Tim's horse affected by weather map lines (9)

I believe the answer is:

isotherms

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'weather map lines' is the definition.
'isotherms' can be an answer for 'lines' (isotherm is a kind of line). I am not certain of the 'weather map' bit.

'tim's horse affected' is the wordplay.
'affected' indicates anagramming the letters.
'tims'+'horse'='timshorse'
'timshorse' anagrammed gives 'ISOTHERMS'.

'by' acts as a link.

Can you help me to learn more?

(Another definition for isotherms that I've seen is " lines on maps".)

I've seen this clue in The Mirror.

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