  Turbulent Rayleigh-B\'enard convection displays a large-scale order in the
form of rolls and cells on lengths larger than the layer height once the
fluctuations of temperature and velocity are removed. These turbulent
superstructures are reminiscent of the patterns close to the onset of
convection. They are analyzed by numerical simulations of turbulent convection
in fluids at different Prandtl number ranging from 0.005 to 70 and for Rayleigh
numbers up to $10^7$. For each case, we identify characteristic scales and
times that separate the fast, small-scale turbulent fluctuations from the
gradually changing large-scale superstructures. The characteristic scales of
the large-scale patterns, which change with Prandtl and Rayleigh number, are
also found to be correlated with the boundary layer dynamics, and in particular
the clustering of thermal plumes at the top and bottom plates. Our analysis
suggests a scale separation and thus the existence of a simplified description
of the turbulent superstructures in geo- and astrophysical settings.
