Cassius and Casca's reactions to the great storm in Act I, sc 3, are very different. Casca is frightened by it and interprets it as the anger of the gods, but Cassius scorns him for being scared. Cassius appears almost exhilarated by the storm and puts another intepretation on it, declaring that it's a warning that Caesar is growing too powerful and that other Romans must act against him. Cassius's real determination to get rid of Caesar is much in evidence here. He also gets Casca on his side and the two plan to recruit Brutus as well. This is where the conspiracy against Caesar, the defining event of the play, really begins to take shape, against the ominous background of the storm.
Casca is upset over the storm and reacts emotionally to it. He is "breathless" and "stares" as if he were frightened. He describes the storm as a "tempest dropping fire," and says he has never seen such a thing before. He believes it either a sign of trouble in heaven or that earth has become "too saucy" and provoked the gods. Although Casca interprets it and other strange signs he has seen as supernatural omens, he does not try to guess what they foretell. He merely says:
When these prodigies Do so conjointly meet, let not men say, “These are their reasons. They are natural."
Cassius also understands the storm as a supernatural warning from heaven that something "monstrous" is about to occur but claims to know exactly what the monstrous event is. He persuades Casca that the problem is the upcoming proclamation of Caesar as emperor and says they have to stand up against him or become slaves. He recruits Casca into the anti-Casar conspiracy. In other words, Cassius interprets the storm to suit his own political agenda, something Cicero had just warned Casca against when he said:
But men may construe things after their fashion, Clean from the purpose of the things themselves.