How does the water vapor in the air movement form snowfall in the sky?

There are two conditions for the formation of snowfall:

1. Water vapor saturation. The maximum amount of water vapor that air can hold at a certain temperature is called saturated water vapor. The temperature at which air reaches saturation is called dew point. When saturated air is cooled to a temperature below the dew point, the excess water vapor in the air becomes water droplets or ice crystals. Because the saturated water vapor content of ice surface is lower than that of water surface, the water vapor saturation required for the growth of ice crystals is lower than that of water droplets. That is to say, water droplets can grow only when the relative humidity (relative humidity refers to the ratio of the actual vapor pressure in air to the saturated vapor pressure of air at the same temperature) is not less than 100%; As for ice crystals, they tend to grow when the relative humidity is less than 100%. For example, when the temperature is -20℃ and the relative humidity is only 80%, ice crystals can grow. The lower the temperature, the less humidity is needed for the growth of ice crystals. Therefore, in the high-altitude and low-temperature environment, ice crystals are more likely to be produced than water droplets.

There must be condensation nuclei in the air. If there is no condensation nucleus, the water vapor in the air will condense into water droplets only if it is supersaturated to the degree of relative humidity above 500%. But such a large supersaturation phenomenon will not exist in the natural atmosphere. So without condensation nuclei, it is difficult for us to see rain and snow on the earth.