Question: A reader has noticed condensation trails have regular "spurts". What causes these?
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If you look closely at the vapour trail, you'll notice they often have a sort rhythmic pattern. Something is causing regular spacing in the trail.
![In the right atmospheric conditions, the water cools, forming tiny ice crystals which trigger the formation of cloud. In the right atmospheric conditions, the water cools, forming tiny ice crystals which trigger the formation of cloud.](/images/transform/v1/crop/frm/silverstone-ct-migration/6bad8e54-5761-433d-b3c4-904303a06926/r0_0_1000_667_w1200_h678_fmax.jpg)
If it was a steam train, you could imagine each puff was generated by a coal engine, but obviously planes don't burn coal. Besides, steam engines turn at a leisurely a few hundred rpm, while jet engines spins at around 10,000rpm, way too fast to produce the spread-out shape we see.
That means the shape of the tail is probably not a direct result of the engine - although the vapour trail itself is mostly caused by engine. The engine produces the trail, but not the puffy shape.
It's counter-intuitive, but burning jet fuel produces water. Numbers vary accordingly, but a Boeing 767 typically produces about 6 tonnes of water per hour. That translates to 6 kilograms of water per kilometre of flight.
That might not seem like much, but you don't need much water to create a lot of cloud. When you add water condensing out of the air, you have more than enough to make a vapour trail.
To be more correct, we should, use term ''contrail''. In the right atmospheric conditions the water cools, forming tiny ice crystals which trigger the formation of cloud. We would never say, as you sometimes hear, ''chemtrail'', because what you're seeing is just water.
The other source of contrails are flight surfaces such as wing tips. This is mostly on humid days where the plane generates a vortex that causes a drop in temperature and pressure. Water already in the air suddenly drops below the condensation point to form cloud.
It's hard to be certain about why contrails have a regular puffy shape, but the most likely explanation is that it relates to aerodynamics. If you observe a plane in a wind tunnel - or a fish swimming in a river, you'll see they generate eddies opposite sides of the body. These travel down the side, and detach in an alternating pattern, left - then right - then left.
If you could follow these you'd see a regular trail behind the plane as it flies. Each cloud puff probably aligns with one of those.
Response by: Rod Taylor, with thanks to AutoCRC research manager Dr Gary White
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