Literal meaning that weather is unpredictable; used to mean that human life is full of sudden changes or misfortunes. First half of a couplet, usually followed by 人有旦夕祸福 rén​yǒu​dàn​xī​huò​fú

Human life is full of unpredictable turns; man's fate is as changeable as the weather. Often used alongside 天有不测风云 tiān​yǒu​bù​cè​fēng​yún to emphasize that misfortune can strike at any moment.