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jeffyen

Why don't clouds break apart when planes fly through?

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This is something that has always puzzled me. When we look at a plane fly into say a cumulus, intuitively, I'd expect the clouds to totally splatter, change shape, move up/down/sideways etc. into an unrecognisable shape. But they don't. They just stay still as if nothing happened. What's the explanation for this from a physics POV? :)

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I would think it's due to hte aerodynamics of a modern airplane, as well as Sir Isaac Newton. :) Take, for example, a boat. When riding through the water, a boat pushes aside water. When the boat passes, the water fills in where it used to be. In a boat's case, there is a visable wake of disturbed water.Now, twist this to airplanes... An aircraft pushes aside air as it flies, and after it passes, the air fills in where it used to be. An airplane does leave a wake in the form of wingtip vortices and other turbulance, but due to the "invisible" nature of air, we can't see it.For this discussion, a cloud should be considered no different from regular air. When an airplane penetrates a cloud, the air moves aside and then fills back in once the plane passes. There is some small disturbance of the cloud shape, but usually no where near enough to be visible from the ground. Remember that when you are riding in an airplane and you penetrate a cloud, you don't really bounce too much. Maybe a little jostle, but nothing huge. This can be taken as a clue that the consistency of the cloud isn't that much different than the consistancy of the clear air.Physics will tell us that due to the small difference in mass between the cloud and clear air, the cloud will have only a very small reaction to an airplane penetrating through it. This, along with the other ideas above, should help explain why an airplane doesn't blow clouds apart. Now - as far as the "wake" of an airplane is concerned - that's easy to see in the right conditions. I work under a departure path for my local airport, and have planes overhead as I walk into work at around 2,000-3,000 feet. One morning last week we had a low layer of clouds and fog, and airplanes, when transitioning this layer left behind two twisting vortices behind them in the cloud. It was quite cool. After a few seconds, the distinct trail would blend back in with the rest of the cloud. Perhaps there are others with REAL science to explain it!-Greg

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You've got it right.One additional thing is that most clouds are reforming on the spot, so even if the plane does move a bit of cloud, more will form behind it. The cloud may actually grow backwards along the aircraft track as the water vapour in the exhaust condenses.I was once on short final in a Cherokee when a 777 took off on a parallel and much longer runway (awesome sight from that angle, let me tell you!). As the 777 neared rotation, I could see it's wake rippling the grass beside the runway. When it rotated, right next to my threshold, I could see the ripples get much, much larger and further away from the plane, and they grew again when it lifted off. I could see ripples in the grass of the runway I was on approach for, which is pretty remarkable since the grass there is only 1/2in long. Needless to say, I was already planning on going around when the tower told me to.Another day I was holding and doing my checklist about 50m or so from the runway while the same scheduled flight took off. The wake ran right over the Cherokee, and even sitting on the ground it bounced around quite dramatically. I could see it coming over the grass, and this was well before rotation. I suspect if I'd been at the rotation point it would have been enough to damage my plane, even just sitting there.Never, ever get close to a large aircraft while flying a small one. Those wakes are hugely powerful.

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