Temperature is a fundamental parameter of sea water. It is often used for observing the various phenomena, e.g. upwelling events, that occur in a water body. Several methods of determining water temperature are available, depending on how they are to be used in practice. The SatBałtyk system specifies two types of temperature: sea surface temperature and the temperature of sea water. The former is the temperature of the thin layer at the water surface, whereas the latter gives temperatures at different depths in the sea.
Coastal upwelling is a current raising deep water up to the surface; such events occur in some regions of the Baltic. Satellite maps of the southern shores of the Baltic often show areas with cooler surface waters, the effect of upwelling. This occurs when water from deeper layers, usually cooler, are brought to the surface under the influence of a current generated by winds blowing parallel to the coastline. In winter the temperature of upwelled waters may be higher than at the surface, so the temperature maps will show this as a warmer area.
A thermal front is a transition zone separating water masses of different temperatures; the temperature gradient across such a front is therefore steep. Thermal fronts in the Baltic are variable in size and stable over time. They usually form at the boundaries between basins of different depths (all-year or seasonal), in areas subject to upwelling (short-term but with a temporally stable position), within a hydrological front, restricting the spread of waters flowing in from rivers (short-term, with frequent changes in position frequently), around eddies etc. The waters on either side of the front often differ not only in temperature but also with respect to other physicochemical features, such as salinity, transparency and other optical properties.
Mesoscale eddies are disturbances in the current field resulting from anomalies of temperature, salinity and sea level on spatial scales from 10 to 100 km and durations from a few days to a month. Local structures with dimensions smaller than 10 km are defined as sub-mesoscale eddies. Eddies of various sizes in the Baltic are often seen on satellite radiometric images (e.g. in distributions of SST, chlorophyll a levels etc.) and on radar images, as sea surface disturbances.