Since ancient times and till the middle of XVIII century it was known in Russia as the Varangian Sea. The Baltic sea is a shallow shelf sea, its prevailing depths are 40-100 m. Its extension for  12° along the meridian specifies noticeable variability of weather conditions of its different areas. Mean air temperature of the south part of the Baltic Sea: in January —1,1°С, in July 17,5°C; of the middle part: in January —2,3°С, in July 16,5°C; of the Gulf of Finland: in January —5°С, in July 17°C, of the northern part of the Gulf of Bothnia: in January —10,3°С, in July 15,6°C.