Definition
A compression wave is a thin region of abruptly increased air pressure, density, and temperature that forms when an aircraft, or part of one, moves through the air fast enough that the air ahead cannot smoothly move out of the way. The air piles up and is compressed into a narrow front. When this front becomes steep enough that the change in pressure is nearly instantaneous across it, the compression wave is called a shock wave.
Plain English
A compression wave is a sudden squeeze of the air, where pressure, density, and temperature all jump up across a very thin layer because the aircraft is pushing through faster than the air can easily get out of the way.
Context Anchor
Seen in high-speed flight discussions, especially when explaining how shock waves form on wings and other aircraft surfaces near the speed of sound.
Derivation
From Latin 'comprimere', meaning 'to press together'. The name describes exactly what is happening: the air is being pressed together into a thin band as the aircraft pushes through it faster than the air can smoothly part.
Why Pilots Care
Recognizing compression waves explains the sudden aerodynamic loads and drag rise that occur as an aircraft nears the speed of sound.
Analogy
Think of a boat moving through still water. At slow speed, water flows smoothly around the bow. As the boat speeds up, water piles up in front and forms a sharper, steeper wave. Air does the same thing at high speed, but invisibly.
Grounding Statement
Near the speed of sound, the pressure changes ahead of the airplane cannot move away fast enough, so the air begins to bunch up.
Intuition Check
Do not think of a compression wave as an ocean wave or as engine compression. Here it means air being squeezed into a higher-pressure disturbance that can build into a shock wave.
Example Sentence 1
As the aircraft accelerated past its critical Mach number, a compression wave formed on the upper surface of the wing.
Example Sentence 2
In the handbook diagram, successive compression waves merged to create the visible shock wave ahead of the nose.