Summary: We consider a gas in a horizontal slab in which the top and bottom walls are kept at different temperatures. The system is described by the Boltzmann equation with Maxwellian boundary conditions specifying the wall temperatures. We study the behavior of the system when the Knudsen number \(\varepsilon\) is small and the temperature difference between the walls as well as the velocity field is of order \(\varepsilon\), while the gravitational force is of order \(\varepsilon^2\). We prove that there exists a solution to the Boltzmann equation for \(t\in (0,\overline t)\), which is near a global Maxwellian, and whose moments are close, up to order \(\varepsilon^2\), to the density, velocity and temperature obtained from the smooth solution of the Oberbeck-Boussinesq equations for \(t\leq \overline t\).