Boundary-layer resistance to heat transfer from plates was studied in a wind tunnel which produced turbulence with streamwise intensity in the range 3.5 to 25% and a longitudinal integral scale of the streamwise turbulence component (L u,x) in the range from 8 to 100 mm. It was found that heat transfer enhancement occurred due to the turbulence, and that at any given intensity, this enhancement was determined by the ratio of L u,x to the characteristic dimension of the plate.