This paper presents results from a boundary layer experiment conducted over a flat site in northwestern Minnesota. Wind and temperature fluctuations near the ground were measured using fast-response instrumentation on a 32 m tower. Measurements between 32 m and the inversion base were made with MRU probes attached to a kite balloon. The daytime convective boundary layer is well-mixed with 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 0.1zi, following free convection prediction, but approaches a limiting value of 1.5zi in the upper half of the boundary layer. The characteristic wavelengths for u and v are maintained at approximately 1.5zi down to heights very close to the ground. This limiting wavelength corresponds to the length scale of large convective elements which extend to the top of the boundary layer. The behavior of the temperature spectra above 0.1zi cannot be generalized in the same manner. Below that height, the θ spectra follow behavior observed in the surface layer; zi=0.1zi is also the upper limit for the free convection predictions of the w and θ variances. The high-order moments and the structure parameters reveal the strong influence of entrainment at heights above 0.5zi. The convective boundary layer is defined as the part of the atmosphere most directly affected by solar heating on the earth's surface. In mid-latitudes over land, this layer typically reaches a height of 1–2 km by midafternoon. Its upper limit is often delineated by a capping inversion. This layer exhibits a near-constant distribution of wind speed and potential temperature, obviously a consequence of the strong vertical mixing produced by convection. The name "mixed layer" is therefore used synonymously with the convective boundary layer in much of the literature on the subject. The wind speed and temperature profiles of Fig. 1 are fairly typical of daytime convective conditions. Almost all the wind shear and all the potential temperature gradient in the boundary layer are confined to a very shallow region close to the ground. The sharp increase in wind speed across the capping inversion appears consistently in many daytime runs and has possible implications for momentum and heat transport in the upper regions of the boundary layer. The boundary layer over land may be idealized as a three-layer structure in terms of the parameters considered relevant to the turbulence in each. Proceeding upward from the surface, we have: 1) The surface layer where wind shear plays a dominant role. Here Monin-Obukhov similarity applies and the controlling parameters are z, τ0, Q0 and g/T. The scaling velocity and temperature for this layer are, respectively, u*=(τ0/ρ)^1/2, T*=-Q0/u*. Dimensionless groups formed with u* and T* becomeThis paper presents results from a boundary layer experiment conducted over a flat site in northwestern Minnesota. Wind and temperature fluctuations near the ground were measured using fast-response instrumentation on a 32 m tower. Measurements between 32 m and the inversion base were made with MRU probes attached to a kite balloon. The daytime convective boundary layer is well-mixed with 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 0.1zi, following free convection prediction, but approaches a limiting value of 1.5zi in the upper half of the boundary layer. The characteristic wavelengths for u and v are maintained at approximately 1.5zi down to heights very close to the ground. This limiting wavelength corresponds to the length scale of large convective elements which extend to the top of the boundary layer. The behavior of the temperature spectra above 0.1zi cannot be generalized in the same manner. Below that height, the θ spectra follow behavior observed in the surface layer; zi=0.1zi is also the upper limit for the free convection predictions of the w and θ variances. The high-order moments and the structure parameters reveal the strong influence of entrainment at heights above 0.5zi. The convective boundary layer is defined as the part of the atmosphere most directly affected by solar heating on the earth's surface. In mid-latitudes over land, this layer typically reaches a height of 1–2 km by midafternoon. Its upper limit is often delineated by a capping inversion. This layer exhibits a near-constant distribution of wind speed and potential temperature, obviously a consequence of the strong vertical mixing produced by convection. The name "mixed layer" is therefore used synonymously with the convective boundary layer in much of the literature on the subject. The wind speed and temperature profiles of Fig. 1 are fairly typical of daytime convective conditions. Almost all the wind shear and all the potential temperature gradient in the boundary layer are confined to a very shallow region close to the ground. The sharp increase in wind speed across the capping inversion appears consistently in many daytime runs and has possible implications for momentum and heat transport in the upper regions of the boundary layer. The boundary layer over land may be idealized as a three-layer structure in terms of the parameters considered relevant to the turbulence in each. Proceeding upward from the surface, we have: 1) The surface layer where wind shear plays a dominant role. Here Monin-Obukhov similarity applies and the controlling parameters are z, τ0, Q0 and g/T. The scaling velocity and temperature for this layer are, respectively, u*=(τ0/ρ)^1/2, T*=-Q0/u*. Dimensionless groups formed with u* and T* become