The gas accelerates to a final exit velocity which depends on the pressure and temperature at entry to the nozzle, the ambient pressure it exhausts to, and the efficiency of the expansion.[5] The efficiency is a measure of the losses due to friction, non-axial divergence as well as leakage in C-D nozzles.[6] Airbreathing engines create forward thrust on the airframe by imparting a net rearward momentum to the air by producing a jet of exhaust gas which has a speed that exceeds that of the aircraft. The jet may or may not be fully expanded as described in section "Reasons for C-D nozzle underexpansion and examples". On some engines that are equipped with an afterburner the nozzle area is also varied during non-afterburning or dry thrust conditions.