What is this weather called when the whole sky is completely blocked by clouds?

The whole sky is completely blocked by clouds, and the weather at this time is called all-day or all-cloudy. At this time, the sun can't be seen, and the outdoor natural light is diffuse light from the sky. There is no shadow behind the object, and the sky brightness distribution is relatively uniform and stable.

The weather with clouds accounting for less than 30% of the whole sky is called sunny. On sunny days, ground lighting consists of direct light and diffuse light in the sky. The diffuse light in the sky changes rapidly except when the solar altitude angle is small, and it is almost unchanged at other times, while the direct light increases rapidly with the increase of the solar altitude angle.

Therefore, the proportion of illuminance formed by direct sunlight on the ground to the total illuminance increases with the increase of solar altitude angle, and the shadow is also obvious.

Definition standard

The definition criteria of "sunny", "partly cloudy", "cloudy" and "cloudy" are simple. In the meteorological industry standard, the percentage of cloud area in the sky is taken as the criterion: it is sunny when the cloud amount is 0% to 10%; 10% to 30% partly cloudy; 30% to 70% is cloudy; More than 70% is cloudy. It is not difficult to see that sunny days are not absolutely "cloudless in Wan Li". Partial cloudy is an essential item in observation, but it is not commonly used in forecast because it is close to sunny day.

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