Definition
A low-pressure area on a weather chart whose isobars (lines of equal pressure) form one or more complete, closed loops around the center. The pressure at the center is lower than the surrounding air, and the closed contours indicate that the low has detached from the main flow of pressure systems and is circulating on its own.
Plain English
A pocket of low air pressure that shows up on a weather map as fully enclosed rings, like a bullseye. The air around it is rotating around the center rather than passing through, so the system tends to sit in one place and bring persistent unsettled weather.
Context Anchor
Seen in weather briefings, surface analysis charts, and forecast discussions when identifying organized low-pressure systems that may affect a flight route.
Derivation
‘Closed’ here refers to the isobars or contour lines forming a complete loop on the chart, with no open ends. The term contrasts with an open trough, where the lines bend but do not close. Knowing this helps the pilot read a weather chart correctly: a closed low is a self-contained system, while an open low is part of a larger flow.
Why Pilots Care
Signals concentrated weather such as clouds, turbulence, or icing that may require rerouting.
Grounding Statement
On a weather map, picture a low-pressure center with pressure lines forming complete rings around it; that ringed pattern is what makes it a closed low.
Intuition Check
Closed does not mean an airport is closed, and low does not mean low altitude. Here, closed means the pressure lines form complete loops, and low means the air pressure is lower than the surrounding area.
Example Sentence 1
The briefer pointed to a closed low over the Midwest and warned that the low ceilings would likely persist for another two days.
Example Sentence 2
A closed low moving across the area brought widespread ceilings below our minimums.