The Grashof number is a dimensionless number which approximates the ratio of buoyancy force to the viscous force acting on a fluid.

Gr = ( g β (Ts - Tinf) L3 ) / ν2

g gravity
β volumetric thermal expansion coefficient
Ts source temperature
Tinf quiescent temperature
L characteristic length
ν kinematic viscosity

See also Reynolds number, fluid dynamics, buoyancy.