In the following two-dimensional momentum equation for natural convection over a surface immersed in a quiescent fluid at temperature T∞ (g is the gravitational acceleration, β is the volumetric thermal expansion coefficient, ν is the kinematic viscosity, u and v are the velocities in x and y directions, respectively, and T is the temperature)

$\mathrm{u}\frac{\partial \mathrm{u}}{\partial \mathrm{x}}+\mathrm{v}\frac{\partial \mathrm{u}}{\partial \mathrm{y}}=\mathrm{g}\beta (\mathrm{T}-{\mathrm{T}}_{\infty })+\nu \frac{{\partial }^2\mathrm{u}}{\partial {\mathrm{y}}^2}$

the term gβ(T - T∞) represent