I believe that the most fitting answer for this question would be imagery. It would not be a simile, as it is not a comparison and does not use the words "like" or "as." It is also not a metaphor because, again, it is not a comparison. It is not personification because it does not give inanimate objects human actions or traits. It would be imagery because it describes how the morning was overcast without actually stating so; it instead uses sensory information to make this point. Hope this helps.
