How does Eric Metaxas use imagery in Bonhoeffer: Pastor, Martyr, Prophet, Spy?

Examples of Imagery:

"Bonhoeffer's sermons challenged the congregations both spiritually and intellectually. In his first sermon he leaped into his favorite subject, the difference between a faith based on our own moral efforts and one based on God's grace ... one can only imagine some of the Barcelona businessmen puzzling over this earnest twenty two year old, freshly descended from the ivory tower. And yet there was an undeniable vitality to what he was saying; he rarely lost their attention." Chapter 5, p. 77

"Bonhoeffer had been in New York a mere nine months, but in some ways it seemed a lifetime. When he left, the Nazis were a tiny gray cloud on the horizon of an otherwise clear sky. Now, black and crackling with electricity, they loomed nearly overhead." Chapter 8, p. 120