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.
Plain English
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.
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.