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

‘Dust of Snow’ means the small particles of snow that can be felt in the air after or during snowfall. The poet is very sad and is sitting under the hemlock tree. Suddenly a crow shakes the tree and the dust of snow falls on the poet. This incident changes the poet’s mood as he starts to think about nature and beauty.