Aircraft punch 50-kilometre-wide holes in clouds

Giant circular holes in clouds are caused by aircraft flying through. The planes create small holes a few tens of metres across, which can then expand to a width of tens of kilometres in a few hours.

Such holes have long been linked to aircraft, but until now no one could explain how they got so big. "It was a mystery," says Andy Heymsfield of the National Center for Atmospheric Research in Boulder, Colorado.

Heymsfield and his colleagues used a satellite called GOES to track 92 cloud holes over Texas over 4 hours in January 2007. The holes grew substantially within an hour, before slowly shrinking. The majority of the holes reached a diameter of between 10 and 50 kilometres.

It was clear how the holes got started. The clouds were supercooled, meaning their water was in liquid form despite being below 0 °C. Water can stay supercooled for a long time if left alone, but any disturbance turns it into ice. A plane flying through is more than enough to trigger freezing, at which point the resulting ice crystals fall away, leaving a hole. But that should only form a small hole.
Heat from ice

Heymsfield wondered if a side effect of making ice was causing the holes to grow. When liquid freezes it releases a little heat. This would cause the warmer air around the ice crystals to rise and the surrounding air to fall, starting a circulating current. As the falling air moved into a warmer zone its previously supercooled liquid water would evaporate.

The circulating air would carry this effect outwards, disrupting more of the cloud and triggering further evaporation. A cascade would be set off, causing the rapid expansion of the hole.

To test this idea, Heymsfield ran a detailed computer model of the internal workings of a cloud. He introduced a line of ice crystals such as that produced by an aircraft and watched as a hole grew to a diameter of 4.4 kilometres in 90 minutes.

Read more at New Scientist