Mohn has shown how the inequalities of what he terms the densitysurface can be found from the salinity and temperature; and he calculates that the level of the Skagerrak should be about 2 ft.
The fresher enclosed seas include the Malay and the East Asiatic fringing seas with 30 to 34.5 per mille, the Gulf of St Lawrence with 30 to 31, the North Sea with 35 north of the Dogger Bank diminishing to 32 further south, and the Baltic, which freshens rapidly from between 25 to 31 in the Skagerrak to 7 or 8 eastward of Bornholm and to practically fresh water at the heads of the Gulfs of Bothnia and Finland.
This channel is usually included in the Baltic. The part of it west of a line joining the Skaw with Christiania fjord receives the name of Skagerrak; the part east of this line is called the Kattegat.
According to recent computations the total area of the Baltic, including the Skagerrak and Kattegat, is 166,397 sq.
In spring and summer the water from the Baltic is sufficiently abundant to inundate the whole surface of the Kattegat and Skagerrak, but in winter the sources of the Baltic current are for the most part dried up by the freezing of the land water.
All the waters which enter the Skagerrak or Kattegat as undercurrents can be found at the surface of the North Sea.
The deepest water stratum in the Skagerrak is certainly of oceanic origin; it has been found to suffer changes of long period, and it is probably not always composed of water derived from the same part or the same depth of the North Atlantic; this water is, as a rule, deficient in oxygen.
The "North Sea" water, of 34 to 35 pro male salinity, does not appear at the surface in the Skagerrak, except as a strip along part of the coast of Jutland, but it is always found as an undercurrent overlying the oceanic water.
The bank-water of 32 to 34 pro male salinity is found all along the continental coast of the North Sea and North Atlantic, and it may therefore enter the Skagerrak either from the North Sea or from the north along the coast of Norway.
On the west and south coasts of Sweden, and in the Skagerrak south-east of Norway, navigation is interfered with by ice only in severe winters, and then the ice is usually drifting, compact sea-ice being very rare.