Plain English Meaning
A low-pressure weather system is filling when its central pressure is rising and the system is getting weaker. The opposite of a low deepening.
Full Definition
In meteorology, the increase in atmospheric pressure at the center of a low-pressure system over time. A filling low is weakening, as its central pressure rises toward that of the surrounding air, reducing the pressure gradient that drives its winds and weather.
Context Anchor
Seen in aircraft fabric covering, fabric inspection, and repair instructions when the direction of the cloth matters.
Derivation
From the everyday sense of 'filling' — adding something to a container until it is no longer empty. A low-pressure area is, in effect, a 'dip' in atmospheric pressure; as air pressure rises back toward normal, the dip is being filled in.
Why Pilots Care
Signals improving conditions that may allow a flight to proceed or reduce the need for an alternate airport.
Intuition Check
Filling does not mean adding fuel or putting material into a space here. In fabric work, it means the threads that run crosswise across the cloth.
Example Sentence 1
The forecaster noted that the low over the Midwest was filling, so we expected the headwinds to ease by afternoon.
Example Sentence 2
With the low filling quickly, the pilot elected to depart as planned rather than delay for better weather.