An atmospheric escape process whereby heavy molecules achieve escape velocity through collision with lighter particles. If the outflow is extremely rapid, adiabatic cooling results in cold temperatures which slow the particles down. Extremely rapid flow of H from a primitive atmosphere will carry away heavy gases in what is known as blowoff. A heavy atom immersed in an upward-flowing light gas experiences two upward forces: a drag caused by the relative motion, and a buoyancy force. The downward force is gravity. There is a critical mass below which atoms will be accelerated upwards. At higher levels, however, the drag force will decrease and a new equilibrium will be reached.