An almost perfect circular hole sometimes appears in a cloud layer, or a long straight channel. A cloud does not tear and does not get cut. So what is a hole doing there?
An aircraft crossed it. Water droplets in a cloud can stay liquid well below freezing, and air flowing around a wing expands and cools fast, so some of them freeze. The ice crystals then grow at the expense of the droplets around them and fall out as snow. What is left is a round opening, or a long channel along the track, which keeps growing after the aircraft has gone.
An almost perfect circular hole in a cloud layer looks like an oddity, and it comes from an aircraft crossing the cloud.
Water droplets in a cloud can stay liquid well below freezing, as , because freezing needs a nucleus and a clean cloud has too few. It is an unstable state waiting for a disturbance.
does the rest. As air flows around a wing or a propeller tip it expands and cools fast, exactly like gas escaping a pressurized balloon, and some of the droplets freeze.
Then a self-feeding process begins: water vapor sticks to ice more readily than to water, so comes at the expense of the droplets, and the crystals fall out as snow or fine rain. The opening left behind can widen for an hour and more after the aircraft has gone. settled it: three quarters of those examined matched a particular aircraft. The process needs suitable conditions, so most flights make no hole, and the effect is local rather than global.
wat. A wing crossing an unusually cold cloud can freeze droplets, drop snow crystals out, and leave a hole behind it that keeps widening.
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An aircraft can seed a cloud without meaning to and cause local precipitation, although the overall effect on climate is tiny.
How do we know?
The researchers combined satellite imagery, radar, flight tracks and models. They tied three quarters of the holes examined to particular aircraft and followed how they spread.
What is still unknown
The process needs a cloud layer with supercooled droplets and suitable conditions, so most flights make no hole. The study found a local effect and not a significant global change.
Learn something?
Where to next
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