"It is very windy outside."
着 goes after an action verb to show that the action is continuing. In the current preferred answer (外面的风很大), there is no action. This is just a description, literally "The wind outside is big." You can express the same meaning using the verb 刮 (to blow). I just answered 外面刮很大风 and it was accepted (literally, "Outside a big wind blows.") I didn't add 着 after 刮, but you could.