For two consecutive days during spring 1997, the wind field over the Baltic Sea has been studied. The strength of the geostrophic wind speed is the major difference in synoptic conditions between these two days. During both days, the mesoscale wind field over most of the Baltic Sea is quite heterogeneous; the modifications primarily being caused by the land-sea contrasts. On the day with the weaker wind speed, sea-breeze circulations develop. As a consequence, the wind direction at lower levels is more or less opposite to the geostrophic over large areas of the Baltic Sea and the surface wind speed decreases with offshore distance. Wind speed maxima caused by the sea breezes are found along the east coasts in the studied area. For the other day, the slow growth of a stable internal boundary layer over the sea also gives a surface wind speed decrease with offshore distance from the coast.