photo #8376550

When the atmosphere at cruising altitude is cold enough and moist enough, the water vapour mixed in with the other gasses of an aircraft’s exhaust can form into droplets or ice crystals that are visible from the ground as condensation trails, or contrails. Whether these lines of cloud persist – or even appear in the first place – is highly dependent on the moisture conditions in the atmosphere up at cruising altitude. And this varies greatly from day to day and from place to place. This is why we sometimes see contrails and sometimes don’t.If the conditions up there are dry, any droplets that form in the exhaust will evaporate away again almost immediately. But in other circumstances, like ahead of an approaching weather front, the upper air has much more moisture in it. Here, contrails form easily and persist as long, straight clouds as their droplets freeze into ice crystals. Their ice crystals grow in size as the surrounding moisture freezes onto them, splintering and acting as seeds onto which yet more moisture freezes. In such conditions, contrails can linger for hours and spread into broad bands in the high winds. We refer to such ice-crystal clouds resulting from human activity as Cirrus homogenitus. Persistent and spreading contrails like this are the first sign of an approaching weather front. In a matter of hours, they’ll have been subsumed into the high layer of ice crystals called Cirrostratus that marks the front’s arrival.Contrails spotted near Manchester, England by Jonathan Cottam (Cloud Appreciation Society Member 43,897). He says that he and his wife Susie had a game of ‘atmospheric noughts and crosses’.