The third one, but the long form of negation would be more natural like "날씨가 춥지 않아요." e.g. to be cold = 춥다 / to be hot = 덥다 The long form(neutral) ☞ 춥 + 지 않아요 / 덥 + 지 않아요 The short form(emphasized) ☞ 안 + 추워요 / 안 + 더워요 \^o^/