Weather and sea states;
Best time to sail in Gulf of Bothnia is summer, between end-April and early-September. In most of the Baltic Sea ice will be clear by end of April. However, N end of the Gulf of Bothnia and E of Finland may be ice-free by end-May. Ice starts to form around the coast by December.
Fog can also be an important issue for the area. During summer, 5 – 10% of fog cover won’t be surprising. Mean daily temperatures from June to August vary from 18˚C in the Gulf of Bothnia to the 25˚C in the southern regions. Temperature increases rapidly at May after clearance of ice.
During summer, prevailing wind direction is from SE at the W side and from NW at the E side. Therefore, it is advised to sail north from W side and sail S from the E side of the Gulf of Bothnia. However, northerly winds in the summer are quite usual. It is advised to plan your trip according to current weather conditions.
Large continental areas around the Gulf of Bothnia currently add fresh waters through rivers. Therefore, these areas can have unexpected current. In fine settled weather the natural flow in Gulf of Bothnia is anti-clockwise of about 0.5 knot. On the other hand, sailors should keep in mind that currents can be wind driven and unpredictable.
Tides are negligible. However, sea level can change due to strong winds, change in barometric pressure and amount of inflow from rivers. This sea level change can reach up to 0.6 meters.
Compiled from guide of RCC Pilotage Foundation