The northwest african depressions affecting the south balkans

Abstract 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.