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Q:

Where does that cloud come from when planes break the sound barrier?

A:

The cloud that you sometimes see when a plane goes faster than the speed of sound is actually due to the dramatic decrease in pressure that the air feels as it passes the shock wave.

 

When a plane reaches the speed of sound, it creates a shock wave. All the air directly in front of this shock wave is at a very high pressure, but as soon as it passes the shock wave, the pressure decreases dramatically and instantly. This dramatic decrease in pressure causes the moisture in the air to condense and form a "cloud". This cloud occurs in what is known as the Prandtl-Glauret singularity.

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