Ahab's Madness

Ahab's Madness

He lived in the world, as the last of the Grisly Bears lived in settled Missouri. And as when Spring and Summer had departed, that wild Logan of the woods, burying himself in the hollow of a tree, lived out the winter there, sucking his own paws; so, in his inclement, howling old age, Ahab's soul, shut up in the caved trunk of his body, there fed upon the sullen paws of its gloom (Chp. 34, pg. 131)!

Ahab scarcely appears on deck until his rousing speech about Moby Dick, but is frequently characterized as a mad and tormented soul. How does Ishmael already know so much about Ahab's character? Should we suppose that he has heard stories from the rest of the crew? In any case, the characterization of Ahab seems to be very blunt and sudden; Melville occasionally describes him but seems to do so rather haphazardly and without any concrete examples to explain his madness. Is Ahab's madness, as well as his character in general, supposed to be somewhat mysterious or do I just not fully understand it?