blue skies为什么是复数

　　Blue skies are not always a guarantee of fine weather.

　　蔚蓝的天空并不永远保证晴朗的天气。

　　Once there were blue skies, with white clouds high above.

　　曾经，这儿有蓝天，白云高高悬挂。

　　The rain finally let up to reveal blue skies.

　　雨终于停歇了，展显出蓝色的天空。

　　Blue skies are not always a guarantee of continuing fine weather.

　　蔚蓝的天空并不能永远保证天气持续晴朗。