原定于周日的发射工作因浓云密布而推迟。
That is, smoothly for a launch that was set for Sunday, but postponed because of thick cloud cover.
下雨前浓云密布天空。
书里并没有说天空浓云密布，只是有点烟雾弥漫，天空浑然一色。
The sky is not described as heavily overcast, but rather sort of hazy, so that it's a uniform color.
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