We demonstrate the ability of a single-wavelength backscatter lidar to provide information on the uptake of water vapor by aerosol in a well-mixed, cloud-capped, boundary layer. Aerosol hydration has important consequences for the effect of aerosols on the Earth's radiation budget. A vertically pointing, airborne lidar is used to measure vertical profiles of aerosol backscatter beneath a stratocumulus cloud deck. In situ aircraft thermodynamic measurements are used to derive simultaneous profiles of relative humidity (RH) under the assumption that the boundary layer is well mixed. The change in backscatter is derived as a function of relative humidity over the range ~85% RH to ~98.5% RH. In situ measurements of the aerosol size distribution and composition are used to calculate the expected enhancement in backscatter due to equilibrium uptake of water vapor. Comparison between lidar backscatter enhancement as a function of RH and that derived from the in situ aerosol size distribution and composition measurements shows good agreement. Conditional sampling on strong updrafts/downdrafts indicates that aerosol backscatter tends to be higher in downdrafts than it is in updrafts for the same RH range. This is consistent with the concept of inertia of the larger hydrated particles to growth/evaporation at short timescales but may also be due to a bias in the way that lidar-derived cloud base is interpreted in updrafts versus downdrafts. Calculations of enhancement in total scatter due to water vapor uptake with enhancement in backscatter suggest that the effects agree to within ~20% of one another for RH 95%.