Thermal gradients in nanomaterials can cause surface mass transport phenomena. However theatomic fluxes are challenging to quantify and the underlying atomic mechanisms are complex. Using low energy electron microscopy we have examined in operando, under a thermal gradient of 10^4 K/m, the thermomigration of supercooled Si(111)-1×1 advacancy islands. The islands move in the direction of the thermal gradient at 0.26±0.06 nm/s. This reveals that the adatoms move towards the cold region and the effective force exerted on Si adatoms is 1.4±0.4× 10^− 8eV/nm. We quantify the heat of transport of Si atoms Q∗= 1.2±0.4 eV and show that it corresponds to the combined effects of adatom creation at step edges and adatom diffusion on atomically flat terraces.