The following table shows some typical daily maximum temperatures and the average monthly average extremes. In the late Spring and Summer, following the ice melting, sea temperatures rise rapidly. To the south of Gotland 5 or 6 in May becomes about 16 degrees in August. In the Gulf of Finland a temperature average of just 1 deg C in May rises to 16 or higher approaching St Petersburg in August. Fog and Thunder Fog is most frequent over the open sea in April and May while the seas are still cold but the air temperature is rising.
Thunderstorms are likely during any time of the year.
Typical numbers of days per month during May to August with thunderstorms range from 2 or 3 at Copenhagen and Stockholm to 4 or 5 towards the east near Riga, St Petersburg and Helsinki. The average currents are very weak. They are driven largely by the flow of water from rivers into the Baltic and by the ice melt. The evaporative effect, that creates the easterly surface current through the Strait of Gibraltar into the Mediterranean, is relatively weak in these latitudes. The net result is a flow out of the Baltic through the Kattegat.