A windsock is that orange or striped cloth tube on a pole that you see beside a runway. It is open at both ends, so wind blows straight through it and it swings round to point away from where the wind is coming from. Pilots read it at a glance: the direction it points tells them which way the wind is going, and how full and how flat it stands tells them roughly how strong it is — hanging limp means still air, sticking straight out means a strong wind. You also see them at small airfields, at chemical plants (so people know which way a gas leak would drift), on bridges, and on highways in windy places. In English the word is sometimes used lightly for anything that flaps and fills with air the same way, and people joke about a thin person in a big coat looking like a windsock, but that is not fixed idiom, just play. It is written as one word, sometimes 'wind sock'. A more formal alternative in aviation writing is 'wind cone' or 'air sock', but 'windsock' is what everyone actually says.

風向袋、風向筒（機場那個橘色布筒）