Weather map lines (7)
I believe the answer is:
isobars
'weather map lines' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for isobars that I've seen before include "can show what pressure folk are under" , "Lines on map indicating atmospheric pressure" , "Lines of equal atmospheric pressure" , "Map lines of equal pressure" , "Lines linking places of equal barometric pressure" .)