Definition
A weather front located aloft rather than at the surface, occurring where two air masses of different temperature and humidity meet above the ground. The boundary between the air masses does not extend down to the surface, so its effects are felt only at altitude.
Plain English
A boundary between two different air masses that exists up in the atmosphere but does not reach the ground.
Context Anchor
Seen in weather discussions and forecasts when clouds, precipitation, icing, or rough air are being explained by a boundary above the surface.
Derivation
Upper' indicates the front is positioned aloft rather than at the surface, distinguishing it from the typical surface fronts shown on standard weather maps.
Why Pilots Care
It can produce turbulence, clouds, or icing at altitude even when surface weather appears benign.
Grounding Statement
Picture two different air masses meeting in a layer above you while the ground below does not show a clear surface boundary.
Intuition Check
“Front” does not mean the front of the airplane or just a line on a surface map. Here it means a weather boundary between air masses, and “upper” means that boundary is above the ground.
Example Sentence 1
The forecaster warned of an upper front near 18,000 feet that could produce moderate turbulence along the route.
Example Sentence 2
Although the surface map showed no fronts, the upper front produced a layer of altocumulus clouds at flight level.