To further explain what LATS is getting at... Acoustics are generally more sensitive to autmospheric changes than electrics. Since they are made to resonate more they use lighter, more dry woods than electrics... since you can't put heavy lacquer finishes on acoustics. What happens, since the wood of an acoustic is usually more dry, when the air gets really humid the wood will absorb the moisute in the air. This causes the wood to expand, throwing the guitar out of tune, mis-adjusting the truss rod, and in some worse-case scenarios... warping the guitar neck or cracking at the neck joints. What LATS (as well as me) is assuming, is that your truss rod has probably been readjusted by a change in humidty or temperature. Causing the neck to expand and causing your neck to have more back-bow than it normally should. This will cause the strings to buzz when you fret the guitar at the lower frets (1-5 or so). Take your guitar to a tech and have them adjust the truss-rod. You can get an entire set-up for around 50 dollars.