  We report measurements of the instantaneous viscous boundary layer (BL)
thickness $\delta_v(t)$ in turbulent Rayleigh-B\'{e}nard convection. It is
found that $\delta_v(t)$ obtained from the measured instantaneous
two-dimensional velocity field exhibits intermittent fluctuations. For small
values, $\delta_v(t)$ obeys a lognormal distribution, whereas for large values
the distribution of $\delta_v(t)$ exhibits an exponential tail. The variation
of $\delta_v(t)$ with time is found to be driven by the fluctuations of the
large-scale mean flow velocity and the local horizontal velocities close to the
plate can be used as an instant measure of this variation. It is further found
that in the present parameter range of the experiment the mean velocity profile
measured in the laboratory frame can be brought into coincidence with the
theoretical Prandtl-Blasius laminar BL profile, if it is resampled relative to
the time-dependent frame of $\delta_v(t)$.
