We examine experimentally the boundary layer beneath a Rankine vortex, which consists of central vortex tube surrounded by an irrotational free vortex. We do this by setting up a tornado like vortex in a large water tank, and then examine the resulting boundary layer using Particle Image Velocimetry. The boundary layer beneath a Rankine vortex has attracted considerable attention in the past, as it is thought to play an important role in the dynamics of a tornado [1]. In particular, Burggraf et al. [2] have performed an approximate asymptotic analysis of the steady, laminar boundary layer beneath a free vortex, predicting a complex, two-layer structure for the boundary layer. Their predictions have been found to be in qualitative agreement with experiments on the flow surrounding a tornado-like vortex, but to date there is no detailed, quantitative comparison. We provide such a comparison here. We find that the analysis of Burggraf et al. is in good agreement with our measurements when the boundary layer is steady, but discrepancies start to arise as the Reynolds number increases and the flow becomes unsteady.