'These vapour trails create clouds which, experts claim, can block out sunlight for millions. This is the reason that our skies appeared unusually blue when the Icelandic volcano Eyjafjallajokull was erupting, and all flights over Britain were banned.The phenomenon occurs when aircraft fly above 25,000ft, where the air temperature is around minus 30C. This causes water vapour emitted by the engines to crystallise and form the familiar white streaks across the sky, known as contrails.
These can be short-lived. But if there is already a significant amount of moisture in the atmosphere they can linger for hours, as the excess water vapour from the engines tips the surrounding air past its saturation point. This acts as a catalyst to speed up the natural process of cloud formation. Cirrus clouds – the wisp-like formations seen at high altitude –begin to form around the contrails.'Read more: