In the beginning of the poem, when the Lady is content to remain in her tower, alone, cursed, and isolated, but happy, the weather is calm,
"Little breezes dusk and shiver" (Tennyson)
But once she gets a glimpse of Sir Lancelot, she becomes determined to leave her prison. The weather reflects how she looks at Lancelot, he is a knight in shining armor, the poem uses the shining sun as a symbol of what he represents to her.
"The sun came dazzling thro' the leaves,
And flamed upon the brazen greaves
Of bold Sir Lancelot." (Tennyson)
After she decides to leave the tower and seek out Lancelot, the weather reflects the tone, it is stormy, and disturbed by the Lady leaving the tower and activating the curse, she will die.
"In the stormy east-wind straining,
The pale yellow woods were waning,
The broad stream in his banks complaining.
Heavily the low sky raining." (Tennyson)
The weather becomes mournful, sullen, as the Lady steps into the boat. It becomes very cold, symbolizing the slow death of the Lady as she floats to Camelot.
"Lying, robed in snowy white
That loosely flew to left and right --
The leaves upon her falling light --" (Tennyson)
"Heard a carol, mournful, holy,
Chanted loudly, chanted lowly,
Till her blood was frozen slowly,
The use of weather as a metaphor of, or to mimic the action in a work of literature is a common, easily identifiable, and generally effective literary device. The Lady of Shallot, in Part III of Tennyson's poem, narrows her focus from the various passers-by to one individual knight, Lancelot, who is described as riding through "blue unclouded weather." In Part IV, the sunlight is replaced by a "low sky raining" as she completely abandons her craft to watch him, bringing on her own death.