A cloudlike streamer frequently observed to form behind aircraft flying in clear, cold, humid air. Condensation trails may persist and encourage the formation of a layer of cirrus clouds. Condensation trails may form by either of two distinct processes. First, addition of water vapor to the swept path of the aircraft inevitably accompanies exhaust of combustion products from the engines. If the humidifying effect of this addition overbalances the concomitant addition of the heat of combustion, exhaust trails may form depending on mixing with air from the environment. The thermodynamics of this process is such that the effect becomes important only for rather low temperatures of the order of those encountered near the tropopause, so this type of condensation trail is only usually observed for high-altitude flight. On occasion, exhaust provides needed condensation nuclei, but this effect has not been fully investigated. Second, in air that is clear, but almost fully saturated, the aerodynamic pressure reduction that accompanies flow of air around propeller tips and around wingtips can so cool the air as to induce condensation and form aerodynamic trails. The latter propeller-tip trails and wingtip trails are seldom as dense as are exhaust trails. Under some conditions the pressure reduction lowers the temperature below that for homogeneous condensation of ice and the trail consists of ice particles even at ambient temperatures as warm as −15°C. Wingtip trails only occur with aircraft of such heavy wing-loading as to yield very strong tip vortex circulations. Interceptor planes pulling out of dives, and hence imposing temporarily heavy wing-loading, may produce transient tip vortex trails. Faint vortex trails may appear aft of the corners of flaps during aircraft landings.