The situation studied in this paper relates to air flow from a heated land surface out over a water surface with much lower temperature. Observations from the Baltic Sea, where this kind of situation prevails for two thirds of the time, indicate that several distinct flow regimes occur. Simulations with a numerical boundary layer model for various combinations of geostrophic wind speed, Vg, and bulk temperature difference between land and sea, Δ&THgr;, have been performed. They show that, initially, stratification is always stable in the air layers near the sea surface but after some time transition to a near-neutral layer capped by an inversion takes place. The simulations, as well as the results of a simple analytical study, indicate that the transition takes place at a certain time of over water transport, te, which can be calculated when the bulk parameter Δ&THgr; is known. Measurements of boundary layer temperature and wind profiles at three sites in the Baltic Sea area strongly support this result. Thus, for measurements representing traveling times less than te, derived with the above mentioned expression, a stable boundary layer is always found. For traveling times larger than te, a mixed layer is found (provided Δ&THgr; is not large enough to cause quasi-frictional decoupling). Turbulence measurements made at the three sites simultaneously with the profiles show that (u * /Vg)103 is between almost zero and 25 for the stable cases and between 25 and 46 for the mixed layer cases.¿ 1997 American Geophysical Union <