What are some events that cause ishmael to have a "feeling of dread"?

Why, asks Ishmael, does the color of these (white) animals serve to heighten the terror we experience when we imagine confronting such dangerous creatures?

Rowing into the mist with the storm coming and encountering the whale.

The many instances when captain Ahab loses his sense of reason and peruse the whale. (like when he plows the ship head on into a storm)