Why the moss is not always on the north side
The old advice works only where sunlight is the main thing drying the bark. In a narrow valley the wettest side of a trunk is the side facing the stream or the prevailing rain, and the moss follows the water, not the compass.
Counted twenty oaks along the lower path today. Eleven had the heaviest moss on the side facing the stream, which happens to be south-east. So much for navigation.