Wave height warnings refer to significant wave heights. Individual waves can be from one and a half to two times higher than the reading given in the warning.
The lowest warning level is in use from May to October. The other two levels for periods of up to 5 days are in use throughout the year.
When the significant wave height reaches 2.5 metres, sea traffic – for instance, the fast ferries between Helsinki and Tallinn – may be disrupted. However, it is good to remember that the significant wave height need not be higher than one metre to make boating more difficult.
Four metres is a rare wave height, for instance in the Gulf of Finland. Off Helsinki, there are on average ten hours a year when such wave heights are recorded.
In the Finnish sea areas, wave heights reach the highest risk level only in the Northern Baltic Sea and south-eastern part of Sea of Bothnia. Such wave heights can pose problems, or even danger, to the navigation of larger vessels as well.