This is the process of the formation of ice crystals in cold clouds. In this process water goes from liquid to vapor and then from vapor to solid.
This process leads to precipitation, usually in the form of rain or possibly freezing rain.
This microscopic ice crystals attract water vapor, increasing their size. Since the size of ice crystals is increasing, the vapor pressure is dropping. These ice crystals become so large and heavy that they begin to fall towards the Earth’s surface. As they do, they pass through the lower warmer areas of clouds, attracting even more water vapor, and growing larger.