The image is of the morning sky over London one hot day in June. Why do the contrails end so evenly and abruptly?
David Muir
Edinburgh, UK
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Condensation trails, or contrails, are formed when the water vapour present in the exhaust gases of aircraft undergoes freezing or condensation.
In the atmosphere, regions of air can differ in temperature, humidity and density, and this is in a constant state of flux. Warm, lighter air can ride up over cold, heavier air, and cold, heavier air can drive underneath warmer, less dense air. This is very clear during storms and hurricanes.
When planes are flying through relatively moist air, contrails last for a longer time. When they fly through dry air, contrails remain for a shorter time because they rapidly evaporate.
The contrail picture in question captures an occasion where planes have been flying through invisible boundaries that sit between moist and dry air. This leaves contrails that end abruptly and look like giant dashes in the sky.
Chris Daniel
Colwyn Bay, Conwy, UK
A contrail is produced when a jet engine鈥檚 exhaust gas 鈥� mainly the water vapour 鈥� freezes. This creates a linear cloud.
While contrails are usually continuous, they can be intermittent (as is the case in the picture) if the air temperature and humidity varies across their lengths.
This appears to be a single, discontinuous contrail, spread in the direction away from the camera that captured it. I can tell this has occurred because the upwind edge of the contrail is sharp and well-defined, while the opposite edge is smeared.
A less likely explanation for this patterning is that the picture contains contrails that were formed by several aircraft travelling in formation towards or away from the camera.
Natalie Roberts
Watford, Hertfordshire, UK
Contrail is an abbreviation of 鈥渃ondensation trail鈥�, which gives a clue as to how these human-made lines in the sky form. When a jet engine is running, it produces three common emission products: water vapour, carbon dioxide and a soot made of incompletely burned hydrocarbons. The water vapour from the jet engine, along with the water vapour already present in the atmosphere, can cluster around the soot particles, which act as condensation nuclei. Essentially, soot particles become catalysts for droplets of water or ice crystals to form rapidly.
The regular, parallel nature of the contrails in the picture can be explained by the fact that commercial airplanes all use the same flight paths, especially when travelling in and out of airports. They also land and depart at regular intervals. The contrail lines in question aren鈥檛 actually sitting within a flight path, but instead have all been blown away by a steady wind.
It is just like dropping ping-pong balls into a stream at regular intervals, all from one place. You will get a line of balls floating away, not a pile in front of you.
The sudden disappearance of the contrails can be explained by another feature of airplanes: they travel high in the sky, but ascend from and descend to airports. As the atmosphere鈥檚 make-up isn鈥檛 homogeneous, planes will cross layers of air that have different temperatures, humidities, wind speeds and wind directions.
These contrails appear to have formed in a windy layer of humid air, before the rising or descending planes entered a layer of much dryer air. This caused the lines in the sky to end abruptly.
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