Your question of course relates principally to Act I scene 3, and the conversation that Cicero and Casca have together, in which they discuss a variety of strange portents or events that are happening, and try and interpret them. Firstly they comment on the massive tempest that is going on around them, then Casca tells Cicero that there was a firestorm, before going on to tell him that a slave's hand was seen to burn incredibly fiercely but left no burn mark, and that he passed a lion at the Capitol and burning men were seen, and that an owl (a nocturnal animal) was seen during noontime in the central market. Note what he says about these portentous sights:
When these prodigies
Do so conjointly meet, let no men say,
"These are their reasons, they are natural,"
For I believe they are portentous things
Unto the climate that they point upon.
Clearly, Casca believes that such signs or strange events indicate that signal ominous events are about to happen and clearly are supernaturally related to major turmoil and upheaval that is about to occur. Of course, we recognise that these signs relate to the plan of the conspirators to assassinate their leader, Caesar.