Contrails, short for condensation trails, are the white streaks often seen in the sky behind aircraft. The international cloud atlas, which classifies clouds, has a category just for them: cirrus homogenitus, an example of man-made clouds.

Contrails contribute to climate change, adding to the warming by the carbon dioxide emitted by aviation. Although the exact amount of warming caused by these wispy-looking clouds is uncertain, what is understood now suggests that reducing the number of contrails has the potential to reduce the climate impact of flights.

Contrails are made of ice crystals. These reflect sunlight, causing the Earth’s surface to receive less energy, but at the same time trapping some of Earth’s outgoing infrared radiation. Depending on the balance between those two opposite effects, a net loss of energy or a gain of energy, individual contrails can be either warming or cooling over their lifetime, but warming dominates once averaged over the global, annual contrail population.

Contrails form behind aircraft at around an altitude of 10-11km. They only form in........

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