In natural convection, the Grashof number plays a similar role as Reynolds number in forced convection. The Grashof number is defined as:

where g is gravitational acceleration, b is the volume expansivity, (Ts - Tamb) is the surface-fluid temperature difference, L is the characteristic length and n is kinematic viscosity.

Grashof number can be interpreted as the ratio of buoyancy forces to viscous forces. Thus, low Gr means laminar flow and high Gr means turbulent flow.