Using laser-Doppler anemometry, we experimentally show that the instability of the rest state in the problem of mixed convection (i.e. free and forced) cannot be favorably compared with the theoretical curve corresponding to neutral stability, but compares rather well with the convective instability curve. Subsequently, we measure the velocity amplitude in the thermoconvective rolls at different locations in the direction of the main flow, and compare with the prediction of an amplitude equation, describing idealized two-dimensional dynamics.