This study presents a discussion of the main characteristics, and at the same time gives a detailed classification, of the north-west African depressions within the framework of the close relationship between the 500-hPa flow and the origin, development, and movement of surface depressions.The classification relies mainly on the similarities and differences present in the 500-hPa flow patterns when each north-west African depression first appears on the surface synoptic chart. For a detailed discussion and, especially for a record of the synoptic conditions that are favourable for the formation of north-west African depressions, mean 500-hPa height and anomaly charts and mean temperature charts, for the years 1970-1974, have been drawn for each subcategory into which the 79 cases fall.For each of these subcategories, mean charts have also been drawn for 850-hPa height and temperature, mean sea-level (MSL) pressure, and MSL pressure anomalies. The classification resulted in the north-west African depression being divided in five main categories A, B, C , D, and ST. Category A is divided into four subcategories, the depressions of which appear only during spring. Categories B and C are divided into two subcategories, B1, B2 and C1, C2, and these too occur only in spring, except for subcategory C2, which also appears in autumn. Category D is also divided into two subcategories, which appear only during winter. Category ST (stationary depressions) includes only four cases out of 79 north-west African depressions. These are shallow, warm-cored atmospheric circulation systems having n o correspondence with the 500-hPa flow and which appear in late spring and early autumn.The study showed that the north-west African depressions d o not exhibit significant differences from other extratropical depressions. Their origin, development, and movement largely follow (except for category ST) Sutcliffe's and Petterssen's traditional theories of development.
This paper deals with the north-west African or the Atlas depressions, named after the region of their formation.The subject is examined from the synoptic climatology point of view, and is based on mean sea level (M.S.L.) pressure and 500 hectopascal (hPa) heights of the five consecutive years from 1970 to 1974. Attention is drawn to the following topics.1. The positions of the depression centres are determined when they are generated and afterwards. So the numbers of the occurrence of their centres in the subregions into which north-west Africa is divided and the tracks of these centres are obtained. 2. These depressions are divided into four categories. The first category is divided into four subcategories, whereas each of the remaining categories is divided into two subcategories. These categories are based on the 500 hPa flow patterns. 3. For each subcategory, mean charts of the 500 hPa height for the first occurrence of its depressions have been prepared. For these mean charts, the relative vorticity advection has been calculated. These charts suggest the region of the first occurrence of the depressions is mostly in a region of positive advection of vorticity. 4. The percentage of the number of depressions from each subcategory affecting the weather of Greece can be determined. For each percentage, the mean M.S.L. pressure and 500 hPa height charts showing the time at which these depressions start affecting the weather of Greece are drawn. Thus, the synoptic situations affecting Greece and resulting from the north-west African depressions are determined. Finally, the rain, wind and temperatures from each north-west African depression subcategory occurring in Greece and especially in the Athens major region are discussed.
The characteristics of the sea breeze in the Attica region of Greece, in which Athens is located, have been studied for occasions ofweak synoptic-scale pressure gradient. The analysis is based on synoptic observations from six meteorological stations, three on the coast and three inland. The three inland stations and one of the coastal stations lie almost in a straight line at different distances from the coast. For each meteorological station, the basic characteristics of the sea breeze were determined, i.e.,(1) The mean number of sea-breeze days for each calendar month.(2) The monthly mean wind speed for each synoptic hour.(3) The times of onset and cessation of the sea breeze.(4) The monthly vector mean wind, and its constancy* for each synoptic hour.(5) For days on which there was a sea breeze at Helliniko (the coastal reference station), the percentage number of days on which there was also a sea breeze at the given station.An attempt was also made to determine further characteristics, such as the inland penetration of the sea breeze, its depth, the spatial and temporal variation of wind speed and direction, and the existence of the return flow.Finally, the properties of the land breeze are briefly outlined.
In this paper the so-called extention of the Azores" anticyclone towards the Balkans and Greece is studied by means of the synoptic evolution of mean charts for m.s.l., thickness, 500 mb and their anomalies. The period under consideration is the twelve years from 1961 through 1972 for the two month interval July-August, which is the main season of the occurrence of the Etesian winds over the eastern Greek seas having their existing in the combination Of the Balkans high pressure and the permanent low pressure system over the eastern Mediterranean Sea. The synoptic evolution of the above mentioned charts proved that the mobile anticyclones or the wedges of the Greek area, seemingly form one common system with the Azores' anticyclone but their generating factors are different from these of the Azores' permanent anticyclone which implies that the term "extension" should not be used. Zusammenfassung Findet eine Ausdehnung des Azorenhochs bis zum Balkan tats~chlich statt?In der vorliegenden Arbeit wird die synoptische Entwicklung der sogenannten Ausdehnung des Azorenhochs bis zum Balkan und Griechenland untersucht. Dazu dienen Kartendarstellungen for mittlere Verh/iltnisse im Boden-nnd 500 mb-Niveau mit den dazugeh6rigen relativen Topographien. Die Untersuchungsperiode erstreckt sich auf zw61f Jahre fur die ausgew~hlten Monate Juli und August. Zu dieser Zeit treten am h~iufigsten die Etesien tiber der )ig~iis auf, die ihre Existenz dem hohen Druck tiber dem Balkan und dem tiefen tiber dem 6stlichen Mittelmeer verdanken. Die synoptische Entwicklung zeigt, dat~ gemeinsam mit dem stationiiren Azorenhoch wandemde Antizyklonen bzw. Hochdruckkeile fiber Griechenland auftreten, deren Bildung und Entwicklung sich aber yon der des Azorenhochs unterscheidet. Es sollte daher der Ausdmck ,,Ausdehnung" des Azorenhochs vermieden werden.
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