Clouds are like signs of weather, which means

Ontology: cloud; Vehicle: signboard. Similarities: Both of them have indication function. But the signboard is more obvious, close to life and easy to understand. Therefore, the signboard is used as a metaphor, which is vivid. The word "signboard" is used figuratively here.

The original meaning refers to the brand hanging in front of the store to indicate the name of the store or the goods sold. As a symbol of the store, it is used here to vividly illustrate the close relationship between clouds and weather. The appearance of clouds is closely related to the change of weather. By observing different forms of clouds in the sky, we can know what kind of weather will be there.