Vapors, released at high altitudes from rockets or the Space Shuttle, may travel many hundreds of kilometers before coming to rest. An understanding of the effects produced by these releases requires knowledge of the vapor flow. In the tenuous upper atmosphere, the injected gases expand from a collisionless to a collision-dominated state. In this paper, this process is described by the Boltzmann equation with the Krook collision integral. The theory is applied to supersonic releases into a nonuniform atmosphere. The model predicts elongation and heating of the vapor trails due to collisions. The theory may be applied to a wide range of experiments whose objectives include the formation of ionospheric depletions, the triggering of instabilities, the generation of gravity waves, and the injection of neutrals as luminous tracers.