Abstract Results from a boundary layer experiment conducted over a flat site in northwestern Minnesota are discussed. Wind and temperature fluctuations near the ground were measured with AFCRL's fast-response instrumentation on a 32 m tower. Measurements between 32 m and the inversion base zi were made with MRU probes attached at five different heights to the tethering cable of a 1300 m2 kite balloon. The daytime convective boundary layer appears to be well-mixed with evidence of significant heat and momentum entrainment through the capping inversion. The spectra of velocity components are generalized within the framework of mixed-layer similarity. The characteristic wavelength for w increases linearly with height up to z = 0.l zi following free convection prediction, but approaches a limiting value of 1.5 zi, in the upper half of the boundary layer. The characteristic wavelengths for u and v are maintained at approximately 1.5 zi down to heights very close to the ground. This limiting wavelength correspo...