How can a guitar string be out of tune higher when you pick up a guitar the following day.

Factual Questions
1

I have a PRS SE 245, which has a wrap around bridge.

Every day I pick it up it is close to in tune but if it is out of tune it tends to be higher that the intended note.

I would think that the string relaxing and slip would cause it to detune down over time.

Note I have considered room temperature but I have an automated thermostat that I log and the house has been at a stable 66 degrees within 1.5 degrees for 3 hours before and during my practice session.

I am also quite sure that no one else is touching the guitar unless my dog has been playing it during the day.

By what action would a guitar go out of tune upwards in frequency?

2

Humidity. The temperature might be the same but changes in humidity can affect the tuning as well.

3

Higher humidity swells the wood, increasing the length between the nut and the bridge, thus stretching the string tighter for a higher pitch?

4

I will start to track humidity also, that is quite plausible.

Thanks.

5

I’ve noticed that my guitar nearly always goes out of tune to the high side in the summer (when the humidity is high), and to the low side in winder (when the humidity is low).

6

I have several mountain dulcimers that display a rather consistent pattern of out-of-tuneness, seeming to be more temperature oriented. If they are tuned on a hot day, and then the temperature drops, they will both go sharp. I think this is because a drop in temperature will cause the string to try to shrink, therefore increasing the tension and thusly the pitch. Vice versa results in them going flat

7

While I don’t know with absolute certainty that the strings are not thus affected, I think it’s much more likely that it’s the wood that’s responding to the temperature, and possibly to an attendant change in humidity.

8

I’ve always assumed that the vibration caused by picking a string heated it up slightly, expanding the metal and loosening the string. It would then cool off and tighten up when left alone. Just a WAG, though. Humidity makes sense and seems to be the consensus.

9

Changes in temperature and humidity, and the nature of the instrument and strings. My acoustic Kamaka ukulele, with synthetic strings, always goes flat as it sits. My Konablaster solid body electric ukulele, with flat wound steel strings, always goes sharp. They’re both baritones.

10

I have this problem pretty consistently. It isn’t specific to any time of year or place. I am going to pick up my guitar in a few minutes and I can guarantee that 5 out of 6 strings will be sharp. My accoustic guitar is so steady it’s freakish in its own way. I can transport that thing Galway across town in 110 weather and when I pick it up, it will be in tune. My electric is always sharp and it gets sharper as I play it.

Of course, my electric is a knock-off of a Tiawanese knock-off. My accoustic isn’t anything fancy, but I think it came off the line on a good day.