This study deals with potential changes in the relative humidity associated with global warming and their implications on heat stress along the coastal region of the Mediterranean in the summer season. It is based on the assumption that the regional warming will enhance the lower-level stability due to the thermal inertia of the sea with respect to its overlying air. The enhanced stability implies more effective trapping of the near surface moisture, and as a result-further increase of the relative humidity. The marine boundary layer over the Mediterranean is modeled. The central feature of the model is the marine inversion capping the marine moist air, which intensity is positively correlated with the stability. Simple calculations indicate that if the temperature increases, while the stability remains unchanged, the near-surface relative humidity would not be affected. But, an increase in the stability would result in an increase in the near-surface relative humidity. This prediction is validated through observed trends of the respective fields, using the NCEP/NCAR reanalysis data and soundings from the eastern Mediterranean. The results are consistent in indicating an increase in the near-surface temperature, the lower-level stability and the relative humidity over the eastern part of the Mediterranean, but not in its western part. The results for the eastern Mediterranean support the expectation for an aggravation of heat stress beyond that imparted by the temperature rise.
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Acknowledgements This study was supported by the Israeli Science Foundation (ISF grant No. 764/06) and the Israeli Ministry for Environmental Protection (grant No. 6-801).