Hi there. The following extract is from a novel. I couldn't understand what the writer wants to mean by saying "achingly clear". How would you paraphrase it? And why does the writer use "an" in the sentence. Don't we use "an" before nouns. Here, however, it is used before an adjective. I will appreciate your help.
The road -- actually little more than a series of ruts worn by hooves and wheels -- follows the river where it plunges down a series of shallow falls. Under the birches patches of moss gleam emerald in the sunlight. Fallen leaves, crystallized by the night's frost, crackle under my feet, whispering of the coming winter. The sky is an achingly clear blue. I walk quickly in my anger, head high. It probably makes me look cheerful.