Contrails (//; short for "condensation trails") or vapor trails are long, thin artificial clouds that sometimes form behind aircraft. Their formation is most often triggered by the water vapor in the exhaust of aircraft engines, but can also be triggered by changes in air pressure in wingtip vortices or in the air over the entire wing surface.1 Contrails are made of water in the form of a suspension of billions of liquid droplets or ice crystals.

Depending on the temperature and humidity at the altitude the contrails form, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several miles wide. The resulting cloud forms may resemble cirrus, cirrocumulus, or cirrostratus, and are sometimes called cirrus aviaticus. Persistent spreading contrails are thought by some, without overwhelming scientific proof, to have a significant effect on global climate.

Condensation from engine exhaust

The main products of hydrocarbon fuel combustion are carbon dioxide and water vapor. At high altitudes this water vapor emerges into a cold environment, and the local increase in water vapor can raise the relative humidity of the air past saturation point. The vapor then condenses into tiny water droplets which freeze if the temperature is low enough. These millions of tiny water droplets and/or ice crystals form the contrails. The time taken for the vapor to cool enough 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 condense rapidly. Exhaust contrails usually form at high altitudes; usually above 8,000 m (26,000 ft), where the air temperature is below −36.5 °C (−34 °F). They can also form closer to the ground when the air is very cold and has enough moisture.4

Moreover, scientists say that clouds are formed due to pollution particles, e.g soot from engine exhaust, which serve as condensation nuclei. These particles condensate the separate water molecules into clouds every time the conditions are right and, thus, pollute the sky much more seriously than pure water and CO2 would do alone.[citation needed]

Condensation from decreases in pressure

As a wing generates lift, it causes a vortex to form at each wingtip, and sometimes also at the tip of each wing flap. 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 landing of the Space shuttle.

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.

During high-thrust settings the fan blades at the intake of a 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. For more information see the Prandtl-Glauert singularity effect.

The tips of rotating surfaces (such as propellers and rotors) sometimes produce visible contrails.5

Contrails and climate

Contrails, by affecting the Earth's radiation balance, act as a radiative forcing. Studies have found that contrails trap outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) at a greater rate than they reflect incoming solar radiation (negative radiative forcing). NASA conducted a great deal of detailed research on atmospheric and climatological effects of contrails, including effects on ozone, ice crystal formation, and particle composition, during the Atmospheric Effects of Aviation Project (AEAP).6 Global radiative forcing has been calculated from the reanalysis data, climatological models and radiative transfer codes. It is estimated to amount to 0.012 W/m2 (watts per square meter) for 2005, with an uncertainty range of 0.005 to 0.026 W/m2, and with a low level of scientific understanding.7 Therefore, the overall net effect of contrails is positive, i.e. a warming effect.8 However, the effect varies daily and annually, and overall the magnitude of the forcing is not well known: globally (for 1992 air traffic conditions), values range from 3.5 mW/m2 to 17 mW/m2. Other studies have determined that night flights are mostly responsible for the warming effect: while accounting for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative forcing. Similarly, winter flights account for only 22% of annual air traffic, but contribute half of the annual mean radiative forcing.

A 2015 study found that artificial cloudiness caused by contrail "outbreaks" reduce the difference between daytime and nighttime temperatures. The former are decreased and the latter are increased, in comparison to temperatures the day before

and the day after such outbreaks.10 On days with outbreaks the day/night temperature difference was diminished by about 6F° in the U.S. South and 5F° in the Midwest.

Distrails

Where an aircraft passes through a cloud, it can clear a path through it; this is known as a distrail (short for "dissipation trail"). The plane's warm engine exhaust causes existing water droplets to evaporate, leaving a clear wake through an otherwise cloudy sky.

Clouds form when invisible water vapor (H2O in gas phase) condenses into microscopic water droplets (H2O in liquid phase) or into microscopic ice crystals (H2O in solid phase). This may happen when air with a high proportion of gaseous water cools. A distrail forms when the heat of engine exhaust evaporates the liquid water droplets in a cloud, turning them back into invisible, gaseous water vapor.

飛機雲（英語：contrail），又名凝結尾。它是一種由飛機引擎排出的濃縮水蒸氣形成的可見尾跡（人為捲雲）。當炙熱的引擎排出廢氣在空氣中冷卻時，它們可能凝結形成一片由微小水滴構成的雲。如果空氣溫度足夠低的話，飛機雲也可能由微小的冰晶構成。[註 1]

從機翼尖端或襟翼拖曳出的翼尖漩渦有時因為漩渦核心的水氣凝結的原因，也是部分可見的。每一個漩渦都是一大片旋轉着的空氣，在漩渦中心的氣壓很低。這種翼尖漩渦和引擎排出的廢氣沒有關聯。翼尖漩渦有時也被稱作蒸汽尾跡。

引擎廢氣引起的凝結

碳氫燃料燃燒後的主要產物是二氧化碳和水蒸氣。在海拔較高處的低溫的環境下，局部水蒸氣的增加可以使空氣中的水蒸氣含量超過飽和點。這些蒸氣之後會凝結成微小的水滴並／或小沉積成為冰晶。成千上萬的小水滴和／或冰晶形成了飛機雲或凝結尾。雲的主要組成部分是在空氣中漂浮的水份。在高空過度冷卻的水蒸氣需要一種觸發條件以激發它們的凝結或沉澱。引擎廢氣中的微粒正是起着這種觸發條件的作用，促使空氣中的水蒸氣快速的轉變成冰晶。飛機雲或凝結尾一般在海拔 8000 米（26000 英尺）以上出現，那裏的溫度低達-40°C（-40°F）[5]。

氣壓降低引起的凝結

飛機的機翼會引起機翼附近的氣壓下降，從而導致溫度下降。氣壓和溫度下降的綜合效應會導致空氣中的水凝結並形成後緣渦流。這種效應在潮濕的天氣較為常見。後緣渦流常見於起飛和着陸期間客機的襟翼後方，穿梭機着陸期間，以及在執行高強度演習的軍用噴氣機上部翼的表面。此外，在渦輪風扇引擎通風口周圍區域的氣壓會比周圍空氣的氣壓低，並可能導致在高推力裝置的通風口形成冷凝霧。

這些類型的蒸汽尾跡與其他由噴氣燃料燃燒引起的凝結尾形成對比。噴氣機引擎廢氣產生的凝結尾常見於高海拔地區，並且出現在每個引擎的後部。與之相反，由於氣壓下降造成蒸氣尾跡常見於濕度較高的低海拔區域，並且在翼梢和襟翼後部而不是在噴氣機引擎後部。

飛機雲或凝結尾和氣候的關係

飛機雲或凝結尾在地球輻射平衡上，扮演着正面的輻射驅動力。研究發現，相比於反射進入地球的太陽輻射（負面輻射驅動力），飛機雲或凝結尾更多的阻礙、由地球和大氣層放射出的長波輻射離開地球（正面輻射驅動力）。因此，飛機雲的總體的網狀效應是「正」的，也就是說主要是使氣溫上升的效應。[6]但是，這種效應在每天和每年的跨度上都各不相同，並且總體上的這種效應大小度量並沒有為人熟知：以 1992年的空中交通情況來說，這種效應值的估計從 3.5

mW/m²到 17 mW/m²。另一項研究顯示，夜間飛行對這種讓氣候變暖的效應負有最大的責任：日間飛行只對這種效應的產生起到了 25%的作用，而夜間飛行卻對這種效應起到了 60%到 80%的作用。同樣的，冬季飛行只佔到了每年飛行交通量的 22%，卻對年平均正面輻射驅動力卻起到了一半的作用[7]。

反飛機雲（擴散尾）

當飛機在雲層中穿過時，飛機可以在雲層中清理出一個通道穿過，這就是所謂的反飛機雲。因為飛機產生的飛機雲不可見（飛機雲通常形成於 26,000英尺以上的高空，具體由溫度和其他因素所決定）。如果雲層較薄的話，反飛機雲就像穿過雲中的一條隧道.[13]。