We all know that clouds are made up of many small droplets of water and small ice crystals, and that raindrops and snowflakes are made up of these small droplets and small ice crystals that grow larger. So how does snow form?
In water clouds, cloud droplets are small water droplets. They increase in size to become raindrops mainly by continuing to condense and collide and merge with each other.
Ice clouds are made up of tiny ice crystals. When these tiny ice crystals collide with each other, the surface of the ice crystals heats up and melts a bit, and they get coated with each other and freeze up again. This is repeated many times, and the ice crystals increase in size. In addition, there is also water vapor inside the cloud, so the ice crystals can continue to grow by condensation. However, ice clouds are generally very high and not very thick, where there is not much water vapor, condensation grows very slowly, and there are not many opportunities for mutual collision, so they cannot grow to a large size and form precipitation. Even when precipitation is caused, it tends to be evaporated on the way down and rarely reaches the ground.
The most favorable for cloud droplet growth are hybrid clouds. Mixed clouds are made up of small ice crystals and supercooled water droplets*** together. When a cloud of air is saturated with ice crystals, it is not yet saturated with water droplets. At this time the water vapor in the cloud to the ice crystal surface condensation, while the overcooled water droplets in the evaporation, then the ice crystals from the overcooled water droplets "adsorption" of water vapor phenomenon. In this case, the ice crystal growth is very fast. In addition, supercooled water is very unstable. When touched, it freezes. So, in a mixed cloud, when a droplet of supercooled water collides with an ice crystal, it freezes and adheres to the surface of the ice crystal, causing it to grow rapidly. When the small ice crystals grow large enough to overcome the drag and buoyancy of the air, they fall to the ground, and this is called a snowflake.
In early spring and late fall, the air near the ground above 0 degrees Celsius, but this layer of air is not thick, the temperature is not very high, will make the snowflakes do not have time to melt completely on the ground. This is called "wet snow", or "rain and snow". This phenomenon is called "sleet" in meteorology.
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