There's a particular school of thought among internet-based conspiracy theorists, which says that long, thin strips of cloud in the air are actually 'chem-trails; the residues of chemicals that Governments spray on their people in an attempt to somehow control them. In reality, they're just unusual cloud formations, sometimes caused by the effect of planes passing through cloud at high speed.
These particular trails of cloud are an all-natural creation though. Nicknamed 'morning glory cloud', they appear regularly in Northern Australia, where a combination of temperature and wind turns clouds into rolls, taking on the appearance of long cigars. They're such a common sight in this part of the world that they've become a tourist attraction; especially during Fall months, where they occur most regularly. Some companies have even started up, offering visitors to go up into the air on a glider and sail through them!