DAVINA: This line echoes the witches’ remark in the first scene, "Fair is foul, and foul is fair."

RALPH: Perhaps Macbeth means that the weather, which had been so fair, has suddenly turned foul – not realizing that the change is a sign of the nearby presence of the witches.

DAVINA: Or perhaps he’s just commenting on the contrast between the bad weather and the good outcome of their battle.

RALPH: In any case, these two lines highlight one of the themes of the play – that things which seem to be good may, in fact, be bad; and vice versa.