Swimmers regularly find Baltic water noticeably warmer in summer than Atlantic water at the same latitude. Three physical properties of the basin explain the difference.
Depth determines how much water must be heated
Solar energy arriving at the surface is mixed downwards by wind and waves. The shallower the water column, the less mass that energy has to warm.
The Baltic is shallow over large areas, so a summer's worth of sunshine raises the temperature of the whole column substantially rather than warming only a thin surface layer.
An open ocean mixes the same energy into a far deeper column, which is why its surface temperature responds slowly to a warm season.
Restricted exchange keeps the heat in
The basin connects to the wider ocean through narrow, shallow straits, which severely limits how much water can be swapped in either direction.
Without a strong inflow of cold ocean water, warmth accumulates locally through the summer instead of being carried away and replaced.
The same restriction works in reverse in winter, when the sea loses heat rapidly and large areas can freeze, giving the region a wide annual swing.
Low salinity changes the water's behaviour
Heavy river inflow and limited ocean exchange leave the surface water only mildly saline, far below open ocean levels.
Lower salinity raises the freezing point and alters density, which contributes to the sea's tendency to form ice in cold winters despite being open water.
It also affects layering, because fresher surface water floats on saltier deep water, creating a stable boundary that slows vertical mixing.
Layering produces sudden cold water
That stable boundary means the warm surface layer can be thin. A swimmer descending a short distance may pass abruptly into markedly colder water.
Strong offshore winds can also push the warm surface layer away from the coast, allowing cold water to rise in its place within a single day.
This upwelling explains why a beach can drop many degrees overnight with no change in air temperature or weather, and it is a genuine safety consideration.
What this means for planning a swim
Water temperature here follows the season with a lag, so late summer is generally warmer than early summer even when the air is cooler.
Sheltered bays and shallow lagoons warm fastest, while exposed coasts facing prevailing winds are the most likely to experience sudden upwelling.
Checking a local water temperature reading rather than assuming continuity from the previous day is the practical habit on this coast.