Cassius's reply to Casca's observation about the storm--that it represents the heaven's response to the events about to take place--is significant. Cassius tells Casca that he is "dull," that that "those sparks of life / That should be in a Roman [he does] want." In other words, Cassius says Casca is being foolish in responding to the storm in this way. In the previous scene Cassius tells Brutus, "The fault, dear Brutus, is not in the stars but in ourselves that we are underlings" (140-141) again, as he does with Casca, dismissing "the heavens" as an indicator of or influence on or having responsibility for the events on earth. Cassius is a man of action, and Casca more cautious, here expressing his fear that the conspirators are not doing the right thing.
The storm is symbolic of the crisis in Rome and for Caesar that will follow. Casca, in lines 53-58 warns Cassius. Casca connects the treachery on earth to the anger in the heavens:
But wherefore did you so much tempt the heavens?
It is the part of men to fear and tremble
when the most might gods by tokens send
Such dreadful heralds to astonish us."
That the atmosphere is building tension-the atmosphere around Caesar. Cassius is of jealous of Caesar and recognizes that he can manipulate Brutus, who already has some concerns as to Caesar's threat to Rome. Casca is another force building his case against Caesar-his account of Caesar's seizure is altered to raise doubts. There is a storm building for Caesar.