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While I think temperature is certainly important, Humidity is more important pending extremes. 30 C with low humidity probably isn't nearly as harmful as 30 C with high humidity. 45-50% humidity is ideal for violins. I would rather see even 60% instead of the other way round' and have 30% humidity.
If I had 30 C I would likely cool to at least 76F or 24 C and try to maintain 50% humidity or as close to that number as possible. You could get away with a higher temp than that, but I personally don't like to get above that range for any stored wood valuables and stay at 45-50% relative humidity.
Same thing applies to lower temps. going much below 7 C might do things to the glue joints, though an instrument will take a low temp better than higher temps. Violins left in attics for years generally need a lot of work.
In my work I control temperatures for lots of large educational spaces including music buildings that house expensive instruments. While this changes slightly from winter to summer, a daytime occupied temperature would be in the range of 68 F cool to 74 F heat. The occupant has a + or - 2 F ability to change their own temp. During evening hours those spaces set back to 55-78 give or take and then come back online staggered to save energy. In spaces housing violins, cellos, Steinway pianos etc. we stay at the occ setting 24/7. We use mechanical humidification in those spaces. Things get lower than I would like at times concerning humidity.
Depending on where in the world you live you might need to either add or reduce humidity at optimum temps. If the violin is too dry it can crack, too wet and things can warp violin. Interestingly, an ideal temp/ humidity for a violin is close to what's ideal for us. We like somewhere around 68-72 F(20-22 C) and 50%RH.....I should say MOST of us prefer that.
In my own home, I spot humidify and try to keep my temp in the 70F 21 C range. Good for me, good for violins.
We people in Britain have to be careful about threads like this, as there are climatic conditions we can't even imagine. I'd guess that it's hot and humid you have to avoid, as hide glue can unstick in those conditions - I have heard of a ukulele being left in a car and literally falling to pieces. Maybe that is what has happened to @Fiddlerman's violin in Florida?
Cold and dry can make some woods split, I think - Leon Goossens', supposedly seasoned, oboe split on a tour of Iceland one winter, I seem to recall from his autobiography. We have Norwegians and ex-Swedes (Pierre again, lol) here who can comment on that! One of our uke forum members is a Scot who lives in Helsinki, but he has been quiet for a year or two. I'd guess it's the thicker wood that doesn't like the cold, so the neck might be a worry.