Definition
The airspace between the transition altitude (where pilots climbing set their altimeters from the local QNH to the standard pressure setting of 1013.2 hPa / 29.92 in Hg) and the transition level (the lowest usable flight level above the transition altitude). Within this layer, climbing aircraft reference flight levels and descending aircraft reference altitudes based on the local altimeter setting.
Plain English
It is the slice of sky between the height where you switch from the local altimeter setting to the standard one when going up, and the lowest flight level you can use when coming back down. It exists so climbing and descending aircraft can change altimeter settings cleanly without colliding in altitude reference.
Context Anchor
Seen in instrument procedure and altimeter-setting discussions, especially when operating in areas that use QNH, the local pressure setting that makes the altimeter read field elevation when the aircraft is on the ground.
Derivation
‘Transition’ comes from the Latin transire, meaning ‘to go across.’ The layer is literally the band of airspace you cross while transitioning from one altimeter reference (local QNH) to another (standard pressure), or vice versa.
Why Pilots Care
Maintains proper vertical separation between aircraft and prevents altitude errors caused by pressure differences in instrument flight.
Analogy
Think of it like a short changeover zone between two road systems. One side uses one set of height rules, the other side uses another, and the transition layer gives aircraft room to change from one to the other safely.
Intuition Check
Do not read “transition layer” as just any layer of air where conditions are changing. In this context, it specifically means the altitude band used for changing altimeter settings between local pressure and standard pressure.
Example Sentence 1
Climbing through the transition layer, the pilot reset the altimeter from the local QNH to 1013 hPa and began reporting flight levels.
Example Sentence 2
ATC issued the transition level so the aircraft could descend through the transition layer using the correct local altimeter setting.