Have you ever noticed that moss tends to grow thicker on the north side of a tree? Or maybe you've heard that moss only grows on the north side of a tree and later wondered if it was true. Well, it all depends on where you live.
If you live in the middle latitudes of the northern hemisphere, the moss will grow thicker on the north side. It may sound like an urban legend, but it's quite real. And the reason is simple: there's more sun on one side. In the middle latitudes of the northern hemisphere, the south sides of things get more sunlight in general throughout the year. Surprisingly, moss grows better in the shade, not the sun.
It can be a subtle effect. A more obvious example is mountainsides. Since anything, no matter how large, gets more sun on its south side, the northern sides of mountains are cooler and less dry. That's why northern mountain slopes in the American west are often visibly greener than southern ones. Another example is ski slopes. If you take a look at where ski slopes are located, you'll find them on the northern sides of mountains because it's cooler.