Portugal is mainly characterized by a Mediterranean climate (''Csa'' in the South, central interior, and the Douro river valley; ''Csb'' in the North, Central west and Vicentine Coast), temperate maritime climate (Cfb) in the mainland north-western highlands and mountains, and in some high altitude zones of the Azorean islands; a semi-arid climate in certain parts of the Beja District far south (''BSk'') and in Porto Santo Island (BSh), a warm desert climate (BWh) in the Selvagens Islands and a humid subtropical climate in the western Azores (Cfa), according to the Köppen-Geiger Climate Classification. It is one of the warmest countries in Europe: the annual average temperature in mainland Portugal varies from  in the mountainous interior north to  in the south and on the Guadiana river basin. There are however, variations from the highlands to the lowlands: Spanish biologist Salvador Rivas Martinez presents several different bioclimatic zones for Portugal. The Algarve, separated from the Alentejo region by mountains reaching up to  in Alto da Fóia, has a climate similar to that of the southern coastal areas of Spain or Southwest Australia.