Why sometimes the weather on the other side of the mountain is different?

There is only a mountain across, but what about the climate on both sides of the mountain? Landform? Vegetation? The soil is so different? At the foot of Jin Jia Mountain on this side of Baoxing County, there are lush vegetation on both sides of the highway, lush virgin forests, and a green and pure white stream nearby, such as silver, and the sound of water is gurgling. After spring, the scenery here is enchanting and charming, but the climate is really pleasing. It's either raining or foggy. After crossing the pass, I saw another world: the blue sky is boundless, the sun is shining, the clouds are flawless, and the air is transparent and fresh. Looking ahead, there are towering icebergs and snow peaks, overlooking the earth under your feet, and the rugged yellow land is desolate and bleak. The mountains here are bare, the hills are barren, the climate is extremely dry, the hot air blows hard, and the streams are cut off? Who would have thought that the climate difference between the two sides was so great just across a mountain? No wonder there is such a local proverb: "After crossing a mountain, it is another day" and "Two days are separated by a mountain"?