Why doesn't it rain?

Because there is moisture in the air, when the air rises to a certain height, it will cool to form small water droplets, and the small water droplets collide with each other to form large water droplets, which together become clouds in the sky. Clouds will float with the flow of air (that is, wind). When the water in the cloud reaches a certain weight, it will fall and form precipitation. As for sometimes falling, it mainly depends on the horizontal flow speed of air. When the wind speed is high, it will take away a lot of water vapor in the air, so that it can't condense into certain water droplets, so it doesn't necessarily rain even on cloudy days.