Guitars need to be watched in extremes of summer weather. A long dry period will result in the strings going flat ( b ) as the instrument shrinks a little as it dries out. When the weather breaks the wood that the instrument is made of will try and drink in as much moisture as it can and will expand fractionally pulling the strings tighter and therefore sharp ( # ). I’ve even known owners of nylon strung guitars to wake up the morning after a storm to find that, due to this, strings have “mysteriously” broken during the night.