Minecraft wiki entry for Rain, associated text: At a certain biome-dependent elevation, the rain becomes snowfall, resulting in huge natural mountains having snow on the peaks while it is raining on the ground. Each biome has a certain temperature, and temperatures below 0.15 turn rain into snow. The temperature drops by 0.0016 per meter above sea level, so at high elevations, it can be cold enough to snow despite there normally being rain instead.