The temperature of the sea in Croatia is interesting for study and has been systematically recorded and monitored for over 30 years. The Adriatic Sea is a warm sea. During summer, the surface temperature of the open sea in Croatia ranges from 23 to 25 degrees Celsius (73.4 to 77 degrees Fahrenheit), while the deepest waters have a temperature greater than 11 to 13 degrees Celsius (51.8 to 55.4 degrees Fahrenheit). In general, the temperature of the sea in Croatia decreases from the surface towards the bottom. Temperatures are slightly higher near the coast, especially in shallow and enclosed bays. During the warm part of the year, especially in summer, there is a temperature jump in the layer from 10 to 30 meters from the surface, and it decreases the fastest at a depth of several meters. At a depth of 70 meters, the temperature of the sea in Croatia is 18 to 19 degrees Celsius (64.4 to 66.2 degrees Fahrenheit).

During the fall, the sea in Croatia begins to cool on the surface. In winter, the surface temperature of the sea in Croatia varies from 14 to 16 degrees Celsius (57.2 to 60.8 degrees Fahrenheit), with a minimum of around 12 to 10 degrees Celsius (53.6 to 50 degrees Fahrenheit). In winter, the southern Adriatic is warmer than the northern, and the temperature difference between the northern and southern Adriatic is 8 to 10 degrees Celsius (46.4 to 50 degrees Fahrenheit). The shallow waters of the northern Adriatic, ports, bays, and river estuaries cool down to 4 to 6°C (39.2 to 42.8 degrees Fahrenheit) in winter. There is no sea ice in the Adriatic Sea. In spring, the sea begins to warm up and soon reaches a temperature of 18 to 22 degrees Celsius (64.4 to 71.6 degrees Fahrenheit) suitable for swimming. During summer, the sea in Croatia is the warmest. The maximum temperatures occur in July and last until mid-September, ranging from 23 to 26 degrees Celsius (73.4 to 78.8 degrees Fahrenheit). During summer, the sea is cooler than the air, preventing significant warming, while in winter, the sea is warmer than the air and warms it up. Thus, the sea in Croatia acts as a regulator of air temperature, mitigating temperature extremes in summer and winter.

The coastal zone, especially near mountainous areas, is rich in underwater sources of fresh water called “vruljas”. Famous vruljas are located along the coast in the Velebit and Brac channels, Rijeka, Kastela, Mali Ston, Boka Kotorska Bay, and Velika Vrata. In summer, relatively cold water flows from these sources and quickly mixes with seawater. However, under certain meteorological conditions, fog can form in these areas. During summer, the cold “bura” wind can quickly cool the surface layer of the sea, keeping the sea temperature 2 to 4 degrees Celsius (35.6 to 39.2 degrees Fahrenheit) lower as long as the bura is blowing. The temperature of the sea in Croatia is measured daily at over one hundred locations along the Croatian coast. The temperature of the sea in Dalmatia, the temperature of the sea in Istria, and the temperature of the sea in Kvarner have their peculiarities and together help illustrate the complex picture of the sea temperature in the Republic of Croatia.