Theoretical and experimental results are presented which show that the thermal force on an aerosol particle in a temperature gradient depends on the velocity of the particle relative to the gas as well as the temperature gradient in the gas. It is shown that any motion of the particle along a line parallel to the temperature gradient tends to induce a local increase in the temperature gradient and thus an increase in the thermal force. Thermal settling velocity calculations which are made neglecting this motion−induced temperature gradient were found to be as much as fifty percent low when compared with experimental data. Calculations which account for this induced gradient, on the other hand, were able to predict the experimental data within three percent.