Why does it sometimes have the sun and sometimes it doesn't?

The earth always revolves around the sun at the same time. The rotation of the earth produces the phenomenon of alternating day and night. The hemisphere with its back to the sun enters the day and the hemisphere with its back to the sun enters the night. The sun is a star, always burning and shining, so sometimes there is no sun during the day, but the sun is blocked by clouds and can't be seen.

Sometimes you can see the sun during the day, and sometimes you can't see the sun. In meteorology, such weather can be called cloudy. As shown in the figure below (second symbol in the first row):

According to the meteorological industry standard, sunny, semi-cloudy, cloudy and cloudy days are distinguished by the percentage of cloud area in the sky;

0%- 10% cloud cover is sunny; 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'. 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.

Because the high clouds have low temperature and are mostly composed of ice crystals, they are very bright; Low clouds are mostly composed of water droplets, which are slightly gloomy. 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".

"Sunny to cloudy" and "cloudy between sunny days" are easily confused. The change from sunny to cloudy means that it will appear sunny first and then turn cloudy gradually during the forecast period, with the focus on the process of weather change, that is, the weather will change; Cloudy between sunny days means that it is sunny most of the time, sometimes cloudy, and sometimes the amount of clouds increases.

"Cloudy to cloudy" indicates the possibility of bad weather, and the cloud cover gradually exceeds 70%. If the water vapor conditions are well coordinated, cloudy days are often accompanied by local showers; If the weather system is not well coordinated, it will only rain on cloudy days.