Cited in Wang Shifu's Yuan-dynasty 《西厢记》 (Story of the Western Wing, Act 3 Scene 2): 碧天万里无云 ('the azure sky, ten thousand li without a cloud'). Reinforced in Feng Menglong's Ming-era 《喻世明言》.
usage
Near-synonym of 晴空万里 - both are staples of weather forecasts and scenic descriptions. 万里无云 emphasizes the absence of clouds; 晴空万里 emphasizes the clarity of the sky.
