▲sky属于自然界独一无于的事物（类似的有sun, moon, earth等），其前通常连用定冠词。如：

Lightning lit up the night sky. 闪电照亮了夜空。

The sun was low in the sky now. 太阳此刻低挂在天空。

Swallows darted about in the sky overhead. 燕子在头顶上飞来飞去。

A helicopter appeared in the sky above them. 一架直升机出现在他们上空。

The constellations in the night sky cannot be directly equated with the heroes of Greek mythology. 夜空中的星座不能直接与希腊神话中的英雄相等同。

▲但是，当它们受到描绘性形容词的修饰时，如果不是特指，则其前要用不定冠词，此时的a含有 a kind of 的意思。如：

A cloudy sky is not always a sign of rain. 阴天不一定是下雨的预兆。

All I recollect is a grey sky. 我所记得的只是灰色的天空。

The sun beat down all afternoon from a clear sky.  整个下午晴空当头, 骄阳似火。

The weather broke and thunder rumbled through a leaden sky. 天气骤变，雷声隆隆响彻乌云密布的天空。

The hues of the leaves in autumn are beautiful against a blue sky. 秋天，色彩丰富的树叶在蓝天的映照下非常美丽。

In sharp contrast to her mood, the clouds were breaking up to reveal a blue sky. 乌云渐渐散开，露出了蓝天，这和她的情绪形成了鲜明的对照。