天有不测风云

天有不测风云

Literally means:

a storm may arise from a clear sky

Actually means:

something unexpected may happer may happen any time

It means that the weather is changeable and unpre used to remind people of worries, warning them not to be blindly optimistic, or it is applied when the speaker feels sorry for someone's unexpected ill foortune.

天气变化无常，不知道什么时候会刮风下雨。比喻人的命运、祸福变化无常，难以预料。多用来提醒人们要有优患意识，不要盲目乐观，或用来对突然遭遇的变故表示感叹。