A noctilucent cloud crosses the sky of northern China on Friday, April 27, 2018. [Screenshot: Sina Weibo/jwb]CAS explained that the cloud was actually a type of condensation trail. The line-shaped clouds are produced by aircraft engine exhaust, or changes in air pressure.
CAS says this contrail had its unusual appearances because it formed in the upper atmosphere where the clouds are still reached by the day's sunlight, but the sun is below the horizon for observers. This characteristic is what sets "noctilucent" clouds apart.
Although the cloud formation was popular with observers, CAS warns that noctilucent clouds are regarded as a sign of global warming.