In Act I, Scene 1, the witches chant “Fair is foul and foul is fair.” The shows that what was once fair will become destructive. The thunderstorm underscores this transition. A once peaceful sky has become turbulent. Macbeth, a once loyal subject, will turn on Duncan.
In Act II, Scene 4, the old man mentions many different events which illustrate that things are out of place. This is the pathetic fallacy. The pathetic fallacy is when nature reflects or empathizes with human destruction, suffering or love. In this case, the old man notes that a falcon was killed by a mousing owl hawk. Ross mentions that the tame horses have gone wild. And in the opening scene of the play, the thunderstorm reflects the witches’ words and foreshadows evil events to come. The old man ends this scene with:
God’s benison go with you, and with those;
That would make good of bad, and friends of foes.
The old man offers God’s blessing to Ross and to those who are behaving in ways that are contrary to their nature. Simply put, Macbeth was loyal and good; now he’s bad. If things are generally going haywire, then anything behaving contrarily, making good of bad or vice-versa, is suspicious.