The effect of impurity particles on the forced convection velocity in a drop is analyzed. The liquid motion in a drop is due to droplet streamlining by the air flow. The experiments are carried out in a wide range of air velocities and droplet sizes and compared with approximate numerical solutions. A simple analytical model for determining convection inside the droplet is proposed for the first time. A semi-empirical model is proposed to generalize the experimental data by introducing the function considering the density of impurities (solid particles in the form of contaminants) on the droplet surface. The novelty of the work lies in the newly formulated hypothesis, justifying the 40-50 times underestimation of the experimental data below the theoretical calculation. For the case of motion of a drop of one liquid in another liquid, the experimental data satisfactorily correspond to the obtained analytical expression. The obtained expressions may be used for a wide range of problems related to the interfacial boundary (liquid - gas). (C) 2019 Elsevier B.V. All rights reserved.