- Novel experimental results on the hydrodynamic forces acting on a small heated particle at low Reynolds and Grashof numbers are presented. The temperature difference between the particle and the ambient causes free convection flow around the particle. As a result, the drag force exerted on the particle by forced flow deviates significantly from the Stokes law. This deviation is represented quantitatively for three basic directions of forced flow, namely assisting, opposing, and crossing with respect to free convection flow. Validity of the Stokes formula at mixed-convection creeping flow conditions is discussed.