How does st elmos fire help ahab trick the crew later that night?

In "Moby-Dick" by Herman Melville, Captain Ahab uses St. Elmo's fire as a sign to manipulate the crew's superstitions. By creating a sense of mystery and foreboding surrounding the phenomenon, Ahab instills fear in the crew and uses it as a tool to control their actions towards achieving his goal of hunting Moby Dick.