What is a “dust of snow”? What does the poet say has changed his mood? How has the poet’s mood changed?

A “dust of snow” could mean a freckle of snow or it could also refer to the little white flowers that can be found on hemlock tree. This “dust of snow” changed the poet’s mood. The poet’s mood changed from that of dismay to joy. He was holding the day in regret when this dust of snow fell on him and this simple little thing brought him some joy.