Clement weather is lovely and mild. You might describe a beautiful, warm summer evening as clement.
Clement can describe a pleasant climate — "The thing about Hawaii is it's such a clement place, always." — or even a merciful, gentle person. It's much more common to hear the antonym of clement, inclement, describing terrible weather. Clement first appeared in the 13th century as a surname, or last name, from the Latin clementem, "mild or placid," and was used to describe nice weather by the 17th century.