Anyone who has looked up to watch an airplane fly has seen the white trail it leaves behind in the sky. Where do these elongated cloud lines appear on a clear day? Science has an answer to this question, even if conspiracy theories are absent.
The reason is the same as that which explains the haze produced in cold weather. Airplanes form a vapor trail because, after the kerosene combustion process, the gases expelled by the engine come out at a much higher temperature than outside. The strong temperature contrast at an altitude of more than 30,000 feet (-50 degrees Celsius in the atmosphere) causes the water contained in this mixture of substances to condense immediately.
Aircraft form a vapor trail because the gases expelled by the engine exit at a much higher temperature than the outside temperature.
Specifically, engine exhaust gases release carbon dioxide, oxides of sulfur and nitrogen, unburned fuel, metallic particles, and soot. It is this last element that provides the conditions for water vapor condensation, with tiny droplets condensing on its surface. Another factor contributing to this effect is the expansion of the gas as it leaves the aircraft, because inside the engine the molecules are more compressed.
A meteorological indicator
Depending on the aircraft's altitude, temperature, and humidity in the atmosphere, contrails can vary in thickness, extent, and duration. In fact, the nature and persistence of these trails can be used to predict atmospheric conditions, as Jenn Stroud Rossmann, professor of engineering, explains, in Scientific American. For example, a thin, short-lived trace indicates air with little moisture aloft, a sign of good weather, while a thick, long-lived trace is a sign of moist air aloft and can be an early indicator of thunderstorms.
A scientific paper estimated in 1998 that man-made cloud cover from aircraft contrails was 0.1% of the planet's surface, not including cirrus clouds that form from the more persistent contrails.
Recent research has also suggested that the ice particles in contrails cause the greenhouse effect and contribute to global warming by becoming part of the insulating layer of moisture and gases in the atmosphere. Scientists had the opportunity to test this hypothesis after the September 11, 2001 attacks in the United States, when a complete shutdown of commercial air traffic resulted in contrail-free skies, so their effects on the environment could be rigorously quantified.
A 1999 report by the Intergovernmental Panel on Climate Change (IPCC) confirmed a correlation between increased cirrus clouds and aircraft emissions. An increase in cirrus cloud cover would tend to increase global surface temperature. The same study estimates that engine emissions associated with air traffic account for 3.5% of the impact of all human activities on climate change.