Boundary layer length scales in thermal turbulence

We present measurements of local temperature fluctuations from a convection cell of aspect ratio 1, using gas at room temperature, for Rayleigh numbers (Ra) ranging from 5×105 to 1×1011. The thermal boundary layer thickness is measured; its scaling is close to Ra-2/7 for Ra2×107. A second length scale is measured through the cutoff frequency of the power spectrum, which we relate to the velocity of the large scale circulation. The Rayleigh number dependence of this length changes at Ra=2×109.