The natural imagery in Banquo’s speech contrasts the unnatural events being described.  The paradoxical nature highlights Macbeth’s treachery.

Banquo is a fascinating character.  He clearly knows much more about what is going on than Macbeth thinks he does.  In Act II, Banquo cannot sleep because he is thinking about the witches’ prophecies.  He believes that unnatural events are contriving against the kingdom.  Notice the contrast the lines you have supplied have with Banquo’s prior speech outside Macbeth’s castle with Duncan in Act I, Scene 6.

This guest of summer,

The temple-haunting martlet, does approve(5)

By his loved mansionry that the heaven's breath

Smells wooingly here. (enotes etext pdf. p. 21)

In both speeches, Banquo discusses nature in a disturbing way.  He describes the martlet as “temple-haunting” and mentions heaven’s breath at the same time.  The juxtaposition shows that Banquo is reflective, and something is really bothering him.  The unnatural events of the witches and what he senses about Macbeth.  Note that when Banquo asks Macbeth if he has been thinking about the witches, Macbeth lies casually dismisses them by saying, “I think not of them” (p. 27).

Thus, the paradox of heaven and the unnatural events highlights Macbeth’s treachery, and provides insight into Banquo’s soul.  We feel uneasy, as if he is our voice, and we know there are bad things to come.

For more read: http://www.enotes.com/macbeth/act-ii-summary-analysis

For the enotes full text pdf: http://www.enotes.com/macbeth-text