Definition
A flow of air in which density changes significantly with pressure (compressible) and no heat is exchanged with the surroundings (adiabatic). At higher airspeeds, especially approaching and above roughly Mach 0.3, air can no longer be treated as having constant density, and any temperature changes within the flow come from the air being compressed or expanded rather than from heat flowing in or out.
Plain English
Air moving fast enough that it squeezes and stretches noticeably as it flows, and any heating or cooling inside that flow comes from the squeezing itself, not from outside heat. This is the model used to describe how air behaves around a fast-moving aircraft.
Context Anchor
Seen in performance-speed and high-speed airflow discussions, especially when the handbook explains pressure changes and airspeed behavior at higher speeds.
Derivation
Adiabatic comes from the Greek 'adiabatos,' meaning 'not to be passed through' — referring to heat not passing through the boundary of the air parcel. Compressible simply means 'able to be squeezed.' Together they describe air that changes density as it flows, with no heat crossing in or out.
Why Pilots Care
Correct understanding prevents errors in airspeed, lift, and drag calculations once airflow reaches speeds where density is no longer constant.
Grounding Statement
Picture air rushing over a fast jet's wing: it gets squeezed in some places and stretched in others, and that squeezing alone is what warms or cools it — no outside heat is added or removed during the brief moment it flows past.
Intuition Check
Adiabatic does not mean the temperature stays the same; it means heat is not being added or removed. Compressible does not just mean air can fit in a smaller space; here it means the change in how tightly packed the air is matters to the airflow.
Example Sentence 1
Above about Mach 0.3, engineers model the airflow around the aircraft as adiabatic compressible flow rather than treating the air as incompressible.
Example Sentence 2
Designers use adiabatic compressible flow equations to predict drag rise near Mach 0.8.