In this blog post, we’ll explore the formation mechanisms of the contrails frequently observed at Seoul National University’s Gwanak Campus and examine the physical principles behind the various types of clouds that appear around airplanes.
How Are Airplane Clouds Formed?
These days, if you look up at the sky over Seoul National University’s Gwanak Campus, you’ll often see unusual clouds stretching out in long, straight lines. Unlike ordinary naturally formed clouds, these clouds extend in a very straight line; this is because they are mostly contrails that form along the flight path of an aircraft. Of course, there are cases of naturally formed long clouds, such as the “Morning Glory Clouds” observed in Australia, but the exact causes of their formation have not yet been fully elucidated. On the other hand, most of the straight-line clouds frequently seen at Seoul National University’s Gwanak Campus are directly related to aircraft operations.
The Basic Principles of Contrails
Contrails are generally referred to as “contrails” or “condensation trails.” They are formed when high-temperature exhaust gases emitted from aircraft engines come into contact with the extremely cold air at high altitudes and cool rapidly. Aircraft fuel consists primarily of hydrocarbon compounds, and the combustion process produces large amounts of water vapor along with carbon dioxide. The exhaust from the engines contains not only this water vapor but also fine particles, resulting in a higher concentration of water vapor than in the surrounding air.
At high altitudes, where temperatures are extremely low, the hot exhaust gases from the engine cool rapidly as they mix with the surrounding air. When the air can no longer hold the water vapor during this process, the vapor condenses, forming a cloud composed of tiny water droplets or ice crystals. Additionally, the fine particles generated during combustion act as condensation nuclei, facilitating the formation of water droplets and ice crystals.
Two conditions are particularly important for the formation of contrails. First, the exhaust gases must cool very rapidly; second, the atmosphere must be relatively stable. If rapid cooling does not occur, the exhaust gases mix quickly with the surrounding air, lowering the water vapor concentration per unit volume and making condensation difficult. Furthermore, if atmospheric turbulence is severe, the air mixes rapidly, preventing the conditions for condensation from being maintained, so contrails do not form easily.
For these reasons, the frequency of contrail sightings varies depending on the season and flight altitude. Even in summer, contrails can form sufficiently at very high altitudes, such as cruising altitudes, but at relatively low altitudes, temperatures are often not low enough for contrails to form easily. Since a significant number of aircraft passing over Seoul National University’s Gwanak Campus are passenger planes flying at relatively low altitudes while approaching or taking off from Gimpo International Airport, it is often difficult to observe contrails from these aircraft during the summer; conversely, contrails are more easily visible from aircraft flying at higher altitudes.
Other Types of Clouds Caused by the Upper Surface of an Airplane, Supersonic Flight, and Wing Tip Vortices
Clouds forming around airplanes are not caused solely by engine exhaust. Various types of clouds can all be explained by a common physical principle: the cooling effect resulting from the rapid expansion of air. In fluid dynamics, the phenomenon where pressure decreases as the speed of a fluid increases is generally explained by Bernoulli’s principle. When air expands rapidly, its temperature drops due to adiabatic expansion, and during this process, water vapor in the air condenses to form clouds.
The first type is the cloud that forms on the upper surface of an airplane’s wing. As the air flowing over the wing moves faster than the air beneath it, the pressure on the upper surface decreases, resulting in lift.
In particular, as the angle of attack increases, the airspeed over the wing increases further, and the pressure and temperature drop even more. Under these conditions, water vapor in the air can condense, forming white clouds on the upper surface of the wing. This phenomenon is more pronounced at high flight speeds or in environments with high atmospheric humidity.
The second type consists of clouds that form as an aircraft approaches or exceeds the speed of sound. Sound is a wave in which the compression and expansion of air are continuously transmitted. When an aircraft approaches or exceeds the speed of sound, pressure waves overlap to form a powerful shock wave, and in certain areas, the air expands rapidly and cools instantaneously. At this moment, water vapor can condense, forming temporary white clouds. The cone-shaped or band-shaped clouds commonly seen during supersonic breaks are caused by these pressure changes and the condensation phenomenon. For reference, the speed of sound varies depending on temperature; near sea level, it is approximately 340 meters per second.
The third type consists of clouds formed by wingtip vortices generated at the tips of the wings. Strong vortices form at both ends of the wings, and the pressure at the center of these vortices is lower than that of the surrounding area. During this process, the air at the center expands and cools; if sufficient humidity is present, water vapor condenses, forming a spiral-shaped cloud. This wingtip vortex condensation is particularly common in aircraft such as bombers or fighter jets that generate significant lift or perform sharp maneuvers.
The white clouds visible on the upper surface of fighter jets or high-speed aircraft in photographs, the white clouds that appear momentarily near the speed of sound, and the spiral clouds trailing from the wingtips like a tail are all representative examples formed by the three mechanisms described above.
Conclusion
In summary, most of the straight-line clouds commonly seen at Seoul National University’s Gwanak Campus are contrails formed when water vapor emitted from aircraft engines is rapidly cooled and condenses in the cold upper atmosphere. On the other hand, condensation clouds appearing on the surface of an aircraft, condensation occurring near the speed of sound, and condensation caused by wing tip vortices are all based on the same physical principle: water vapor condenses due to rapid changes in air pressure and temperature.
When specific flight paths repeatedly pass over a city, contrails can be observed continuously along those routes; however, the frequency, duration, and shape of contrails vary significantly depending on the season, flight altitude, atmospheric humidity, and atmospheric stability. Therefore, if you spot long, white clouds stretching across the sky, considering both the day’s weather conditions and the operating environment of the aircraft passing overhead will help you gain a more fascinating understanding of this natural phenomenon.
