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Plenty of answers answer the question that it happens but I can't find a good answer as to why the wind shifts during the passing of a front. For an extreme example see: a graph of an abrubt wind shift

(in Dutch, x is hours and y is wind direction in degrees)
Where a passing cold front moved the wind directon from south to northwest.

The other question provides a very broad answer as to why wind changes in general and the answers there are applicable in general. My question is about the specific circumstances of a thermal front and what causes the change in the balance of forces to suddenly allow or force the direction of the wind to change direction.

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    $\begingroup$ @gansub sadly no, that answer is about the regular wind inside a system without fronts passing. $\endgroup$
    – Borgh
    Dec 15 '20 at 14:55
  • $\begingroup$ a front is not some extraordinary phenomenon. It has to obey the NS or primitive equations with some balance condition rolled in. $\endgroup$
    – gansub
    Dec 15 '20 at 17:01
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    $\begingroup$ So yeah, what is that balance condition? It seems that a front either provides or releases some form of friction to, quite suddenly, change the balance of forces that makes the wind blow in a certain direction. Is it just the boundary between two air masses that provides this resistance? $\endgroup$
    – Borgh
    Dec 15 '20 at 17:41
  • $\begingroup$ en.wikipedia.org/wiki/Frontogenesis and glossary.ametsoc.org/wiki/Semigeostrophic_theory $\endgroup$
    – gansub
    Dec 16 '20 at 2:24
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    $\begingroup$ Mate if a wikipedia article answered my question I wouldn't be here. You seem to know the answer, please walk me through it. $\endgroup$
    – Borgh
    Dec 16 '20 at 11:45
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An answer I got on Reddit which seems to make sense is that fronts are usually accompanied by a pressure change which locally reinforces the "normal" geostrophic wind which changes the wind direction.

If anyone has a more in depth answer I'd love to hear it.

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