The white visible effect that comes out of the aircraft engine is known as "vapor trail" or "contrail". This phenomenon occurs when the water vapor in the exhaust of the aircraft engine cools and condenses rapidly at high atmospheric temperatures. This condensed water vapor forms ice crystals and as a result, long white streaks form in the sky. Persistence of these vapor lines is from a few seconds to a few minutes and depends on two main factors: low temperature and high humidity in the upper atmosphere in which the aircraft flies. Also, the amount of fuel consumed and the type of fuel can also have an effect on the formation and size of these lines. Water vapor and carbon dioxide are the products of complete combustion of hydrocarbon fuel in the combustion chamber. Soot and all kinds of unburned hydrocarbons and particulate matters (PM) and nitrogen oxides are other pollutants that are emitted from gas turbine engines but are not necessarily visible.