What is the nature of the disc-like ‘barrier’ that this plane is piercing as it breaks the speed of sound? The caption mentions water vapour. If that’s correct, what is actually going on?
This is called a “Prandtl-Glauert” cloud. The basic idea is that near the speed of sound, the pressure/temperature differentials in the airflow around the aircraft get very large, so that condensation occurs. It’s a sort of amplified version of a lenticular cloud, to put it very simply. The nearer you are to the speed of sound, the more likely you are to get one of these.
It is not quite correct to say it occurs “as it breaks the speed of sound”; it does not represent a barrier between sub-sonic and super-sonic speed. However, it does occur at speeds near the speed of sounds, and is called the Prandtl-Glauert condensation effect. The cloud is a result of condesation that occurs when an area of lower pressure forms behind the shocks wave of compressed air that piles up in front of the jet. Although the picture looks like the jet is passing through the cloud, it actually moves with the jet, and skilled pilots can cause the cloud to move fore and aft by varying the speed of the aircraft. More information on this phenomenon may be found here, as well as a video of the effect in action.
Ooooh, here is a SPECTACULAR movie of a plane creating one. You can see it’s intermittent due to local conditions, and the plane carries it along with itself.
Thanks! Very helpful and very interesting!
ASWOME! thanks for the info, folks! I love learning cool science stuff!
Wow, that really was spectacular! Thanks for posting this link.
How common are lenticular clouds? I don’t recall ever having seen one.
I see them with some regularity, buth here in Cape Town, and further inland. It’s all to do with mountains and prevailing winds. Do you live near mountains?
If you hang around in the mountains, they are fairly common.
I live in Florida. We have a dearth of mountains. Dark and ominous thunderclouds we got aplenty.
Is it quite correct to say that the cloud is moving? I would suspect that it’s more that the local water vapor condenses and then re-evaporates as the plane passes, such that the “moving cloud” would not be a single cloud at all, but a wavelike succession of different clouds.
KarlGauss
Wow - interesting thread you started there !!
And congrats to all the Dopers that found the answers so fast.
Strictly speaking, you are of course, correct. However, that observation would be true of many cloud formations (specifically the lenticular clouds linked above). The cloud is condensation as moist air passes through a region of lower pressure, such as the venturi created when an air stream passes over a mountain. The air is constantly moving, but the cloud appears to stay still (relative to the mountian), as moisture condenses and evaporates continuously as it moves through the region of lower pressure above the mountain. The cloud around the jet gives the appearance of motion relative to the viewer, because the region of low pressure that creates it is associated with the jet.