Describe the climatic condition that makes Calpurnia worry.

In act 2, scene 2, Calpurnia, Caesar's wife, is plagued by nightmares. She has dark presentiments of doom, foreseeing the bloody murder of her husband. She repeatedly cries out the word "murder" in her sleep. Calpurnia, not normally given to superstition, has been disturbed by the ill-omened events that took place earlier that night: ghosts wandered around the city; a lioness gave birth in the street; men rose from the dead; and to cap it all, the dark, brooding sky was split asunder by a terrifying flash of lightning. Calpurnia is utterly convinced that these events spell doom for Caesar, and so she begs him not to go to the capitol on the ides of March. Out of a mixture of arrogance and defiance, Caesar ignores Calpurnia's warnings, as well as those of the augurs, and sets off for the senate for his date with destiny.
In this scene, Shakespeare uses lightning to create an atmosphere of doom and foreboding, providing a suitable background to the terrible events that are about to take place. If we cast our minds back to the opening scene of Macbeth, we see him using the same effect. Whether or not we believe in omens and prophecies, we can still recognize the dramatic significance of the kind of violent weather conditions that so terrify Calpurnia.