What is unusual about the description of summer in Sonnet 18 is that the speaker mentions the negative aspects of summer. Usually when we think of summer we think of the beautiful sun and warm temperatures, but here the speaker mentions the "rough winds" that shake the early flower buds, and the cloudy skies when the "gold complexion (of the sun is) dimmed." The speaker comments on the fact that while summer is warm, it can sometimes be too hot to be enjoyable. The ultimate point he makes about summer is that summer is imperfect and too short of a season, and like everything of nature, it eventually fades to something not as pleasant (like fall and later winter.) This is what he means when he says that "every fair from fair sometimes declines, By chance of nature's changing course untrimmed." When you consider the premise he makes in the first line that he is going to compare the person to a summer's day, you might expect a list of pleasant images, but remember that the second line says, "thou art MORE lovely and MORE temperate." The speaker is paying a very high compliment when one considers the good and the not so good aspects of summer.