Why is it raining?

In the water cloud, all water droplets are small water droplets. They are mainly increased by continuing to condense and collide with each other. Therefore, in water clouds, if cloud droplets are to increase to the size of raindrops, first of all, the clouds need to be thick, dense and rich in water, so that they can continue to condense and grow; Secondly, there needs to be strong vertical movement in the water cloud, so as to increase the chances of multiple collisions and mergers. In a thinner and more stable water cloud, there is not enough opportunity for cloud droplets to condense and grow together, which can only cause cloudy days and is unlikely to rain.

In different clouds, the distribution of cloud droplet size is different, and the reason of uneven cloud droplet size is the transportation of water vapor in the surrounding air and the evaporation of cloud droplets. The factors that make cloud droplets grow are condensation process and collision and merger process. In the case of condensation, the size of cloud droplets is uniform, but due to the supplement of water vapor, some cloud droplets grow, and the result of merger is that larger cloud droplets continue to grow and become raindrops. Raindrops fall under the action of gravity. When there is an updraft, raindrops will be subjected to an upward force, which will slow down their falling speed and some small raindrops may be picked up. Only when raindrops increase to a certain extent can they fall to the ground and form rainfall.