How does the weather in the brief opening scene of "Macbeth" reflect the human passions revealed in the rest of the act?
The thunder and lightening in the opening scene with the introduction of the three witches is symbolic of the havoc that they intend to wreck on human nature, particularly using Macbeth as their emissary of destruction.
Shakespeare uses nature, the disorder that is created by Macbeth's actions, killing a rightful, honorable king, Duncan, to illustrate how closely human feelings and actions are tied to the function that nature plays in the world. Lennox discusses the crazy night that just past, the night that Macbeth killed Duncan, with the Porter at his home.
"The night has been unruly. Where we lay,55)Our chimneys were blown down, and, as they say, Lamentings heard i’ the air, strange screams of death, And prophesying with accents terrible Of dire combustion and confused events
New hatch'd to the woeful time. The obscure bird(60) Clamor'd the livelong night. Some say the earth Was feverous and did shake." (Shakespeare)
The destructive forces that Macbeth unleashes when he disrupts the order of nature in Scotland is exemplified by the chaotic destruction that nature renders in response.
There are storms that destroy homes, raging violence is exhibited in the animal community, Duncan's horses are said to cannibalize each other in one scene.
The stormy weather in the opening scene of Macbeth is suitably portentous, providing us with a terrifying glimpse of what's to come. Right from the outset, the forces of nature are presented as wild and rebellious, with the power to disrupt and destroy. This is particularly appropriate as it's what Macbeth will himself end up doing. He too is a force of nature who will turn the social order upside-down in his quest for power.
But he also relies on the supernatural forces harnessed by the Weird Sisters to help him to achieve his ambitions. The thunder and lightning in the opening scene are a natural reflection of the deadly supernatural powers of the witches. The stormy weather, and the dark, gloomy atmosphere that it provides, heightens the sense that what we're about to witness is profoundly unnatural; it's as if the weather is showing its anger at Macbeth and the witches. Indeed, it's instructive that Shakespeare never mentions any other kind of weather in the play except thunder and lightning; and even then, he only does so before the witches make an appearance.
The brief opening scene of Macbeth very aptly introduces the readers/spectators to the upside down world of evil ambition over-riding scruples of conscience. Three Witches appear on a desert place away from human habitation in the midst of thunder and lightning, the foul weather resulting from their incantation being symbolic of foulness of moral convulsion in Duncan's Scotland underlying the fairness epitomised by the noble and heroic Macbeth. Whenever the Witches appear, they bring with them inclement weather that signals commotion and disorder in the natural world of man:the loyal & fair turning treacherous and foul, the host betraying the guest, age-old trust of hospitality & kinship being violated. The eerieness of weather, the Witches' reference to the 'hurlyburly', their proposal to meet Macbeth on a heath, the mewing of the gray cat and the sound of the paddock, all contribute to the tragedy of conflicting passions, confusing the fair & the foul:foulness underlying the facade of fairness. The opening scene thus shows pollution and inclemency--bad weather--suggesting deeper pollution in the human world.