QUESTION = What causes the trail behind jets at high altitude? GENERATION = When an airplane flies over, it creates a disturbance in the air as it pushes against it. This disturbance will cause a small amount of water to vaporize. This water creates a bunch of tiny little water droplets which condense and form the contrail. A similar thing happens at ground level when a car goes over the same spot. It creates an area of low pressure under the car, which, due to friction, causes a small amount of water to evaporate. This water forms a bunch of little water droplets which also condenses and create the contrail. A more complicated way to explain it is that it's *technically* because the plane is moving very fast. It creates a lot of motion for a small change in velocity, which causes a bunch of water droplets to condense and form a contrail. Why does it happen at altitude? It has to do with the density of the air. There is no free air at sea level, so no free water to form. Because the density is much lower at high altitudes, the water droplets aren't as dense, so they aren't as visible. RETRIEVAL #0 = Contrail---them back into invisible, gaseous water vapor. Section::::See also. BULLET::::- Chemtrail conspiracy theory BULLET::::- Cirrus cloud BULLET::::- Cloud chamber – particle detector that works on similar principle BULLET::::- Environmental impact of aviation BULLET::::- Fallstreak hole BULLET::::- Global dimming BULLET::::- Ship tracks BULLET::::- Skywriting BULLET::::- Space jellyfish Section::::External links. BULLET::::- Contrails.nl: Pictures of contrails and Aviation Cirrus (-Smog), from 1995 on BULLET::::- Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate BULLET::::- Reference site for debunking weird stories about contrails BULLET::::- Contrail simulator (Java applet) – interactively shows how temperature and humidity of the surrounding air affect contrail formation and characteristics BULLET::::- NASA Contrail Education BULLET::::- What is a contrail and how does it form?, National Weather Service RETRIEVAL #1 = Contrail---sometimes called cirrus aviaticus. Persistent spreading contrails are suspected to have an effect on global climate. Section::::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 distance behind the aircraft. 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 , where the air temperature is below . They can also form closer to the ground when the air is cold and moist. A 2013–2014 study jointly supported by NASA, the German aerospace center DLR, and Canada's National Research Council NRC, determined that biofuels could reduce contrail generation. This reduction was explained by demonstrating that biofuels produce fewer soot particles, which are the nuclei around which the ice crystals form RETRIEVAL #2 = Contrail (disambiguation)---Contrail (disambiguation) A contrail is a condensation trail caused by an aircraft. Contrail or Contrails may also refer to: BULLET::::- Contrail (company), a defunct video game developer BULLET::::- Contrail (software), a Cloud Federation computing project BULLET::::- "Contrail" (song), a song by Namie Amur BULLET::::- "Contrails" (book), a handbook issued to new cadets entering the United States Air Force Academy Section::::See also. BULLET::::- Distrail BULLET::::- Vapor Trail (disambiguation) RETRIEVAL #3 = Contrail---at cruising altitude with a sample-gathering aircraft flying in trail. In these samples, the contrail-producing soot particle count was reduced by 50 to 70 percent, using a 50% blend of conventional Jet A1 fuel and HEFA (hydroprocessed esters and fatty acids) biofuel produced from camelina. Section::::Condensation from decreases in pressure. As a wing generates lift, it causes a vortex to form at the wingtip, and at the tip of the flap when deployed (wingtips and flap-boundaries are 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 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 RETRIEVAL #4 = Anthropogenic cloud---the different "genera" in three main groups of clouds according to their altitude: BULLET::::- High clouds BULLET::::- Middle clouds BULLET::::- Low clouds Homogenitus clouds will be generated by different sources at those different levels. Section::::Generating process.:High homogenitus. Despite the fact that the three genera of high clouds, "Cirrus", "Cirrocumulus" and "Cirrostratus", form at the top of the troposphere, far from the earth surface, they may have an anthropogenic origin. In this case, the process that causes their formation is almost always the same: commercial and military aircraft flight. Exhaust products from the combustion of the kerosene expelled by engines provide water vapor to this region of the troposphere. In addition, the strong contrast between the cold air of the high troposphere layers and the warm and moist air ejected by aircraft engines causes rapid sublimation of water vapor forming small ice crystals. This process is also enhanced by the presence of abundant nuclei of condensation produced as a result of combustion. These clouds are commonly known as condensation trails (contrails) and are initially lineal "cirrus clouds" that could be called "Cirrus homogenitus" (Cia). Due to the large temperature difference between the air exhausted and the ambiance air generates small-scale convection RETRIEVAL #5 = Mirage---mirages explained BULLET::::- China daily, rare mirage in Penglai BULLET::::- The superior mirage BULLET::::- The inferior mirage BULLET::::- The highway mirage BULLET::::- Fata Morgana Mirage from the Continental Divide Trail RETRIEVAL #6 = Contrail---suggesting that examination of historic weather data could help study these effects. Section::::Head-on contrails. A contrail from an airplane flying towards the observer can appear to be generated by an object moving vertically. On 8 November 2010 in the US state of California, a contrail of this type gained media attention as a "mystery missile" that could not be explained by U.S. military and aviation authorities, and its explanation as a contrail took more than 24 hours to become accepted by U.S. media and military institutions. Section::::Distrails. Where an aircraft passes through a cloud, it can disperse the cloud in its path. This is known as a distrail (short for "dissipation trail"). The plane's warm engine exhaust and enhanced vertical mixing in the aircraft's wake can cause existing cloud droplets to evaporate. If the cloud is sufficiently thin, such processes can yield a cloud-free corridor in an otherwise solid cloud layer. An early satellite observation of distrails that most likely were elongated, aircraft-induced fallstreak holes appeared in Corfidi and Brandli (1986). Clouds form when invisible water vapor ( in gas phase) condenses into microscopic water droplets ( in liquid phase) or into microscopic ice crystals ( in solid phase). This may happen when air with a high proportion of gaseous water cools. A