Humidity changes can throw a neck out of whack pretty quickly, causing too much forward bow (relief) or back bow. Taking your instrument to a different environment may cause the neck relief to change, and when you return home it will most likely follow suit.
If your guitar has excessive fret buzz from the first to seventh frets, your neck may be too straight. If your action seems like it’s gotten higher, the neck may have bowed forward. You can correct this if your guitar has an adjustable rod, but you should know how to adjust it and how to determine when and if it needs adjustment.
Regardless of your desired action height, the truss rod should typically be set the same way on most guitars for optimum playability. Although the truss rod will raise or lower the action, it should be set for a slight forward bow, and you should adjust the nut and saddle heights to get your desired action (we’ll discuss that later). On average, acoustic guitars like 8–10 thousandths of an inch of relief, although you may prefer more or less depending on your playing style. For example, if you play hard-strummed chords mostly in the first position, you may find a little extra neck relief keeps unwanted buzz away. On the other hand, if you have a delicate attack and play all over the neck, you may not need as much relief and could benefit from the consistent feel and lower action of a straight neck.
Here’s how I check neck relief: With the guitar tuned up, hold the high E string down at the first fret with one hand (or use a capo) and at the neck-to-body joint with the little finger of your other hand. Then, with your thumb, press the string down at the seventh fret a few times and notice how far it moves. A feeler gauge is a good tool to use if you don’t trust your eye. If the string moves, this means your neck has relief. It should move about the width of the high string or a tad less. If the string doesn’t move at all, this indicates a dead straight or back-bowed neck. It’s a good idea to check relief on the low string too: a difference of more than a few thousandths between low and high strings may indicate a twisted neck. As I mentioned, a straight neck may work for some players, but back-bow is never a good thing for playability.
If an adjustment is needed, make sure you have the proper adjustment tool or you could damage the truss-rod nut. Check with the manufacturer if you aren’t sure. Some truss-rod nuts are hidden behind the top brace under the fingerboard, requiring a special gooseneck tool and a mirror to locate, while others are easily accessed under a cover on the peghead or are just inside the soundhole under the end of the fingerboard. Usually, tightening the rod will counter the tension of your strings, and loosening will increase the tension, allowing the strings to pull the neck forward. Go slowly, I’d say no more than a quarter turn at a time while checking the relief as you go. If the truss rod doesn’t want to turn, don’t force it! Take it to a qualified repair luthier for a look. I can tell you from experience, broken truss rods are not easily fixed.