Measurements were made to determine the level of origin of air parcels participating in natural convection. Lagrangian measurements of conservative variables are ideal for this purpose. A simple remotely piloted vehicle was developed to make in situ measurements of pressure, temperature, and humidity in the convective boundary layer. These quasi-Lagrangian measurements clearly show that convective plumes originate in the superadiabatic surface layer. The observed boundary layer plumes have virtual temperature excesses of about 0.4 K in a tropical region (Orlando, Florida) and of about 1.5 K in a desert region (Albuquerque, New Mexico). The water vapor contribution to parcel buoyancy was appreciable in Orlando but in Albuquerque was insignificant. These observations indicate that convective available potential energy should he determined by adiabatically lifting air parcels from the surface layer, at screen level.