Thermodynamic structure of the marine atmosphere in the region between 80 and 87‡E along 13‡N over the Bay of Bengal was studied using 13 high resolution radiosonde profiles from surface-400 hPa collected onboard ORV Sagar Kanya during the period 27th–30th August, during BOBMEX-99. Saturation point concept, mixing line analysis and conserved variable diagrams have been used to identify boundary layer characteristics such as air mass movement and stability of the atmosphere. The results showed relatively dry air near the ocean surface between 1000 and 950 hPa. This feature is confirmed by the conserved þeta_v structure in this layer. Further, þeta_v seldom showed any inversions in this region. The þeta_e and þeta_es profiles showed persistent low cloud layers between 900 and 700 hPa. The conserved variable diagrams (þeta_e-q) showed the existence of double mixing line structures approximately at 950 and 700 hPa levels.