In act 1, scene 3, Casca comes running breathlessly to announce that he has seen something so tempestuous and alarming that he believes the gods have been incensed to "send destruction."

Cicero is naturally alarmed to hear this and asks Casca what he has seen to make him feel like this. Casca goes on to elaborate upon the omens he has encountered. In ancient Rome, it was believed that certain signs indicated that the gods were unhappy and that something terrible was likely to happen. These signs would usually be something supernatural or uncanny, but the Romans also set great store by listening to the words of sooth-sayers and examining the entrails of animals for signs before going to battle, so Casca and Cicero do take these omens as very serious portents of danger.

The omens that Casca lists are actually four:

1. He has seen a common slave holding up a left hand which appeared to be burning like a torch, without causing any apparent damage to the hand;

2. He has seen a lion near the Capitol, who "glared" at him but then continued on its way without paying him any attention;

3. An omen which was only reported to him by others was that men were walking up and down the streets while on fire;

4. He has heard a bird of night "hooting and shrieking" in the market place at noon.

Casca argues that while it might have been possible to explain away one of these things, they cannot all be explained away. They are, to him, unnatural and connote bad things to come.

In act 1, scene 3 of William Shakespeare's historical tragedy Julius Caesar, Casca, a Roman nobleman and tribune, and Cicero, a Roman senator, meet in a street in Rome during a raging thunderstorm. Casca seems awestruck by the storm, and he tells Cicero that he's seen many storms in his life, but he's never seen such a violent storm with so much lightning—what he calls "a tempest dropping fire" (1.3.10).

Casca thinks that the storm signifies unrest among the gods, "a civil strife in heaven" (1.3.11), or some wrong done to the gods by the Roman people which has caused the gods to send such destruction on them.

Cicero also seems awestruck by the storm, but in a positive way, asking Casca, "Why, saw you anything more wonderful?" (1.3.14).

Casca then describes to Cicero what he's seen that evening which causes him such concern and distress. Casca says that he saw a slave who held up his hand, "which did flame and burn / Like twenty torches join'd" (1.3.16-17), but his hand remained unharmed. A lion brushed again him in the Capital, Casca says, but it passed him by without attacking him.

Casca tells Cicero that he saw a crowd of a hundred fearful women, "who swore they saw / Men all in fire walk up and down the streets" (1.3.24-25). Casca also says that he saw an owl, "the bird of night" (1.3.26), sitting in the Forum marketplace at noon, "howling and shrieking" (1.3.28).

Casca believes that these are unnatural, unexplainable occurrences, "portentous things" (1.3.32), which reflect the current state of unrest in Rome and which are omens foreshadowing tumultuous events in the city.

Cicero remarks that the state of Rome does seem to be unsettled at the moment, but "men may construe things after their fashion" (1.3.35), meaning that people will perceive and interpret these events based on their own frame of mind and their own frame of reference, which might have nothing at all to do with the events themselves.

Casca's belief is that these events portend upheaval and chaos in Rome, but for Cassius, who Casca next meets in the scene, the terrible storm will pass, and—with Caesar's assassination—Rome will be cleansed of its "yoke and sufferance" (1.2.90), and the tyranny of Caesar's rule over the city will come to an end.

In act 1, scene 3, Casca has a conversation with Cicero, where he describes several omens that he witnessed throughout the night. Initially, Casca tells Cicero that he's never seen such a terrible storm before in his life. Casca describes the storm as a "tempest dropping fire," which is another way of saying there is a terrible thunderstorm with numerous lightning bolts in the sky.

Casca's first omen is when he witnesses a common slave holding up his hand, which appears to be on fire "like twenty torches joined." However, the slave's hand is not burnt and seems immune to the fire. Casca's second omen is when he witnesses a lion staring at him near the Capitol, then strutting past him without bothering to attack. Casca's third omen is when a night owl hoots in the middle of the marketplace during the day. 

Superstitions abound in the Rome of Julius Caesar. In fact, the chaotic state of human affairs is reflected in the many omens of Shakespeare's play.  A key figure in helping Cassius sway Brutus to feel that it is necessary to rid the republic of the tyrant Julius Caesar, Casca describes for Cassius what he has heard and seen in Act I, Scene2.  Then, in Scene 3, Cicero asks Casca what he has observed in the storm of lightening and thunder. 

Casca, visibly shaken, replies that he has seen four omens: 

• a common slave whose left hand was caught on fire, but it "remained unscorched."
• a "surly" lion who was in the center of Rome; it stared at him and passed by without attacking him.
• A hundred ghostly women, huddled together, who in their fear swore that they had seen men on fire, walking up and down the streets.
• An owl (seen the day before) who was incongruously out at noon, "hooting and shrieking."

After hearing Casca, Cassius, who has disputed fate previously with Brutus, berates Casca for his fears, telling him that he, Cassius, bared his chest in the aim of the flash of lightening, daring it to hit him.  Unlike Casca, who perceives these omens as warnings from the gods, Cassius sees them as warnings against Caesar.