Thermal signature of plumes in turbulent convection: The skewness of the derivative

We present experimental evidence that, in the hard turbulence regime of Rayleigh-Benard convection, the temperature fluctuations are produced by buoyancy, despite the presence of a mean horizontal flow at the plates. In a convection cell of aspect ratio 1, we measure the temperature time derivative for Ra from 2 x 10(7) to 1 x 10(11), which is skewed toward the negative in the region outside the cold thermal boundary layer. This skewness of the derivative indicates the presence of thermal fronts, or plumes, which are detached from the boundary layer by buoyancy. At higher Ra, the skewness of the derivative is reduced, which we relate to the transition to a turbulent Reynolds number in the velocity boundary layer at Ra similar to 10(9). We define a time scale using the derivative to characterize these fronts, and find that its minimum value scales as Ra--2/3 over the entire range of Ra in our experiment. [S1063-651X(96)11405-7]