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.