Depending on the atmospheric conditions, white trails of condensed water vapor can remain visible in the sky for hours.
The U.K. Meteorological Office explains that a contrail is formed when water vapor released from airplane engines is condensed into ice crystals. In addition, the U.K. Department for Transport clarifies that "humid air" can cause ice crystals to remain in the sky for longer, "leaving a wider and longer lasting contrail that may be indistinguishable from naturally occurring cirrus clouds."
There are many renditions of the widely debunked chemtrails conspiracy theory. The BBC lists some of the most popular conspiratorial motives behind airplanes allegedly releasing toxic chemicals into the atmosphere. Some believe it is part of a large-scale government program to control the population, while others maintain it is a way to control the weather.
A group of scientists and experts in atmospheric science published a report in the Journal of Environmental Research Letters. 76 of the 77 scientists concluded that "they had not encountered evidence" of the chemtrails conspiracy theory. They further stated that "well-understood physics and chemistry associated with aircraft controls and atmospheric aerosols" could explain the contrails phenomenon.