhibiting different annual variation in the coastal than in the inland stations. This is due to the Etesian winds which in summer affect the penetration of the sea breeze over the land.























Phenomenological features of precipitation series in agricultural regions.

In this paper phenomenological features of daily rainfall series are investigated in homogeneous agricultural regions. A region in central Macedonia of northern Greece is used for the analysis, where the availability of water is mandatory for the agricultural activities of the region. Seven stations are employed with a 10-year record of daily rainfall depths. At first, composite correlation diagrams are used, which justify the regional and spatial homogeneity of the data sets. Temporal characteristics of the rainfall data have shown that the majority of days throughout the year are without rain for all the stations using either mean annual or monthly rain occurrences. Similarly, the small rainfall amounts constitute the majority of the rainy cases. Moreover, harmonic analysis of the annual variation of rain occurrences is attempted using 5-day values of rainfall depths. The five strongest harmonics are presented along with phases, amplitudes, temporal cycles and total variance explained .























Spatial covariability of the climatic parameters in the Greek area.

In this work, 12 monthly climatic parameters of 49 Meteorological stations in Greece are classified according to their spatial covariability, in order to define groups of climatic elements having the same characteristic spatial distribution. The grouping of the parameters is achieved objectively by using the method Factor Analysis. The analysis is applied for the similar winter months, January and February and for the summer months July and August. In winter the parameters are classified in 4 groups as following: (a) maximum, mean and minimum temperature and, negatively, pressure and number of frost days, (b) wind and, negatively, diurnal temperature range, (c) precipitation and (d) cloudiness and, relative humidity. In summer the groups are three: (a) precipitation, cloudiness and, negatively, minimum temperature, (b) maximum and mean temperature, relative humidity and (c) wind. From the above groups the most interesting is (a) of summer, as the comprised parameters, in winter belong to different groups. Finally, the study of the spatial distribution of the above groups showed that some depend on latitude, others on the continentality and others on the leeward or windward character of the various areas.























Climatic fluctuation of temperature and air circulation in the Mediterranean.

The global warming is tested in the Mediterranean area during the last 45 years using SST and 1000/500mb thickness data, in order to avoid urban effects. The area is divided in two parts vis. the West and the East Mediterranean. Since warming is not apparent in the East Mediterranean but only during the last years, the time series of surface pressure and relative geostrophic vorticity were examined for possible explanations. It is found that there exists a continuous and considerable increase of the mean vorticity in the extreme East Mediterranean, meaning increased frequency and/or strength of N. winds there, causing a delay of the warming. The opposite phenomenon appeared in the extreme West Mediterranean where increased S. winds may contribute to the warming there.























Circulation types over the Atlantic, Europe and N. Africa and their temperature distribution.

The main pressure patterns that dominate over the Atlantic, Europe and N Africa in winter and in summer are investigated using grid point surface pressure data for the 100 year period 1890-1989. The method used is Factor Analysis, T-mode, which revealed 5-6 patterns for the winter months and 2 for the summer ones. In winter, the first factor exhibits lower than normal pressure values in high latitudes; the second factor shows positive anomalies over Scandinavia and negative over central Europe. The other factors are very weak and only one or two years depend on them. In summer, the first factor shows positive anomalies in the polar region and negative over the British Isles while the second one exhibits the opposite characteristics. Finally, the the 1000-500hPa thickness anomalies for the years corresponding to each factor are found to be in good agreement with the expected conditions by the described circulation patterns.























On spatial and temporal variations of drought in selected regions of central and southeastern Europe.

Spatial and diachronic characteristics of drought periods are examined. Assessment of droughts is performed by employing the Palmer Drought Severity Index (PDSI). The Palmer's model is applied to monthly data in the period 1951-1990 (1957-83 for Greece) at 27, 19, 16, and 26 stations in Poland, Czech Republic, Hungary and Greece, respectively. Factor analysis is employed to define subareas in each country, inside of which PDSIs covary in time. The same is also performed for Z-index. The station with the highest factor loading in each grouping is used for estimation of the basic statistical parameters and for ARMA modeling techniques.























The variability of the relative geostrophic vorticity in Europe, the Atlantic and N. Africa, during the year.

The intra-annual variation of relative geostrophic vorticity over the Atlantic, Europe and N. Africa is studied using daily and pentad surface pressure and 500hPa height grid point data, for the 20 year period 1970-1989. Factor analysis is used in order to reduce the dimensionality of the 117 grid points. Six factors were retained explaining about 65% of the total variance. The first one, which is the dominant, covers the Mediterranean, N. Africa and the Middle East. These areas display one maximum and one minimum during the year. However, the time of this maximum is not the same in all the above areas, as numerous seesaw phenomena were revealed.























Factor Analysis as a tool for coherence and variability studies in climatology.

In this paper the outlines of Factor Analysis are presented, along with some applications in Climatological Studies. The main achievement of this analysis is at first the objective reduction of the dimensionality of a data set, which then can be handled more easily, in order to reveal its hidden constituent features. The variables denote either space (stations or grid points) or time (years, months or days) or meteorological parameters and are examined for possible covariability. The combinations of each two of these variables result in the various modes of this analysis. So we have the following examples:
S-mode (S for space): The covariability (and the climatic change) of the lower-troposphere temperatures are studied, using monthly grid point 1000-500hPa thickness data for the N hemisphere. The factor scores trends showed that since 1945 only the recent warming is apparent and over the continents.
T-mode (T for time): The atmospheric circulation types over the N hemisphere are investigated, using monthly values of surface pressure for the period 1890-1989. In winter, up to 6 circulation types appeared (4 important) while in summer 2 only. The validity and the significance of the results were confirmed by examining the 1000-500hPa thickness anomalies corresponding to each factor.
P-mode (P for parameters): The covariability of 15 climatological parameters at the National Observatory of Athens is studied using their monthly values since 1931. Four factors are found but two of them are the most significant, viz. the "temperature" and the "precipitation" ones. The other two factors are weaker and variable, depending on the season.