Streaming due to thermal surface tension gradients – the so-called Marangoni effect in isotropic fluids – has been observed in thin droplets of nematic liquid crystals deposited on a glass substrate. The coupling between the molecular orientation and the hydrodynamic flow induces distortions in the molecular alignment. This results in striking optical patterns which are directly related to the distortion of the molecular alignment at the nematic-air-interface. We also demonstrate that this experiment provides a simple test to determine the anchoring conditions of the molecules at the interface. The hydrodynamic equations have been solved and their solutions proved to be in good agreement with the experimental results. In particular a computer simulation has allowed us to describe completely the director field on the free surface and to reproduce numerically the optical patterns observed under the microscope.