What does the weather sign mean?

You can know the weather by observing the clouds.

"Signboard" refers to a sign, which refers to the relationship between clouds and weather. Different clouds will have different weather. Thin clouds in the sky are usually a symbol of good weather.

And those low and thick clouds are often signs of rain and snow, and the weather can be judged according to various signs. The "signboard" here vividly illustrates the close relationship between clouds and weather with vivid rhetoric.