“Is the water in the Baltic Sea warm?” is what many ask when, in summer, they come to Poland’s coast to relax and spend their holiday on the beach. Children love playing in the water, and adults want to cool off in pleasant temperatures during the heat. But… does the sea water temperature always encourage swimming? No—definitely not for dipping in mid-winter.
To understand why the Baltic is not always as warm as we would like, it’s worth looking at the bigger picture. What determines the water temperature in the Baltic Sea, and how does it change?
Due to its unique geography in the northern temperate climate zone, the Baltic Sea has a distinct thermal profile, very different from southern Europe’s warmer waters. It’s relatively cool, shaped by limited water exchange with the Atlantic Ocean and by a large inflow of fresh water from rivers draining into the Baltic Sea.
At the same time, its relatively small average depth and enclosed character mean this body of water has very high temperature variation dynamics. Surface water can warm up quickly during summer sunshine, but it can also cool down rapidly in the autumn–winter period.
Typically, the average water temperatures along Poland’s coast in the summer season range from 16°C to 22°C, and this pattern is strongly dependent on current weather conditions. The most comfortable bathing conditions develop from mid-July to the end of August, when accumulated solar radiation leads to more stable temperature values. From a human physiology and leisure comfort perspective, the Baltic Sea’s water temperature thresholds can be classified as follows:
Below 14°C: Extremely cold water, creating a risk of thermal shock; at this level bathing areas are obligatorily closed.
About 18°C: a threshold for bathing; however, people sensitive to the cold may still feel noticeable discomfort.
Around 20°C: Commonly regarded as a comfortable and pleasant level for adult swimmers.
Above 22°C: Very comfortable conditions, ideal also for families with small children.
It’s worth noting that official statistics are based on morning measurements taken by the Institute of Meteorology and Water Management. On a sunny day, in windless weather, the temperature in the shallow coastal zone can rise in the afternoon by an additional 1°C to 2°C.
The table below presents a detailed breakdown of average water temperatures in the most popular Polish resorts in the summer months, compiled on the basis of long-term measurement data from 2010–2024.
Seaside resort | June [°C] | July [°C] | August [°C] | September [°C] | Seasonal average (holiday period) [°C] |
Puck | 19,1 | 20,3 | 20,5 | 16,6 | 20,4 |
Świnoujście | 18,0 | 19,6 | 19,9 | 17,0 | 19,6 |
Gdynia | 17,0 | 19,4 | 20,2 | 17,1 | 19,4 |
Międzyzdroje | 17,3 | 19,4 | 19,9 | 16,8 | 19,3 |
Hel | 15,9 | 18,5 | 19,6 | 17,0 | 18,6 |
Władysławowo | 15,5 | 18,6 | 18,9 | 15,7 | 18,2 |
Kołobrzeg | 16,1 | 17,9 | 17,8 | 14,8 | 17,6 |
Ustka | 16,4 | 17,8 | 17,6 | 14,8 | 17,6 |
source: https://www.psur.pl/a.aspx?id=183
The distribution of sea-water temperatures in Poland’s coastal zone shows clear geographical variation, driven by the shape of the coastline and the seafloor bathymetry. Shallow areas sheltered from the direct influence of the open sea warm up much faster.
Classic examples are Puck Bay and the Vistula Lagoon. In towns in these areas, such as Puck or Tolkmicko, the water temperature in season regularly exceeds 21°C. In extremely hot years, readings as high as 26°C—close to Mediterranean standards—were recorded there.
Quite different physical conditions determine water temperatures on Poland’s western coast. The Szczecin Bay area, represented by Świnoujście and Międzyzdroje, features the most thermally stable conditions along the entire coastal strip.
Thanks to the favourable shape of the wide, gently sloping seabed, the water is less prone to sudden fluctuations caused by deep currents, reaching a comfortable summer average above 19°C.
For a full understanding of the Baltic Sea’s annual thermal cycle, it’s important to look at temperatures outside the main holiday season.
In spring, the water warms up slowly, showing a significant lag behind air temperatures.
In autumn, the accumulated heat is released to the atmosphere gradually, which means September—and even the beginning of October—can be warmer than May.
In winter, during severe frost, surface waters fall to about 0°C; in northern bays (e.g., Bothnian Bay or Gulf of Finland) this causes frequent freezing.
The table below presents average water temperatures at selected measurement points in the spring and autumn-winter periods, showing the full range of annual thermal changes.
Town | March [°C] | April [°C] | May [°C] | October [°C] | November [°C] | December [°C] | January [°C] | February [°C] |
Gdynia | 3,9 | 7,0 | 11,9 | 12,4 | 8,0 | 4,4 | 2,9 | 2,6 |
Hel | 3,8 | 6,2 | 10,6 | 12.6 | 8.6 | 5.1 | 3.4 | 2.9 |
Kołobrzeg | 4.6 | 8.1 | 12.3 | 10.9 | 7.4 | 3.9 | 2.8 | 2.9 |
Świnoujście | 4.2 | 8.4 | 13.3 | 12.1 | 7.6 | 3.6 | 2.1 | 2.0 |
source: https://www.psur.pl/a.aspx?id=183
One of the most surprising and dynamic physical phenomena in the Baltic Sea is upwelling. This process transports cold, nutrient-rich deep waters up to the ocean or sea surface in the coastal zone. In just 24 hours, it can lower the beach water temperature by several degrees Celsius, causing extreme drops to below 10°C in the middle of hot summer.
The main mechanism driving upwelling is winds blowing parallel to the shore—on the Polish coast, mainly easterly and north-easterly winds. Wind friction against the water surface, supported by the Coriolis effect (the Ekman transport), pushes warmed surface waters toward open sea. A niche forms in their place, and cold, dense water masses are dynamically drawn up from the bottom of the basin.
The phenomenon is highly irregular, caused by varying atmospheric circulation, but it is most strongly manifested in three areas of the Polish coast: near Kołobrzeg, Łeba, and the Hel Peninsula. Strong storms are not needed to trigger it—steady winds of only 2–4 m/s are enough.
Upwelling brings a range of climate, ecological, and tourist consequences:
Advection fog: When hot, humid tropical air moves over a sea surface that has been sharply cooled by upwelling, water vapour condenses. This creates dense advection fog that can completely cover the beach, drastically reducing visibility and lowering the temperature felt on land.
Thermal shock: Suddenly entering water colder than 14°C from a sun-warmed beach can cause blood vessel constriction, disrupt heart function, and even pose a direct risk to life due to thermal shock. For this reason, lifeguards close bathing areas when temperatures drop below 14°C.
Nourishing the water column: biologically, upwelling is highly beneficial. Deep waters bring nutrient salts and organic matter to the surface, boosting phytoplankton and zooplankton, energizing the food chain, and attracting fish and water birds. It also helps curb harmful, toxic algal blooms, which are unpleasant for tourists.
In the long term, historical studies of air temperature in the Baltic Sea basin in 1861–2000 show a clear warming trend (an increase of 0.11°C per decade in the northern part and 0.08°C per decade in the southern part).
Climate change, by modifying the trajectories of air currents and the structure of winds, directly affects the frequency, intensity, and area of occurrence of upwelling events, destabilizing existing natural cycles.
To sum up: for your Polish seaside trip, skip Spanish heat and expect… a lottery. The Baltic may treat us one day to thermal-bath-like temperatures, then the next morning (thanks to upwelling) deliver free cold-water swimming in mid-July. Not a reason to give up your holiday—swap complaints for solutions: bring a beach windbreak for chilly wind, and in your phone keep a proven app with a map of water temperatures.
If you hit a day when the water is a poor 14°C, there’s always a walk with a gelato in hand, collecting amber, and breathing in iodine. After all, we love our Polish coast so much for this unpredictable climate!
