What does the weather forecast mean that there are no clouds in Wan Li?

Words such as "sunny to cloudy", "cloudy between sunny days" and "cloudy to cloudy days" often appear in weather forecasts, but how to distinguish them?

In meteorological industry standards, sunny days, partial cloudy days, cloudy days and cloudy days are judged by the percentage of cloud area in the sky: it is sunny when the cloud amount is 0%- 10%; 10%-30% partly cloudy; 30%-70% is cloudy; More than 70% is cloudy. "It is not difficult to see that sunny days are not absolutely' cloudless in Wan Li'." Cai, a senior engineer of the Central Meteorological Observatory, said, "Partial cloudy weather is a necessary item in observation, but it is not often used for forecasting because it is close to sunny days."

Visually, when it is sunny or partially cloudy, the blue sky is like washing, and there is no cloud or only a small amount of cumulus in Wan Li; Clouds cover most of the sky on cloudy days, mostly cumulus clouds or layered clouds with a small range, through which sunlight can shine on the ground; On cloudy days, a large range of layered clouds cover the sky, which has a great influence on the light transmittance, and generally the sun is not visible.

On cloudy days, white clouds are floating in the blue sky. On cloudy days, there will be a sense of depression that "dark clouds are pressing the city to destroy".