Two sets of Lagrangian measurements were made during the southern Aerosol Characterization Experiment (ACE1) south of Tasmania, Australia, in December 1995. This paper intends to provide an overview of the general conditions encountered during the two intensive observational periods. The measurements by the NCAR C-130 provide the main data set for this study. We also use the sea surface temperature obtained from the R/V Discoverer and the European Center for Medium-Range Weather Forecasts (ECMWF) analyses field for the large-scale divergence field. Emphases of the paper are on the atmospheric and oceanic environment and the boundary layer mean structure during the six flights in the two Lagrangian measurement periods. The large scale features, such as variations of sea surface temperature, synoptic conditions, and large-scale velocity fields, are discussed. These large-scale environments had significant influences on boundary layer turbulence and the inversion structure. The boundary layer mean structure and its evolution along the Lagrangian trajectory are also studied using two-dimensional cross-section plots of vertical and horizontal (along the flight track) variation of potential temperature, water vapor, wind components, and ozone concentration. The most prominent feature of the boundary layer is the two-layered structure observed throughout Lagrangian B and during the last flight of Lagrangian A. The two layers have detectable differences in potential temperature, water vapor, and, to a lesser extent, ozone concentration. These differences make it necessary to study the exchange between the two layers. Low-level cloud structure and cloud microphysics are also discussed. We emphasize, though, that the results on cloud fractions should be used with caution due to the variable nature of the cloud bands observed during ACE1. ¿ 1999 American Geophysical Union