On December 24, 2012, a strange cloud-like double helix discovered just outside of Moscow, Russia.
A number of amateur photos were posted on a Russian website, the netall.ru, that morning.
Cloud-like helix over Moscow
Although not completely known what caused such a phenomenon, there is a fairly extensive research in progress which claims to be some kind of contrail. The contrails or "vapor trails" are very thin artificial clouds formed sometimes behind the aircraft.
Their formation is caused by the water vapor in the exhaust of aircraft engines, but can also be caused by changes in air pressure at the surface of the flap. Depending on the temperature and humidity of the altitude contrail, may be visible for a few seconds, minutes or several hours.