Why is the night sky sometimes bright and the sky dark?

It is very bright at night, and there are two concepts. One is that the sky background is bright, but there are few or no stars; Second, the sky background is dark, but there are many stars and bright. According to what you said, it should be the first case.

This situation is usually formed by the reflection of ground light from the sky, that is, the sky is illuminated by ground light.

There may be several reasons for this.

-Cloudy and cloudy at low altitude. The ground light is reflected by the ground and enters the sky, and then diffused by the bottom of the clouds, so the sky looks brighter.

The air is humid and foggy. When the temperature drops at night, the moisture in the air partially condenses into fog, which makes the ground light diffuse and the sky brighten.

-the air quality is poor and there is smog. Like fog drops, mist particles can diffuse ground light.

-There is dust in the low-altitude air. In floating dust or sandstorm weather, the night sky will be illuminated by the reflected light of dust floating in the air.