Why does it snow?

The reason why it snows is because the temperature is below freezing and the water vapor in the air reaches saturation.

When the temperature drops below freezing, water molecules in the air condense into water droplets or ice crystals to form clouds. If water droplets or ice crystals in a cloud are heavy enough, they may fall from the cloud to form snowflakes. Snow can also form when the humidity in the air is high enough and the temperature is below freezing.

The names of these atmospheric solid precipitations vary from place to place and from person to person. The names are numerous and extremely inconsistent. For the sake of convenience, the International Snow and Ice Committee of the International Hydrological Association convened a specialized international conference in 1949 on the basis of soliciting opinions from experts from various countries.

The meeting adopted a proposal on a concise classification of atmospheric solid precipitation. This concise classification divides atmospheric solid precipitation into ten types: snow flakes, star-shaped snowflakes, columnar snow crystals, needle-shaped snow crystals, dendritic snow crystals, axis-shaped snow crystals, irregular snow crystals, graupel, ice particles and hail .

The formation mechanism of snow

The necessary condition for snow is saturation of water vapor. The maximum amount of water vapor that air can contain at a certain temperature is called the saturated water vapor amount. The temperature at which air reaches saturation is called the dew point. When saturated air cools to a temperature below the dew point, excess water vapor in the air turns into water droplets or ice crystals.

Because the saturated water vapor content of the ice surface is lower than that of the water surface, the water vapor saturation required for the growth of ice crystals is lower than that of water droplets. In other words, water droplets must grow when the relative humidity (relative humidity refers to the ratio of the actual water vapor pressure in the air to the saturated water vapor pressure of the air at the same temperature) is not less than 100%.

As for ice crystals, they can often grow when the relative humidity is less than 100%. For example, when the air temperature is -20°C and the relative humidity is only 80%, ice crystals can grow. The colder the temperature, the less humidity required for ice crystals to grow. Therefore, in high-altitude and low-temperature environments, ice crystals are more likely to be generated than water droplets.

Reference for the above content: Baidu Encyclopedia-Snowing