Editing Contrail (section)

==Condensation from decreases in pressure==
[[File:Propeller tip vortices being generated by P-40N1 Warhawk VH-ZOC at Temora.jpg|thumb|A vintage [[P-40 Warhawk]] with propeller tip vortex condensation]]
{{Main|Wingtip vortices}}

As a wing generates [[Lift (force)|lift]], it causes a [[vortex]] to form at the wingtip, and at the tip of the [[Flap (aircraft)|flap]] when deployed (wingtips and flap boundaries represent discontinuities in airflow). These [[wingtip vortices]] persist in the atmosphere long after the aircraft has passed. The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible; this effect is more common on humid days. Wingtip vortices can sometimes be seen behind the wing flaps of airliners during takeoff and landing, and during [[Space Shuttle]] landings.

The visible cores of wingtip vortices contrast with the other major type of contrails which are caused by the combustion of fuel. Contrails produced from jet engine exhaust are seen at high altitude, directly behind each engine. By contrast, the visible cores of wingtip vortices are usually seen only at low altitude where the aircraft is travelling slowly after takeoff or before landing, and where the ambient humidity is higher; they trail behind the wingtips and wing flaps rather than behind the engines.

At high-thrust settings the fan blades at the intake of a [[Jet engine#Turbofan|turbofan engine]] reach [[transonic]] speeds, causing a sudden drop in air pressure. This creates the condensation fog (inside the intake) which is often observed by air travelers during takeoff.

The tips of rotating surfaces (such as [[Propeller (aeronautics)|propellers]] and [[Helicopter rotor|rotors]]) sometimes produce visible contrails.<ref>{{cite web |title=Photos from the field |url=http://www.verticalmag.com/digital_issue/2014/v13i2/files/2.html |website=Vertical Magazine |access-date=8 July 2014 |page=39 |date=April–May 2014 |archive-date=16 July 2014 |archive-url=https://web.archive.org/web/20140716081046/http://www.verticalmag.com/digital_issue/2014/v13i2/files/2.html |url-status=dead }}</ref>

In firearms, a vapor trail is sometimes observed when firing under rare conditions, due to condensation induced by changes in air pressure around the bullet.<ref name="snipercountry">{{cite web |title=Vapor trail and Bullet trace |url=https://www.snipercountry.com/bullet-trail/ |website=Sniper Country |access-date=13 October 2021 |date=9 August 2018}}</ref><ref>{{cite web |title=Vapor Trail vs Bullet Trace |url=https://www.youtube.com/watch?v=dRWTzTB_8Tg |website=YouTube | date=18 October 2017 |access-date=13 October 2021 |language=en}}</ref> A vapor trail from a bullet is observable from any direction.<ref name="snipercountry"/> Vapor trail should not be confused with [[bullet trace]], a refractive effect due to changes in air pressure as the bullet travels, which is a much more common phenomenon (and is usually only observable directly from behind the shooter).<ref name="snipercountry"/><ref>{{cite web |last1=Norseman |first1=Dave the |title=Language Lessons: TRACE |url=https://www.breachbangclear.com/language-lessons-trace/ |website=Breach Bang Clear |access-date=13 October 2021 |date=15 June 2017}}</ref>