BALTIC SEA, a sea extending between S4° and 66° N. lat. and 9° and 3o° E. long. surrounded by Sweden, Denmark, Ger many, Poland, Lithuania, Latvia, Estonia, Russia and Finland (German Ostsee; Russ. Baltiyskoe More). The main axis runs approximately south-west to north-east and the distance from Kiel to Haparanda is 1,700 kilometres. It connects with the North sea by a channel between south Scandinavia and Denmark; the part north and west of the Skaw is called the Skagerrak and resembles the North sea, the part south of this line is the Kattegat and is the outer part of the Baltic. At its southern end the Kattegat is blocked by the Danish islands save for the Sound, the Great Belt and the Little Belt which communicate with the Baltic proper. There are several deep areas, the Arcona (5o metres) north of Riigen, the deep (1o5 metres) east of Bornholm, sepa rated by the Middlebank from the Gottland (249 metres) which extends north-eastwards to the Gulf of Finland. Beyond the Middlebank, to the south-east, is the Danzig deep (ii. me tres). Along the Swedish coast a deep channel runs northward from outside the island of bland ; this is entirely cut off to the south and east by a bank which sweeps eastward and northward from near Karlskrona, and on which the island of Gottland stands, but it communicates at its northern end with the Gottland deep, and near the junction opposite Landsort is the deepest part of the Baltic (420 metres). On the east is the large shallow Gulf of Riga, not more than 46 metres deep. Lat. 6o° N. marks the boundary between the Baltic proper and the Gulf of Bothnia and on it lie the Aland islands, to the west of which (Alands Haf) the sea is deep (237 metres) whereas to the east it is shallow. In the Gulf of Bothnia depths of more than 200 metres are reached, but north of the Nord Kvarken, situated on a submarine ridge, depths of only ioo metres or less are found in Bothnia-Wick. The most re cent estimate of the area is 422,3ookm., including the Kattegat, and the mean depth is 55 metres.
Bottom Deposits.—In the deeper hollows in the south part of the Baltic the bottom consists almost invariably of either soft brown or grey mud or hard clay, while on the shallow banks and near the low coasts fine sand, of white, yellow or brown colour with small pebbles, is usually found. There are several areas of stony bottom, some of which are dangerous to ships, e.g. "Adler grund" (only 6 metres) between Bornholm and Rugen. These are clearly remains of glacial moraines and they greatly hinder trawling. They may be compared with the Borkum bank in the North sea.
Coast Changes.—At some period during, or between the cold phases of, the Pleistocene ice age an arm of the sea extended across Sweden, submerging a great part of the littoral up to the Gulf of Bothnia, and including the present lakes Vener, Hjel mar and Malar. During this period the waters of the northern Baltic were sufficiently salt for oysters to flourish. Subsequent upheaval restricted direct communication with the open sea to the Danish channels, and the Baltic waters became fresher; the oyster disappeared, but a number of cold salt-water fishes and crustaceans, and even seals, became acclimatized. The Baltic was next elevated into a vast freshwater lake, the Ancylus lake of G. de Geer (named from the remains of the mollusc Ancylus fluvia tilis), which is supposed to have included the whole of the present Baltic area and a large part of Finland, with Lake Ladoga. Then followed a subsidence, which not only re-established communi cation through the Danish channels, but allowed the Baltic to become sufficiently salt for such forms as Littorina littorea (Lit torina period). At this time the Gulf of Bothnia must have suffered greater depression than the Baltic proper, for the deposits of that epoch show a thickness of ioo metres near Hernosand, but only 25 metres in the neighbourhood of Gottland. After this period of subsidence the process of elevation set in which gave the Baltic its present form and physical condition, and appears to be still in progress. The Gulf of Bothnia has been more strongly uplifted than the Baltic proper, just as previously it sank more. Hogbom (1913) showed, through the drawing of isobasic lines (lines of equal elevation), that the centre of the movement is in the Nord Kvarken and that the German Baltic coast has probably not moved up or down since the end of the ice age. Witting (1918) calculated a reduction of depth of about 0.2cm. per annum near Gottland, about o.5cm. near Stockholm, 1cm. or more in the Bothnia Wick. This last has put out of use the former harbours of Vaasa, Kokkola (Gamla Karleby) and Raahe (Brahestad).
Coasts.—The coast of the Baltic is rocky only in the island studded region at the head of the Baltic basin proper—a sub merged lake-district—and the littoral generally is typically morainic. From Slesvig Nord eastwards to Lubeck bay the coast is pierced by a number of narrow openings or Fohrden, the result of encroachment of the sea caused by subsidence. East of Lubeck, as far as the mouth of the Oder, these give place to Bodden, ramified openings studded with islands. Beyond the Oder the coast-line is unbroken as far as the Gulf of Danzig. It is then cut into by the estuaries of the Vistula, the Pregel and the Memel. Here the westerly winds have full play, and the coast is rimmed by a continuous line of dunes, which cut off the two great lagoons of the Frisches Haff and Kurisches Haff by sandspits or Nehrungen.
Baltic Levels.—If we start from a point in the North sea we find that the surface in the Kattegat is ocm. higher, between Riigen and Bornholm 2 2cm., near Stockholm 31 cm., and near Haparanda, as well as near Viipuri (Viborg) and Leningrad, 38cm. and more higher. The westerly winds tend to press the water eastwards but another factor is the large contribution of river water, which is lighter than sea water so that for equilibrium the surface in the Baltic must be above that of the North sea. The mean annual range of level is i i-15cm., the highest point being reached on the average at the end of summer, August being often rainy, the lowest point is usually noted in spring which is usually dry and attended by south-east winds which drive some of the water towards the North sea. Occasional north-east gales in winter, with snow and ice, are apt to cause devastating storm floods on the south Baltic coasts. On Nov. 12-13, 1872, a great storm flood occurred, also in Nov. 1899 when the level of the sea was raised by 3-4 metres and enormous losses resulted. The tidal range at the Skaw is 0.28 metres, near Kiel only 0.07 metres, near Swinemunde 0.01 metres and near Memel 0.005 metres, so that the Baltic may be called tideless.
"The Freshwater Drainage Area."—This is estimated by Spethmann (1912) at 1,65 7,00okm. ; the most extensive river basin is that of the Neva, those of Vistula and Oder follow in size. The Baltic receives much more water by rainfall, discharge of rivers, etc., than it loses by evaporation; hence an enormous sur plus must be got rid of by an outflowing current which is named the "Baltic Stream." This must be a surface current, because it originates from a redundancy of fresh water. On account of the earth's rotation, the main part of the Baltic stream must keep close to the coast of the Scandinavian peninsula. It must be a periodic stream, because the discharge of the rivers into the Baltic varies with the season of the year. In spring and summer the water from the Baltic is sufficiently abundant to cover the whole surface of the Kattegat and Skagerrak, but in winter the sources of the Baltic current are for a great part dried up by the freezing of the land water. From near Stockholm this Baltic stream flows south, along the south Swedish coast it goes west, and in the Sound and the Belts its direction is northwards. The fresher Baltic water can be traced far into the North sea espe cially along the Norway coasts even as far as North Cape. In the Baltic proper it includes the surface waters down to a depth of 3o to 5ocm. and has a salinity of Under this is a thin almost stagnant layer at a depth of 50-60cm., very cold even in summer (36.5°-39°). Beneath this, again, the water is warmer and more salt and belongs to an undercurrent coming in from the North sea. Salinities for this undercurrent are in the Katte gat, in the Great Belt, 13%, in the Gottland deep. It flows quite irregularly into the Baltic and is inhabited by marine organ isms which use up its oxygen content to such an extent that free carbon dioxide has been found in the mid and east Baltic. This is probably a factor of the poverty of the Baltic sea floor in fish-life.
Climate.—Climatic influences of the Baltic on neighbouring lands are slight and unusual. The relatively fresh surface water freezes over large coastal stretches and the melting of this ice in spring retards rise in temperature seriously, as compared with the rise over inland areas. The air over the Baltic in winter is only 2°-31° warmer, in summer only 2°-31° colder than that over the neighbouring lands. The sea near Haparanda has a mean January temperature of +14°, between Stockholm and the Aland island 28.5°, near Bornholm 32°, while the corresponding July tempera tures are 57° (Haparanda), 61° (Aland island), 61.5° (Born holm). This shows that differences between north and south are very great in winter but almost vanish in summer. On the west and south coasts of Sweden _navigation is impeded by ice only in severe winters and then it is usually drift ice, compact sea-ice being very rare at present, as it has been through the 19th cen tury.
Formerly, however, the south Baltic including the Kattegat and Skagerrak froze on several occasions; it was possible to go from Norway to Jutland over the ice in 1294, to cross similarly from Denmark to Pomerania in and 1545• In 1659 Charles X. of Sweden marched 20,000 men across the Sound and the Belts to Jutland, and the Sound last froze in 1740. O. Pettersson thinks that the salinity of Baltic waters was formerly much lower than now and that this promoted freezing. Unless strong icebreakers keep channels open, navigation between Stock holm and Visby stops between the end of December and about April 1 o. In very severe winters the Aland sea is covered with thick ice available for traffic. The south part of the Gulf of Bothnia is covered with ice every winter along the coasts, but rarely, if ever, in its central part. Navigation is interrupted by drifting ice from about the middle of November to the beginning of May, though the port of Hernosand has been known to remain in some years open during the whole winter. The northern part of the Gulf of Bothnia is frozen every winter for 120 to 210 days (near Haparanda). In the Gulf of Finland the sea is closed to navigation by ice for about 150 days in the year; but navigation is rendered possible throughout the winter by the use of ice breakers, except in a very severe winter such as was, for example, 1925-26.