Why is the Nishan Tunnel Yin and Yang?

The "Yin-Yang Boundary" of the Nishan Tunnel is named after the great weather changes on both sides of the tunnel from Ya 'an to Xichang in expressway, Beijing and Kunming. More than 70% of this expressway is built on bridges and tunnels, where there is the legendary "double spiral channel" and the "Yin-Yang boundary" of the Nishan tunnel. When entering the Mudan Tunnel from Xichang side, it was still sunny. Just after 10 minutes, when I came out from Ya 'an side through the 10 km long mud mountain tunnel, it suddenly became dark, as if at night, the mountains were covered with dense fog, the visibility was less than 50 meters, it was drizzling and the road was slippery. This is the legendary "Yin-Yang boundary".