Answer:
Metaphor is being used.
Step-by-step explanation:
Figurative language is a tool that allows the writer to provide a more vivid description of the words that he/she has written. This tool also makes the imagination and understanding of the written work more easily comprehensible by the readers. Metaphor is one such tool, where comparisons are made with things or elements that are impossible to be true in the real world.
The line "Thou art more lovely and more temperate" is taken from the "Sonnet 18" by William Shakespeare. In this sonnet, as common in sonnets, the writer is heaping praises on his lover, and proclaiming her value and admiring her beauty. He also at the same time, is asserting her significance and even compares her to the "summer's day" and beauty of nature, putting her on the same level of the beautiful nature and seasons.
The metaphorical tool used by Shakespeare allows/ rather gives a more refined and classy image of the beauty of the lover, and gives the writer a string hand in trying to capture and proclaim her beauty.