1. Contrails Contrails (short for "condensation trails") or vapour trails are artificial clouds that are the visible trails of condensed water vapour made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals.[1] The wingtip vortices which trail from the wingtips and wing flaps of aircraft are sometimes partly visible due to condensation in the cores of the vortices. Each vortex is a mass of spinning air and the air pressure at the centre of the vortex is very low. These wingtip vortices are not the same as contrails. Depending on atmospheric conditions, contrails may be visible for only a few seconds or minutes, or may persist for many hours which may affect climate.[2] Condensation from engine exhaust The main byproducts of hydrocarbon fuel combustion are carbon dioxide and water vapor. At high altitudes this water vapour emerges into a cold environment, and the local increase in water vapor can push the water content of the air past saturation point. The vapour then condenses into tiny water droplets and/or deposits into ice. These millions of tiny water droplets and/or ice crystals form the vapour trail or contrails. The vapor's need to condense accounts for the contrail forming some way behind the aircraft's engines. At high altitudes, supercooled water vapor requires a trigger to encourage deposition or condensation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapor to rapidly turn to ice crystals. Exhaust vapour trails or contrails usually occur above 8000 metres (26,000 feet), and only if the temperature there is below −40 °C (−40 °F).[3] Condensation in the cores of wingtip vortices from an F-15E as it disengages from a KC-10 Extender following midair refueling Contrails from a Qantas Boeing 747-400, Australia Iridescent contrails

2. Wingtip Vortices Wingtip vortices are tubes of circulating air which are left behind a wing as it generates lift.[1] One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure. To first approximation, these low-pressure regions form with little exchange of heat with the neighboring regions (i.e. adiabatically), so the local temperature in the low-pressure regions drops, too.[2] If it drops below the local dew point, there results a condensation of water vapor present in the cores of wingtip vortices, making them visible.[2] The temperature may even drop below the local freezing point, in which case ice crystals will form inside the cores.[2] Wingtip vortices are associated with induced drag, an essentially unavoidable side effect of the wing generating lift.[3] Managing induced drag and wingtip vortices by selecting the best wing planform for the mission is critically important in aerospace engineering. Wingtip vortices form the major component of wake turbulence. Migratory birds take advantage of each others' wingtip vortices by flying in a V formation so that all but the leader are flying in the upwash from the wing of the bird ahead. This upwash makes it a bit easier for the bird to support its own weight, reducing fatigue on migration flights.[4] Some technical writers use the alternative expression "trailing vortices" because these vortices also occur at points other than at the wing tips.[1] They are induced at the outboard tip of the wing flaps and other abrupt changes in wing planform Condensation in the cores of wingtip vortices from an F-15E as it disengages from a KC-10 Extender following midair refueling A C-27J Spartan showing prop vortices