The ideal conditions for an acoustic guitar are 70-75o F and 40-50% humidity. Of course this is ideal and not always attainable. When I bought my first Martin in 1971, I couldn't afford the case so it was left out. I now have 2 splits as a result. When I have to take my guitars out in hot or cold weather, (in the case of course), I always cool off or warm up my car ahead of time and never leave my guitars in the car unattended. Never put a guitar in the trunk.
If you must expose your guitar to extremes, remove all
the tension from the strings and allow the guitar to return to a good
temperature in its case slowly before tuning up
again. I've been in some music stores that are so hot and humid, the
guitars are already warped and have rusty strings. Make sure your music
store cares for its guitars before you buy one there. Remember: If you
are uncomfortable, your guitar is uncomfortable.
Also see my Tip 69 on bellying.
Bob, Gman ( o )==#