In situ water vapor measurements performed in the upper troposphere and the lower stratosphere, from an aircraft, in the North Atlantic region around 50 ¿N, are analyzed. It is shown that the water vapor content observed in the region located slightly above the local tropopause is consistent with the transport from troposphere to stratosphere at middle and high latitudes. It appears that there is a seasonal variation with a higher water vapor content during summer. The source of this variation could be due to the difference in the relative contribution of the troposphere to stratosphere isentropic and convective transport as well as transport from the upper levels of the stratosphere due to the general circulation. ¿ 1999 American Geophysical Union