(OED)

to break

of weather: to change suddenly, esp. after a long settled period.

Cf. sense to break up 8 at Phrasal verbs.

 --> 8. [ < 8b.] Of frost, (formerly) of an epidemic: To give way, cease. Of any kind of weather: to change.

to break

(of the weather) change suddenly, especially after a fine spell.

"the weather broke and thunder rumbled through a leaden sky"

 : to become fair (see fair entry 1 sense 3) : clear

when the weather breaks

https://www.merriam-webster.com/dictionary/break

a break in the weather

noun phrase

 : a change from unpleasant to more pleasant weather

It rained all day. We waited for a break in the weather, but it never came.

https://www.merriam-webster.com/dictionary/a%...

Perhaps it can mean both those things? Perhaps the meaning is changing/has split in two due to people re-interpreting the word "break"?

I can imagine using it to mean that the weather has broken, i.e. a long period of good or bad weather has come to an end.

Witterungsumschlag (= Wetterumschwung - LEO: Übersetzung im Englisch ⇔ Deutsch Wörterbuch) mag häufig für eine Wetterverschlechterung gebraucht werden, es bezeichnet aber grundsätzlich sowohl einen Wechsel zum Guten als auch zum Schlechten.

I also noticed that there seems to potentially be a slight difference between AE and BE in that the above US sources mention a change to good weather, while the UK sources mention a change in weather "of any kind" and include an example of the weather breaking in the form of a storm beginning.

https://duckduckgo.com/?t=ffsb&q=Unwetter+%22...