The hydrodynamic wind within the upper portion of a planet's atmosphere allows light chemical elements such as hydrogen to move up to the exobase, the lower limit of the exosphere, where the gases can then reach escape velocity, entering outer space without impacting other particles of gas. This type of gas loss from a planet into space is known as planetary wind.[148] Such a process over geologic time causes water-rich planets such as the Earth to evolve into planets like Venus.[149] Additionally, planets with hotter lower atmospheres could accelerate the loss rate of hydrogen.