Radiosonde data collected during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), 1987, the Atlantic Stratocumulus Transition Experiment (ASTEX), 1992, and the Tropical Instability and Waves Experiment (TIWE), 1990, were used to develop four composite thermodynamic soundings of the marine boundary layer for cloud conditions ranging from solid stratocumulus to trade cumulus and sea surface temperatures ranging from 16¿ to 27 ¿C. Average cloud fraction and cloud base height for the composite soundings were estimated using laser ceilometers located at each of the four sites. The average fractional cloudiness varied from 0.83 off the coast of California at San Nicolas Island (33.4¿N, 119.6¿W) during FIRE to 0.26 over the central equatorial Pacific during TIWE (0¿N, 140.0¿W). During ASTEX, two sites in the vicinity of the Azores generally experienced cloud conditions characterized by cumulus clouds penetrating into a stratocumulus layer. At the more northerly site, the island of Santa Maria (37.0¿N, 25.2¿W), the average fractional cloudiness was 0.67 compared with 0.40, 1000 km downstream at the German R/V Valdivia (28.0¿N, 24.0¿W). The two composite soundings from ASTEX and the composite sounding from TIWE indicate decoupled boundary layer structures. These three soundings have a cloud layer that is conditionally unstable and show a systematic increase in relative humidity with increasing fractional cloudiness. The effect of cloud top entrainment on fractional cloudiness was evaluated. For the four composite soundings the fractional cloudiness decreases as the slope of the normalized w-&thgr; mixing line increases (greater instability), but this decrease is 4 times less than that from previous studies. Fractional cloudiness diagnosed from parameterizations using cloud layer relative humidity compares well with the observed mean cloudiness. ¿ American Geophysical Union 1995