Weather is a symbol for mood and a foretelling of the events to come. The way in which the witches use nature does both. They reference the sun-”That will be ere the set of sun”-to say that the setting of the sun is the ending of the story, the battle lost and won. It also means the end of an era(Duncan’s ruling and the pattern of overthrowing relatives), a new day to come(ending with Macduff as ruler).
The witch’s use of nature also adds to their mysticism. The witches are meant to guide and confuse the characters in the play, share personal information with them yet be distant. Later the second witch says-”I’ll give thee a wind” and the first witch says-”Though his bark cannot be lost.” The second witch means to aid the other but she uses their power of nature to highlight their supernatural ways, that they go about the world in a different way. Their language carries over to their diagnosis of their subjects. In a deeper meaning, the image of Macbeth’s inability to shed his bark could mean that he will be unable to change who he is/becomes, or shake the feeling of guilt.