Line of equal barometric pressure (6)

I believe the answer is:

isobar

(Other definitions for isobar that I've seen before include "Line on map indicating barometric pressure" , "Line of equal barometric pressure on a map" , "meteorologist's line" , "Line showing points having equal barometric pressure at a given time" , "sign of pressure" .)

I've seen this clue in the Sydney Morning Herald.

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