Why does the plane leave a trail? June 23rd, 2017

Of course, often in the sky you see this trace is not so "powerful", but there are some points about it that you might not know.

Check yourself...

Often raising our heads to the sky, we see a white stripe on it from a flying plane. The trail it leaves behind is called condensation. By the way, we often call it a contrail, but on Wikipedia opposite "contrail" there is a note "obsolete name". Therefore, we will use the term "condensation". In addition, this name is "speaking" - in this very name lies the answer to the question of what it is.

As a rule, the exhaust gases of jet engines are the direct cause of the trace. They include water vapor, carbon dioxide, nitrogen oxides, hydrocarbons, soot and sulfur compounds. Of these, only water vapor and sulfur are responsible for the appearance of the contrail. Sulfur serves to form condensation points, while the contrail itself can be formed both from water vapor, which is part of the exhaust gases, and from steam, which is part of the supersaturated atmosphere.

Getting into the cold air (and at the altitude at which planes usually fly, the temperature is about -40 degrees), the steam condenses around the particles of the burned fuel and tiny droplets are obtained, like fog, which form a strip in the sky. We can say that it turns out a kind of man-made long cloud. Over time, it will dissipate or become part of the cirrus clouds.

Why is this trace not always visible?

If for such humidity the ambient temperature is below the dew point, then the moisture forms white condensation trails behind the engines. At low altitudes, they consist of water droplets, which usually evaporate quickly, and the trail disappears. But when the plane is flying at high altitude, where the air temperature is below -40 ° C, the steam immediately condenses into ice crystals, which evaporate much more slowly.

By the way, the contrails of aircraft can affect the Earth's climate. If you look at the Earth from a satellite, you can see that in those areas where planes often fly, the entire sky is covered with their traces. Some scientists believe that this is good - the traces increase the reflective properties of the atmosphere, thereby preventing the sun's rays from reaching the Earth's surface. This can reduce the temperature of the earth's atmosphere and prevent global warming. Others believe that it is bad - cirrus clouds arising from the condensation trail prevent the atmosphere from cooling, thereby causing it to warm. Who is right and who is wrong, time will tell.

Want to stop leaving a trace?

Depending on atmospheric conditions and wind speed, a contrail can remain in the sky for up to 24 hours and be up to 150 km long. Scientists from the University of Reading (UK) decided to figure out how to make planes fly without a trace, while maintaining the profitability of transportation.

“It may seem that the plane needs to make a considerable detour to avoid the contrail. But due to the curvature of the Earth, you only need to increase the distance a little to avoid really long trails, ”says Emma Irwin, author of the study, published in the journal Environmental Research Letters.

Their calculations showed that for small short-haul aircraft, a deviation from moisture-saturated areas, even 10 times the length of the contrail itself, can reduce the negative impact on the climate.

“For larger planes, which are emitting more carbon dioxide per kilometer, three times as much variance makes sense,” says Irwin. In their study, the scientists assessed the impact on the climate of airliners flying at the same altitude.

For example, a plane flying from London to New York can only deviate by two degrees to avoid a long wake, which adds 22 km to its path, or 0.4% of the total distance.

Scientists are currently involved in a project that aims to assess the possibility of redesigning existing transatlantic routes to take into account the impact of aviation on climate. To implement the proposals of climatologists means in the future to face problems in the field of economy and safety of air transportation, experts admit. "Controllers need to evaluate whether these flight-to-flight reroutes are feasible and safe, and forecasters need to understand whether they can reliably predict where and when contrail clouds might form," Irwin said.

A plane flying in the sky is a beautiful sight. Especially when he leaves behind a trail that can stretch across the sky. Over time, this trace disappears, it is carried by the winds that reign in the sky. It can be long or short, and sometimes the plane does not leave it at all. What are these phenomena connected with, why does the trace sometimes remain, and sometimes not, and what does it consist of?

Many inquisitive people ask these questions. To understand all the nuances, it is necessary first of all to understand what this trace consists of.

Not smoke from burning fuel

Some might argue that this trail is nothing more than the smoke that remains when fuel is burned, by analogy with car exhaust. Airplane turbines are much more powerful than a car engine, which is why they generate so much smoke. But this answer will be fundamentally wrong, completely illiterate.

Aircraft engines do emit gases left from the combustion of aviation kerosene, but the exhaust of the aircraft is transparent. After all, not a single aircraft in good condition smokes on the runway, during takeoff or landing. If it was exhaust, it would become obvious immediately, and there would be nothing to breathe at the airport. But there are some things that engines do throw out.

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Along with other elements of the gas-air mixture of the exhaust, water is also emitted - in a vapor state. If the aircraft is at low altitude, this is usually not visible. In a situation where the plane has risen high, the water immediately crystallizes, forming white clouds that stretch behind each turbine. This is the key to the trail that stretches behind the planes.

Why is the trace not always visible?

The lower the temperature overboard, the faster and more completely the process of crystallization of the water emitted by the engines takes place. If the plane is flying low, there is no question of lower temperatures, the trace is not visible, or it is barely noticeable. It is worth remembering that the higher the winged car rises, the lower the temperatures drop. In the high layers, the indicator can appear in the region of -40 degrees, and it is quite natural that the moisture here freezes instantly and completely, forming a thick trail. In such temperatures, even a person’s breath freezes - it is worth remembering that literally 50-60 years ago, pilots were given sheepskin coats and warm clothes for flights at any time of the year so that they would not freeze in the cockpits.

Why does the plane leave a trail?

Sometimes long white streaks are visible in the sky, like very narrow clouds. These stripes are woven into bizarre patterns, rush up, and then suddenly break off unexpectedly. Each of us knows that this is the trail of an airplane soaring high into the sky. Going, for example, by taxi to the airport, we can watch how many planes take off and land, but why does a low-flying plane leave no trace of itself, and a plane that has soared so high that it is not visible at all begins to leave traces?

Aircraft trace - the so-called contrail (contrail) - a visible trail of condensed water vapor that occurs in the atmosphere behind moving aircraft under certain atmospheric conditions. The phenomenon is observed most often in the upper layers of the troposphere, much less frequently in the tropopause and stratosphere. In some cases, it can also be observed at low altitudes.

Contrails belong to a separate group of clouds - technogenic or artificial clouds - Ci trac. (Cirrus tractus, cirrus - pinnate, tractus - trace).

The trace got its name from the condensation process that leads to its appearance. Condensation occurs only under such conditions when the amount of water vapor exceeds the amount that is necessary for saturation. These conditions are determined by the dew point - the temperature at which the water vapor contained in the air reaches saturation at a given specific humidity and constant pressure. The degree of saturation is characterized by relative humidity - the percentage of the amount of water vapor contained in the air to the amount required for saturation (at the same temperature). In addition to these conditions, the presence of condensation centers is also necessary. At temperatures up to -30 ... -40 ° C, water vapor passes into the liquid phase during condensation, at temperatures below -30 ... -40 ° C, water vapor immediately turns into ice crystals, bypassing the liquid phase. Also an important role in the formation of the trace is played by the evaporation process, leading to its disappearance.

There are two main reasons for the conditions for condensation and the appearance of a trace: The first is an increase in air humidity when water vapor contained in the exhaust gases of an aircraft engine as a result of fuel combustion is added to atmospheric water vapor. This raises the dew point in the limited volume of air (behind the engines). If the dew point becomes higher than the ambient temperature, then as the exhaust gases cool, the excess water vapor condenses. The amount of water vapor emitted by the engine depends on its power and mode of operation, that is, on fuel consumption. The second reason is the decrease in pressure and temperature of the air above the wing and inside the vortices that occur when flowing around various parts of the aircraft. The most intense vortices are formed at the wingtips and extended flaps, as well as at the ends of the propeller blades. If the temperature drops below the dew point, the excess atmospheric water vapor condenses in the area above the wing and inside the vortices. The degree of reduction in pressure and temperature depends on such parameters as the mass of the aircraft, the coefficient of lift, the magnitude of the inductive drag, etc. Often there are traces formed as a result of a combination of these two causes. The formation of a condensation trail is also facilitated by condensation centers in the form of particles of unburned or incompletely burned (soot) fuel. Along with condensation, the reverse process also occurs - evaporation: particles of condensed water vapor evaporate, and the trace disappears over time. The evaporation rate is affected by the humidity of the air surrounding the trace and the state of aggregation of the trace particles. The drier the air, the faster evaporation occurs. On the contrary, evaporation does not occur when the water vapor is in a state of saturation. Condensed water vapor at an air temperature of -30 ... -40 ° C partially, and at a temperature below -40 ° C completely turns into crystals, the evaporation of ice crystals occurs much more slowly than water drops.

Thus, the possibility of the appearance and lifetime of a condensation trail, as well as its type, depend on the humidity and temperature of the atmospheric air (ceteris paribus). At low humidity and relatively high temperature, there may be no trace at all, since under such conditions the water vapor does not reach a state of supersaturation. The higher the humidity and the lower the temperature, the more water vapor condenses, the slower the evaporation occurs, hence the trail is richer and longer. And at a relative humidity close to 100% and low temperature, the largest amount of water vapor condenses, high humidity prevents the evaporation of trail particles, which leads to the formation of condensation trails that can exist for a long time, often turning into cirrus or cirrocumulus clouds. Since the water vapor in the atmosphere is unevenly distributed, this is the reason for the same "uneven" footprint.

Contrails are formed not only at high flight altitudes (hence one of the erroneous names - "high-altitude trail"). At the ice airfield of the Scott Amundsen Polar Station (altitude 2830 m above sea level), under certain conditions (air temperature minus 50 degrees and below), this trail is formed already on takeoff or landing, and behind turboprop aircraft (C-130 " Hercules" from the "Snow Wing" of the US Air Force), which makes it unnecessary to discuss another misnomer - "jet trail".

Contrails are still an unmasking factor for the activities of military aviation, so the probability of their occurrence is calculated by aviation meteorologists using appropriate methods and recommendations are issued to crews. Changing the flight altitude within certain limits allows you to avoid or completely eliminate the undesirable effect of this factor.

There is also an antipode (opposite) to the contrail - a "reverse", "negative" (very rare names) trail, formed by the dispersion of cloud elements (ice crystals) within the wake under certain conditions. Reminds me of "color reversal" in graphic editors of computer programs, when the blue sky is a cloud, and the trail itself is pure blue space. It is clearly observed in stratus or cumulus clouds of insignificant vertical thickness and the absence of other (higher for an observer from the Earth) cloud layers masking the blue background of the upper atmosphere. It is observed at least as often as contrails, but, due to the mentioned specificity, it is less likely to be expected and less illustrated in publications about clouds and materials from amateurs of observing these phenomena.

A contrail should not be confused with a wake. A wake is a disturbed region of air that always forms behind a moving aircraft. However, the condensation trail, interacting with the wake, reveals in relief the vortex structure of the disturbed air.

According to climatologists, contrails affect the climate, reducing the temperature due to the fact that they degenerate into cirrus clouds, thereby increasing the Earth's albedo.

According to materials:

 Answer:
The answer is obvious - for the same reason that fog or frost appears when breathing in the cold. Hydrocarbon fuel is burned in aircraft turbines, and one of the combustion products is water, more precisely, its steam, heated to a high temperature. Hot water vapor, flying out of the turbine nozzle, immediately begins to condense, forming a filamentous cloud consisting of tiny water droplets or ice crystals, since the temperature at such a height is lower −40 °С. Sometimes the air at altitude is supersaturated with moisture, which cannot condense only because of the absence of so-called condensation nuclei - the smallest particles, such as dust. In such cases, an overflying aircraft, leaving behind soot particles - a product of incomplete combustion of fuel, causes the condensation of supersaturated atmospheric vapors. Therefore, according to the intensity of the white trace from a flying aircraft, one can judge the air humidity in the upper layers of the troposphere, and hence the upcoming weather. A rapidly disappearing or barely noticeable trace indicates that the air at altitude is dry, and the weather will be cloudless. And if a white trail stretches across the sky, then you should expect worse weather.
In photographs taken from satellites, the Earth in many places is covered with a dense white grid of traces from overflying aircraft (photo from the site fiz.1september.ru).

It has been shown that in some cases the traces of a flying aircraft turn into clouds with an area of 4000 before 40000 square kilometers, influencing the climate. Therefore, for example, the suspension of flights over the United States for three days after the tragedy of September 11, 2001 sharply increased the transparency of the atmosphere, and as a result, the difference between the average day and night temperatures increased by 1 °C. Thus, white traces from airplanes serve as one of the factors of the global "blackout" of the planet, counteracting its global warming.

A wide variety of journals that collect and analyze information relating to the achievements and problems of aviation often focus readers on the material aspects of the work and structure of modernized devices such as airplanes, rockets, helicopters and other aircraft. Often, all phenomena that occur with the internal and external structure of the vehicle during the flight are also analyzed. Usually the contrail reflects this. Many people watch beautiful planes that leave a flat line in flight.

The concept of this phenomenon

The contrail is formed in the tropopause. Its appearance is influenced by water vapor, which undergo enhanced condensation. They are present in the combustion products, since hydrocarbon fuel is evenly consumed during combustion. After exiting outside and sufficient cooling, a bright contrail from an aircraft or other aircraft in the air becomes noticeable.

There are special air shows that are advisable to hold only in sunny weather. These events are organized at airfields that have the status of the largest in the world. At this time, a large number of spectators enthusiastically watch the movement of many aircraft, making interesting maneuvers in the air. The main distinguishing feature of such events is the leaving of a bright trail from each vehicle. It is often done so that each aircraft has its own tail color, which helps to get the most striking and memorable effect.

Unlike aircraft, rockets constantly leave behind massive, even often formidable trails that not only look large, but also have a rich color. They are issued from combat aircraft. This procedure can be observed not only when going to special events, but also being on the street or turning on the TV on the channel of interest. So you can see the contrail.

Wing tip vortex

It should be remembered that an aircraft in flight leaves behind a limited and fairly wide area of ​​the atmosphere, which becomes perturbed, its composition changes for a long time. This phenomenon is often referred to as a tangled trail. Usually it appears under the action, since during operation they constantly interact with the environment. The end vortices of the aircraft wings also take part in this process.

If we compare the significantly negative impact on the environment, then the primacy is always given to the tip vortices of the wings. There are many symbols for tangled tracks, but most often they are drawn on special diagrams in the likeness of a sheet with unusual edges, the ends of which are completely twisted, that is, they can be compared with vortices.

Twisting process: scientific reasoning

The twisting process can be easily explained scientifically. There is a clear difference in pressure between both sides of the wings of the aircraft, that is, on their upper and lower surfaces. The air is gradually redistributed from the lower surface, since it has the most increased pressure, to the upper one in order to remain in the area with the lowest pressure.

This redistribution occurs through the tip of each wing, which creates powerful and very noticeable vortices. The force of the pressure difference matters, since it depends on it. It is this value that has a strong influence on the wing. The stronger this effect, the more powerful and relief vortices are formed.

Various brands of aircraft that provide for a wing tip vortex

The speed of air flows sometimes changes, but it can be approximately determined that if the diameter of the vortex wake is about 8-15 m, we should talk about a value of 150 km / h. The tip vortex can be formed in various ways. This process depends on the brand, configuration of the aircraft. The powerful Mirage 2000 and F-16C fighters deserve attention if they move into position when flying with a high angle of attack.

The process of the appearance of the end vortex

The end vortex is visualized thanks to a special tracer generator responsible for the proper representation of the smoke trail. The action of this element is due to a change in the state of the atmosphere, which lasts for quite a long time. Then the circumferential speed of movement gradually subsides, that is, the visual object is lost and disappears.

Under the influence of time, the circumferential speed of the vortex decays, due to which the visual image changes shape until it completely dissolves. The perceived intensity of the whirlwind can last up to about two minutes after the aircraft has passed a particular location. Such a vortex has the ability to significantly affect the flight mode of an aircraft that has entered an atmosphere disturbed by the action of the engine of the previous vehicle.

Long-term observation of the tip vortex

When the vortices interact with each other, they slowly descend and diverge, that is, a perceptible change in the atmosphere disappears. The contrail of an aircraft is an excellent object for observing its transformations. After about 30 - 40 seconds, it begins to change shape, as it is strongly influenced by a whirlwind, which gradually develops. When both inversion and vortex layers intersect, bizarre shapes are created that can be calculated in advance, since various patterns act on the process of their formation.

The number of stripes and the height of the contrail are controlled by the number and location of engines in the system. At the same time, the contrail not only floats in the air, but also constantly changes, creating interesting contours. Most often, twisting of this layer is observed under the influence of the end vortex. All transformations of the layer reflect various aerodynamic processes that are always formed during the flight.

Separated-vortex flows

Sometimes pilots are forced to perform various attacks, which are carried out with a large angle of inclination, which is more than 20 degrees. In this case, the nature of the flow around the contours of the aircraft changes significantly for a while. Separation areas begin to appear, which are mainly fixed near the upper surface of the wing and fuselage. In them, the pressure is greatly reduced, so the concentration and increase of atmospheric moisture immediately begins. Thanks to this aspect, it is possible to observe the flight of an aircraft without the use of tracers.

Conditions for the appearance of the separation-vortex effect

If the angle of attack is too large, a significant cloud halo will form around the aircraft. When the plane flies, this cloud automatically turns into a vortex contrail from the plane. Typically, in bombers, separation regions form near the wings, due to which the appearance of a vortex bundle is clearly observed. This is what a contrail looks like, the photos of which are always fascinating.

Hot traces of missiles

Sometimes when one has to deal with such cases when a stall is observed in the area of ​​the gas-air path located in the rocket power plant. The gas jet departing from is distinguished by a high temperature, therefore, sometimes it enters the air intake of the carrier aircraft, which happens when the device is set to certain modes.

Becomes too uneven in temperature as it is exposed to elevated temperature gases, causing the air entering the engine to become altered. An engine surge is formed, that is, a stall occurs in the system. To identify this process, the main combustion chambers are observed, since the air flow is subjected to longitudinal oscillations, passing through the engine tract, and then marked by the release of a flame from these elements. This is how a contrail from a rocket appears.

Features of the contrail during testing

Often launches of missile weapons are carried out in the concept of testing. An exception is the on-board equipment, which serves for the purpose of recording and storing information. Often, the photographic aircraft is released along with the carrier, while the process of filming is carried out, which allows you to capture the whole phenomenon on camera. You can often find such a contrail from the Buk missile.

It is often carried out at relatively low speeds in order to better capture the entire process. In this case, engine surge is often formed, since hot gases enter the rocket engine in jets, which disables its air intake. Flame ejection is immediately noted, which is typical when a surge occurs. This is how the FSX contrail is expressed.

This incident causes the engine to stop. These features after the study helped to create a number of different systems, the tasks of which include timely diagnosis of surge, taking measures to eliminate it, as well as transferring the engine to the optimal operating mode with constant maintenance of its optimal state. In this case, missile armament expands the scope, while in each mode of engine operation, these aircraft are able to show the most stable state.

in the air

The MiG-29 aircraft was tested, which consisted of refueling. During one of the flights, the release of fuel liquid into the atmosphere was recorded, which was preceded by depressurization of the fuel pipeline. With the help of an aircraft-photographer, this unusual situation was recorded. At the same time, a certain part of the fuel got into the engine, which almost immediately led to its stop due to surge.

In addition to the ejection of the flame, which always happens when the engine surges, there was an ignition of the fuel that went through the air channel. After that, the flame engulfed all the fuel and went beyond the internal structure, but was almost instantly demolished by the oncoming air flow. Because of this situation, an unusual phenomenon appeared, which was called a fireball. This contrail "Buk" is also capable of transmitting.

Bright trace of afterburner

Modern fighter aircraft have an engine that is equipped with adjustable nozzles, classified as supersonic. When the afterburner mode is activated, the pressure at the nozzle exit is much higher than that of the surrounding air masses. If you analyze the space at a considerable distance from the nozzle, the pressure gradually equalizes. This aspect during the movement of the aircraft leads to increased production of gas, which leads to the formation of a bright contrail from the aircraft, which appears when the aircraft is in motion.