More natural phrasing:
* As the sun sets, the dark clouds will lighten. This is the most straightforward and natural way to express the idea.
* The dark clouds will lighten as the sun goes down. This is similar to the previous option, but uses a slightly different verb.
* The dark clouds will lighten with the setting sun. This emphasizes the connection between the clouds and the sunset.
* The dark clouds will be illuminated by the setting sun. This focuses on the effect of the sun on the clouds.
More poetic options:
* The dark clouds will soften as the sun dips below the horizon. This adds a sense of beauty and tranquility.
* The darkening sky will shimmer with the fading light of the sun. This uses more evocative language.
Ultimately, the best way to say it depends on the context and the tone you want to convey.
