well i dont know if it is humidity or hot/cold or maybe a mixture of both but when i go up the hills with my guitar it is perfectly tuned when it goes in the case then when i get into a freezing cold bothy take it out the case trum it once and it is fine but half hour later it sounds awful.
I try to keep it in a warm part of the room but not too near the log fire and it seems fine most, unless I am really cold and have to sit and play next to the fire, then it will go out of tune again.
In situations like this I keep my intelitouch tuner clamped on to keep it tuned as much as possible, but then when i get steaming drunk I dont really care about it going off tune slightly.
The person to invent strings that only need tuned once in their life will be a very rich man indeed.
ye get some that are cut out for the job and others just get by from pretending