All sorts of supernatural portends foretell that something terrible is about to happen in Rome, and this helps creates a mood of unease, terror, and tension.
Casca tells Cicero that for the first time he has watched "a tempest dropping fire": in other words, it rained fire, not water, during a storm. Casca also saw the hand of a slave blazing on fire as if lit by twenty torches, only the fire did not hurt him and his hand did not get burned. Casca also ran into a real lion as he was walking the streets of Rome, which looked at him in a "surly" way but didn't attack him. Further, Casca reports that group of frightened women said they saw men on fire in the streets. Finally, an owl, usually only up at night, was heard hooting in the market in the middle of the day.
These multiple omens lead Casca to believe civil unrest is coming to his country. Cicero says it is difficult to interpret omens and that people interpret them to suit their own needs and desires. In fact, the assassination of Julius Caesar and civil war will soon occur.