They get closer together when it's windy (7)
I believe the answer is:
isobars
'they' is the definition.
Both the definition and answer are plural nouns.
Maybe they are linked in a way I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for isobars that I've seen before include "Lines on map indicating atmospheric pressure" , "Lines linking places of equal barometric pressure" , "can show what pressure folk are under" , "15 is strong when they are close" , "What may be featured on map" .)