AbstractData from fifty‐two aircraft flight legs 100−150m above ground from HAPEX and FIFE are analysed to estimate characteristics of boundary‐layer moisture fluctuations in boundary layers with different bulk stability and surface energy regimes. In HAPEX, considerable effort was devoted to the quality of measurement of moisture fluctuations. The data include repeated 120km flight legs flown at 150m over the same relatively homogeneous terrain, allowing statistical examination of motion characteristics on horizontal scales up to 10km.This study finds two prototype boundary‐layer regimes. With significant boundary‐layer instability, relatively weak surface evaporation and drier air aloft, the entrainment‐drying boundary layer may develop. This boundary layer is characterized by vertical divergence of the moisture flux and significant diffusion of dry air from the top downwards (top‐down). In this type of boundary layer, dry air occasionally reaches the lower boundary layer, leading to negative moisture skewness in spite of positive temperature and vertical velocity skewness associated with warm moist updraughts. In contrast, the moistening boundary layer associated with greater surface evaporation is characterized by positive moisture skewness near the surface.In addition to the above turbulent moisture fluctuations, some of the above data are characterized by 10km moisture variations with horizontal gradients often concentrated in narrow zones of horizontal convergence. Since corresponding signatures of vertical velocity and temperature are weaker, these zones are referred to as ‘mesoscale’ moisture fronts. As a more general feature, moisture variations on scales of 5km and greater are negatively correlated with temperature variations associated with cool moist regions and warm dry regions. On scales of tens of kilometres and greater, such negative correlation may be due to inhomogeneity of the surface energy budget. The negative moisture‐temperature correlation leads to large mesoscale variations of relative humidity and the lifted