Weather map lines (7)
I believe the answer is:
isobars
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">weather map lines</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)