The flow regime in natural convection is governed by the dimensionless Grashof number
[On the other hand, the flow regime in forced convection is governed by a dimensionless Reynolds number]
The Grashof number is the ratio of the buoyancy force to the viscous force acting on the fluid
The Grashof number provides the main criteria for determining whether fluid flow is laminar or turbulent in natural convection
For vertical plates, the critical Grashof number is about 10^9.