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Firstly, the witches seem to talk in rhyming couplets. This adds tension to the writing as it sounds as if they are chanting a spell. Secondly, the thunder and lightning adds chaos to the scene as if there was something abnormal about the place.
The phrase `there to meet with Macbeth' is effective as it makes the witches seem psychic. The line `fair is foul, and foul is fair' is very paradoxical. This contradiction inserts intellect to the witches' characters.
The witches are believed to have familiar spirits. This is shown in "I come, Graymalkin". This makes the reader ponder whether these witches are of this world.
The phrase `hover through the fog'. This makes it seem that the witches can fly and links with the familiar spirits. This adds to the eeriness of the scene.