I teach harp lessons in person here in Houston as well as online with students in all parts of the country, and I frequently get lots of questions from newer harpists about whether it is normal for harps to go out of tune more in the summer, break more strings in the summer, etc. The same questions frequently arise in the winter, too.
I try to reassure everyone that our harps change with changing weather/environmental conditions just like we do. Wood and metal strings expand and contract with temperature changes. If the air is too dry, the wood dries out, just like our skin does.
There is an old adage among harpists that, if you are comfortable, your harp will be too. While this may be somewhat of an oversimplification, it is largely true. You wouldn’t enjoy sitting in a cold draft or in the direct blast from a heater or fireplace. Not for long anyway! You wouldn’t appreciate having to sit in moist, drizzly heat or cold nor in air that is parched and dry. Neither will your harp!
Your tuning pins might tend to slip as the wood gets colder and/or drier because the wood is shrinking and looses its tight grip on the tuning pins. Gut strings will be much more susceptible to changes in temperature and humidity than nylon strings, and you will notice that your metal strings may actually go sharp if they get cold because the metal contracts.
I think that the best rule of thumb is to avoid taking your harp from one extreme to another too quickly. If it is hot, park your car in the shade, roll the windows down for a few minutes, and then get the A/C cranking before you load up your harp. I take everything else I will need and have it right beside the car so it can be loaded in immediately after the harp without having to make any more trips back into the house. Also, plan to arrive at your destination with plenty of time to allow the harp to acclimate to the new temperature before doing a lot of tuning.
If you arrive at a gig, and they want you to sit right under a huge A/C or heat or too close to a fireplace, speak up for yourself and ask that they allow you to set up somewhere else!
Of course, here in hellish Houston, temperatures inside a car can get over 125 degrees very quickly, even on a relatively cool day, so we cannot leave a harp in a car for any length of time most of the year. Leaving your harp in a very cold car is not advised either. I try to get from Point A to Point B, and unload the harp as quickly as possible!
Most of the best advice for taking care of your instrument is common sense. Be sure that the harp is not sitting in a window in full sun nor in the direct blast of an A/C vent or near an exterior door that gets opened and closed a lot. And run a humidifier, even if you live in a humid climate. Your A/C takes the humidity out of your indoor air!
If your harp is going through changes as the weather changes, be patient with it. At a workshop many years ago, I actually saw a young harpist crying, dramatically declaring that her harp hated her because it broke a string! I can assure you that it is nothing that personal. Tuning and replacing strings are all part of what we sign up for when we decide the play the harp!
Keeping conditions as constant as possible for your harp will go a long way toward minimizing tuning and string breakage and maximizing the overall well-being and longevity of your harp!
Take care of your harp, and it will take care of you!