Long white lines have recently appeared in the skies over northern Portugal. Some stay sharply defined, while others spread out like clouds. These marks are known as contrails, short for condensation trails, and differences in atmospheric conditions determine whether they form and how long they remain visible.
Commercial aircraft typically fly between 9 and 12 kilometres above the ground, where temperatures can fall to between 40 and 60 degrees below zero. When hot engine exhaust mixes with this cold air, water vapour can condense around small particles in the exhaust. The resulting droplets freeze quickly into tiny ice crystals, which appear from the ground as white trails. They are not smoke.
Not every aircraft leaves a visible trail, even at a similar altitude. In dry air, the ice crystals quickly turn directly into water vapour, causing a contrail to disappear within seconds. When the air is sufficiently humid relative to ice, the crystals can persist and grow, keeping the trail visible for hours. High-altitude winds may spread them over a wider area, forming cloud-like layers similar to cirrus clouds.
Persistent contrails do not necessarily mean that rain is coming. They show that conditions in the upper atmosphere allow ice crystals to remain, but humidity at around 10 kilometres can differ from conditions at the surface. Contrails can offer clues about the atmosphere, but they cannot, on their own, predict the weather.
Contrails can also affect the climate, particularly when they persist and develop into extensive clouds. They reflect some incoming sunlight during the day, which has a cooling effect, but also trap some of the infrared radiation emitted by Earth, contributing to warming. The European Union Aviation Safety Agency says the overall climate effect of persistent contrails is warming, though its strength depends on several factors. Changes to some flight routes and altitudes are being studied to avoid atmospheric areas especially favourable to their formation.
Claims that ordinary contrails are deliberate chemical sprays, sometimes called “chemtrails,” are not supported by credible scientific evidence. Their formation and persistence are explained by atmospheric physics. Aircraft engines do produce pollution, but that is distinct from the ice crystals that make contrails visible.
Sources: Pplware.