firn line

例句：

1. 1. The firn line is a popular material for making hats and scarves.
   松针线是制作帽子和围巾的流行材料。
2. 2.The birch stripe on the tree adds a unique touch to the landscape.
   树上的松针条纹给景观增添了独特的触感。
3. 3.The firn line was used as a natural insulator in ancient buildings.
   松针线在古代建筑中用作天然隔热材料。