The storm outside isn't anything of the ordinary type; it's raining fire! What's more, as my collegue stated, there is a lion roaming the streets, a man with a flame the size of twenty torches coming from his hand but not burning his flesh, a hundred ghostly women who swore they saw men engulfed in fire roaming the streets and a night owl screeching at midday.
Casca is frightened by these signs and fears that he and the conspirators are being warned by the gods not to act against Caesar, but Cassius insists that the signs can be interpreted any way men choose to see them. He chooses to believe the gods are not warning the conspirators, but rather warning Rome herself against Caesar. To prove his point, he claims to have walked around in the storm and even bared his chest to the thunder. The gods did not strike him down, so he believes they are on the conspirator's side.
In Act I, scene iii, a terrible storm is brewing outside, symbolically reflecting the growing conspiracy afoot to murder Caesar. The unrest over the impending storm is reflected in the characters' unrest in Rome.
He sees a slave whose hand is in flames but is not burned, a lion roaming in the capitol (Rome), and 100 ghost-like women who say they have seen men walking enflamed through the street. Finally, he says he sees a night own at noon hooting in the city. All of these omens seem to be warning that things are not as they should be in Rome.