The Fried coherence length is quantitatively measured by means of two different methods in a convective turbulence tank for wavefront compensation experiments in adaptive optics. The spectra of angle-of-arrival (AA) fluctuations and their probability distribution are given. The fluctuation of the wavefront phase follows a normal distribution. The χ² distribution appears to be a reasonable assumption for variance of AA fluctuations, and the statistical distribution P(r₀) regarding the Fried coherent length r₀ of experimental results is shown. The spectrum of AA fluctuations in the x-direction, being the same as the spectrum in the y-direction, obviously indicates that the turbulence in the tank is an isotropic feature. The experimental results for the Fried coherence length and temperature differences between a heated plate and cooled plate correspond to the similarity relationship in fluid mechanics and thermodynamics experiments in a convective turbulence tank.