The rules of the harp is that if a harpist plays at a wedding that’s outside, she (or he) typically shows up about an hour early not to just set up but so the instrument adjust to the weather and won’t go out of tune while the bridal party is marching. The harp has to be in shade because you risk more damage with it being in the sun. Harpists put that in the contract with writing it out to the bride, that’s how important it is.
So I would try playing the violin in the shade too. Surely you don’t have to wait an hour for the violin to adjust to the weather but it might be best to give it some time and playing at the temperature of 60’s and 70’s is best and if course the less humidity the better. Those are very strong rules for the harp when it comes to taking it outside and since the harp and violin are both made of wood I’m sure the rules apply and some way.