(measure reflecting a fluid system's propensity to convection)
A Rayleigh number (Ra) is a measure of a condition within a fluid
that might cause convection, a ratio between a tendency toward
conduction and a tendency toward convection, each tendency
represented by a timescale.
Sufficiently low, there is no convection, sufficiently high, there
is turbulent convection, and between, there is smoother
convection (via a laminar flow).